Injection Water Treatment.pdf

POL Petroleum Open Learning

Injection Water Treatment Part of the Petroleum Processing Technology Series

OPITO THE OIL & GAS ACADEMY

POL Petroleum Open Learning

Injection Water Treatment Part of the Petroleum Processing Technology Series

OPITO THE OIL & GAS ACADEMY

Petroleum Open Learning

Designed, Produced and Published by OPITO Ltd., Petroleum Open Learning, Minerva House, Bruntland Road, Portlethen, Aberdeen AB12 4QL

Printed by Astute Print & Design, 44-46 Brechin Road, Forfar, Angus DD8 3JX www.astute.uk.com

© OPITO 1993 (rev.2002)

ISBN 1 872041 85 X

All rights reserved. No part of this publication may be reproduced, stored in a retrieval or information storage system, transmitted in any form or by any means, mechanical, photocopying, recording or otherwise without the prior permission in writing of the publishers.

Injection Water Treatment

Petroleum Open Learning

(Part of the Petroleum Processing Technology Series)

Contents

Page

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Training Targets

4

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Introduction

5



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Section 1 - The Reasons for Water Injection



Reservoirs and Rock Properties Reservoir Drive Mechanisms Oil Recovery Pressure Maintenance by Water Injection

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Section 2 - Reasons for Treating Injection Water



Bacteria Suspended Solids Dissolved Gases Dissolved Solids Other Chemical Treatments

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15

Visual Cues

training targets for you to achieve by the end of the unit



test yourself questions to see how much you understand



check yourself answers to let you see if you have been thinking along the right lines



activities for you to apply your new knowledge



summaries for you to recap on the major steps in your progress

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Contents (cont'd) *

Section 3 - Sea Water Treatment Equipment

Page 19

Bacteria Control Sodium Hypochlorite Generation Filtration Equipment Basket Filters Sand Filters Dual Media Filters Cartridge Filters Oxygen Removal Gas Stripping Vacuum Deaeration

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Section 4 - A Typical Injection Water Treatment System 37



Sea Water Intake and Coarse Filters Chlorination Facilities Sea Water Reservoir Fine Filtration Deaeration Water Injection Pumps Water Injection Wells

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Check Yourself - Answers

54

Visual Cues

training targets for you to achieve by the end of the unit



test yourself questions to see how much you understand



check yourself answers to let you see if you have been thinking along the right lines



activities for you to apply your new knowledge



summaries for you to recap on the major steps in your progress

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Training Targets When you have completed this unit on Injection Water Treatment you will be able to : • Define the terms porosity and permeability as applied to reservoir rock • Explain three basic reservoir drive mechanisms • Explain how reservoir pressure is maintained by water injection • Explain the reasons for treating injection water • Describe the function, construction and operation of sea treatment equipment, including filters, sodium hypochlorite generators, and deaerators • Describe a typical injection water treatment • List and describe common chemicals used in the treatment of injection water Tick the box when you have met each target.

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Oil and Gas Injection Water Separation Treatment Systems

Petroleum Open Learning

Introduction

The production efficiency of most oil reservoirs can be improved by injecting water into the layer of water which underlies the oil in the reservoir rock. But why should this be so ? Where do we get the water from and what treatment does the water require before it is injected ? In this unit we are going to look at these questions and try to answer them. The unit is divided into four sections. In Section 1, we will look at the reasons for injecting water into a reservoir in oilfield operations. We will concentrate on the nature of a typical reservoir and the properties which influence the way in which oil is produced. You will see that the most common type of water used for injection purposes is sea water.

Section 2 will focus on the problems which could result from injecting untreated sea water into a reservoir rock, and the methods used to overcome these problems.

In Section 3, I will explain the construction and operation of equipment used in the treatment of injection water.

Finally, in Section 4, we will look at a typical sea water injection system. I will take you through this hypothetical system step by step. This will give you an overall picture of the operation and control of such a process.

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Injection Water Treatment

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Section 1 - The Reasons for Water Injection Before we can go on to consider the equipment used and procedures involved in water injection, it is necessary to look at the reasons for injecting water. This will involve a little bit of reservoir engineering. Don't let the term put you off. We will tackle it in a rather simple way.

This type of reservoir is known as a dome structure. It looks rather like a number of upturned saucers lying on top of each other. Each saucer represents a layer of rock. The rock layer which contains oil, water and gas is the reservoir rock. On top of the reservoir lies a layer of rock called the cap rock. The cap rock forms the upper boundary of the reservoir and stops any fluids from migrating upwards. The whole structure is often called a reservoir trap.

Let’s start by having a look at a typical petroleum reservoir and the way in which the oil is produced:

Reservoirs and Rock Properties

In order for reservoir fluids to be brought to the surface, wells are drilled into the reservoir rock. However, in order for these fluids to enter the wells, they must be capable of flowing through the rock.

Look at Figure 1, which shows a simplified cross section through a typical reservoir structure.

A reservoir rock, therefore, must be capable of holding fluids within itself and allowing the fluids to move through it. The two properties of rock which govern this are:

• porosity



• permeability

Porosity is the property of the rock which enables it to hold fluids within itself. The oil, gas and water are contained in tiny holes in the rock called pores.

Figure 1 6

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Sandstone is a common reservoir rock. It is made Figure 2 is an illustration of a porous and permeable Before we move on, complete the following Test up of grains of sand which are cemented together rock. Yourself question. at the points where they touch. Between the sand grains are void spaces - the pores. The ratio of the volume of the pores to total rock volume expressed as a percentage is the rock porosity. This means that, if you have a sandstone reservoir with a porosity of 25% , for every 4 m3 of reservoir rock, 1 m3 consists of holes and 3 m3 solid sand grains. Another common reservoir rock is limestone. This is a rather brittle rock which contains lots of tiny cracks and fissures. These tiny cracks give the limestone its porosity. Permeability is a measure of the ability of a fluid The following example may help you to visualise to flow through the rock from one pore to another. porosity and permeability in a reservoir. In order for it to be able to do this, the pores must be interconnected. We have all seen an ordinary building brick, but have you ever examined one carefully ? Take a Permeability is measured in d’arcys, named after close look at a brick under a magnifying glass and a French engineer who studied the flow of liquids you will see that the surface is a mass of tiny holes. through filters. He found that the flow increased in A building brick is both porous and permeable. proportion to the pressure increase. However he If you place one in a bowl of water you will see also discovered that the flow was affected by the small bubbles rising from the surface. This occurs thickness, or viscosity, of the fluid. as water flows into brick and displaces the air. In many ways an oil reservior rock is similar, in terms Generally there is a wide spread of permeability of porosity and permeability, to a building brick. values in reservoir rocks.

Test Yourself 1 a) What is the porosity of a rock whose pore spaces occupy one fifth of its total volume? b) What is the likely permeability of a cap rock? c) Which of the following materials is likely to be porous and permeable?

i) sponge ii) glass iii) sandstone iv) slate

You will find the answers to Test Yourself 1 on page 54.

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When wells are drilled into a reservoir, the fact that the reservoir is porous and permeable enables the reservoir fluids to flow towards the wellbores. But what causes this fluid flow ? The presence or absence of pressure within the reservoir has a great deal to do with its ability to produce fluids. It was recognised in the early days of the petroleum industry that, when pressure was high, oil was easily produced. When pressure dropped, however, assistance was needed to help the wells produce. The fact that pressure is required to drive oil to the wellbore is important but it doesn’t fully explain how oil is produced. A complex set of circumstances causes this flow through the pores of the reservoir rock. Most of the oil is driven to the wells by one of three basic mechanisms which utilise existing energy forces within the reservoir. They are referred to as drive mechanisms. We will look at these mechanisms shortly. Before we do this, however, have a look at the list of terms highlighted opposite. They are common terms used when discussing oil reservoirs.

Gas / oil ratio - This is a term which relates the volume of gas produced from a well to the volume of oil production. It is usually abbreviated to G.O.R. and has often been expressed in units of cubic feet of gas per barrel of oil (cu.ft. / bbl). It is now more common to use the SI Units of cubic metres of gas per cubic metre of oil (m3/m3). Oil in place - is an estimate of the total amount of oil in the reservoir. Recoverable oil - is an estimate of the amount of oil which may be produced from the reservoir and recovered for sale. Recovered oil - means the actual amount of oil which has been removed from the reservoir.

When you are familiar with the terms above we can move on to the drive mechanisms.

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Reservoir Drive Mechanisms The three basic mechanisms are:

• dissolved gas drive



• gas cap drive



• water drive

They are considered to be natural drive mechanisms. No outside assistance is introduced. Let’s consider each of them in turn.

Dissolved Gas Drive Figure 3 is an illustration of a reservoir with a dissolved gas drive. In this type of reservoir, the energy to drive the oil to the wellbores comes from gas dissolved within the oil. This gas is liberated from the liquid as bubbles when the pressure in the reservoir declines. Being highly expansive, the bubbles of gas provide the energy to push the oil towards the wells as the pressure continues to decline. Reservoirs behave characteristically during their producing lives. The trends of these characteristics for a dissolved gas drive reservoir have been included in the table opposite.

Characteristics

Trends

Reservoir Pressure

Declines rapidly and continuously

G.O.R.

Low initially then rises rapidly to a maximum and finally falls

Water production

Very little

Recoverable oil

5 to 30 percent of original oil in place Table 1 - Dissolved gas drive reservoirs

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Gas Cap Drive Let's move on to gas cap drives. Figure 4 is an illustration of such a reservoir. In many situations, oil has accumulated where there are considerable volumes of gas present. In such situations, all of the gas may not dissolve in the oil at the temperature and pressure of the reservoir. The undissolved gas will then migrate to the top of the reservoir, forming a layer of free gas above the oil. This layer of gas is called a gas cap. The gas cap is usually in a compressed state. As such it becomes a source of energy to drive the oil to the well bore and lift it to the surface. The characteristic trends of a gas drive reservoir are given in the table opposite.

Characteristics

Trends

Reservoir Pressure

Falls slowly and continuously

G.O.R.

Rises continuously in wells higher in the reservoir

Water production

Low

Recoverable oil

20 to 40 percent of original oil in place Table 2 - Gas cap drive reservoirs

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Water Drive Finally let's look at water drive mechanisms, as illustrated in Figure 5. The greatest natural source of energy available to produce oil is the water which underlines the oil in certain reservoirs. In these reservoirs, the porous rock occupies tremendous volumes. However, the oil bearing part of the rock will be only a relatively small proportion of the total. The water occupies a very large volume compared with the oil. We generally consider water to be incompressible. It can be compressed to a very small extent, however, and when such tremendous volumes are involved, this compressed water can have quite an influence on the behaviour of the oil reservoir. As oil is produced, the pressure in the reservoir tends to decline. This decline is counteracted by the expansion of the water, as it moves to replace the produced oil. These types of reservoir are usually the most efficient. Once again I have included a table which shows the characteristics of water drive reservoirs.

Characteristics

Trends

Reservoir Pressure

Remains high

G.O.R.

Remains low

Water production

Increases gradually to significant amounts

Recoverable oil

35 to 65 percent of original oil in place

Table 3 - Water drive reservoirs Of course, in the natural world, reservoirs are seldom found which fit exactly into one of the classifications we have just been looking at. Combinations must occur which further complicate the picture. Before moving on, have a go at the Test Yourself question on the following page.

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Oil Recovery

Test Yourself 2 Identify the type of drive mechanism present of the following reservoirs : 1.

There is very little water production and we expect to recover up to 35 percent of the original oil in place.

2.

The G.O.R. was 0.5 m3 of gas per m3 of oil when oil production began. It rose to 3 m3 per m3 after 10 years and is now 1 m3 per m3.

3.

The reservoir pressure has declined continuously, there is little water production and we expect to recover only 18 percent of the original oil in place.

4.

The expected oil recovery is only 55 percent of the original oil in place.

5.

Water production is low and the G.O.R. has been rising especially in wells which are completed higher in the reservoir.

You will find the answers to Test Yourself 2 on page 54.

Look again at the expected oil recovery from the three types of drive mechanism.

• dissolved gas drive

expected recovery — 5 to 30 % of original oil in place



• gas cap drive

expected recovery — 20 to 40 % of original oil in place



• water drive

expected recovery — 35 to 65 % of original oil in place

Even the most efficient natural drive is likely to leave more than 35% of original oil in place in the reservoir ! It would make sense to try to improve the recovery of oil in these circumstances. So, how could we get more oil from a reservoir than is possible when relying on natural drive mechanisms ? We have seen that, with high reservoir pressure, oil production is improved. If the reservoir pressure could be maintained we could expect an increase in recoverable oil. This could be done by one of two ways.

• inject gas into a gas cap



• inject water into the water layer

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We have also seen that the most efficient natural drive mechanism is the water drive. It would seem reasonable to assume, therefore, that pressure maintenance by water injection would be beneficial. In days gone by, water was injected later on in the life of a reservoir after maximum recovery had been obtained by natural drive mechanisms. It was referred to as secondary recovery. Nowadays water injection is planned to start as soon as the field comes into production and is an integral part of many production facilities.

Pressure Maintenance By Water Injection Figure 6 is an illustration of an artificial water drive being used on an oil reservoir. As the oil is removed from the reservoir via the oil production wells, water is injected into the water layer beneath the oil. The water layer is often called the aquifer. The injection of water into the aquifer helps to maintain the reservoir pressure, many of the problems which decrease the efficiency of the reservoir are avoided. It is usually necessary to inject more water than the volume of oil being removed - in some cases as much Some reservoirs produce oil at over 16,000 m3 per day and water may be injected at a rate of over 30,000 m3 per day. Imagine. This amount of water being forced through the reservoir rock, every day for as 2 m3 injected per m3 produced. the life of the reservoir.

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Summary of Section 1 In this section we have looked at the nature of oil and gas reservoirs. We started by considering the shape of a basic dome type reservoir. From there we saw that the two rock properties which influence the way in which oil and gas are produced are: porosity permeability We then considered reservoir drive mechanisms. Here you saw there are three basic mechanisms which utilise natural forces to drive fluids through a reservoir towards a well. These are:

• dissolved gas drive



• gas cap drive



• water drive

You saw that relying on these drives can be inefficient in terms of oil recovery. Even the best drive is likely to leave behind more than 35% of the original oil in place. It would make sense to try to improve the ultimate recovery. One way of improving this recovery would be to maintain the pressure in the reservoir for as long as possible. This could be done by injecting gas into the gas cap or water into the water layer. I pointed out that the most efficient drive mechanism is water drive. Here the reservoir pressure tends to remain high and oil recovery is maximised. It would appear, then, that to simulate a natural water drive by injecting water into the aquifer could prove beneficial.

In the next section we will see where this water comes from and why it is necessary to treat it before injection.

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Injection Water Treatment

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Section 2 - Reasons for Treating Injection Water In Section 1 you saw that the injection of water into an aquifer could increase the recoverable oil from a reservoir. It may be necessary to inject considerable amounts of water. I gave an example of over 30,000 m3 of water being injected daily. The actual amounts required will depend on the size of the field and the extent of the natural water drive. But in any case we will still be looking at a very large amount being injected continuously. Where do we get all this water ? In some land locations water supply can be a problem. Surface supplies such as lakes or rivers can be used if available. If not, water wells may have to be drilled or produced water used. Offshore, there is no problem of supply. Oceans full of sea water are available. In fact, sea water may be pumped to land locations for injection purposes. As this is the most common type of water injected into oil reservoirs, we will concentrate on sea water treatment throughout the rest of this unit. If we were to take raw sea water, however, and inject it directly into the reservoir, we would very quickly find ourselves in trouble. Sea water requires a great deal of treatment before we can inject it and this is what we are going to look at in this section. We will look at the problems associated with the injection of sea water and the reasons for treating it. So let’s first look at the make-up of sea water,

and what it is. It is basically everyday water which contains the following

• bacteria



• suspended solids



• dissolved gases



• dissolved solids

Each of the above substances poses its own particular problem in water injection systems. Let’s consider each in turn and think of ways in which the problem may be overcome.

Bacteria Sea water contains tiny micro-organisms called bacteria. They are very simple life-forms which are split into two categories. They are

• aerobic bacteria



• anaerobic bacteria

Aerobic bacteria require oxygen in order to survive. They are responsible for producing the green slimes which we see on sea walls, weirs and other highly oxygenated areas of water. This type of bacteria does not cause serious problems in itself. However, it consumes oxygen and creates anaerobic conditions which can lead to the growth of anaerobic bacteria. Anaerobic bacteria thrive in conditions where there is no oxygen. They are responsible for producing the black slimes which we see in sewers and other dark and dank places where the oxygen content may be low or zero. A special type of anaerobic bacteria is called sulphate reducing bacteria. These bacteria reduce the sulphates present in the water. (The term ‘reduce’ means to remove oxygen or add hydrogen.) In the case of sulphate reduction, the result is the production of hydrogen sulphide. This gas, which has the formula H2S, is very corrosive and extremely toxic. It is essential to try to limit the production of H2S in oilfield operations. Certain bacteria are slime forming as you saw earlier. These slimes can cause fouling in pipework and, if injected, cause plugging in the reservoir. In view of this, I think you will agree that it is necessary to maintain relatively low numbers of bacteria. In particular, it is necessary to control the growth of sulphate reducing bacteria.

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In addition to bacteria, other marine organisms can enter the system. Hard shell creatures such as barnacles and mussels enter as embryo. These can accumulate and grow on pipes, restricting flow. Dead shell debris can be carried to the reservoir once again to plug the reservoir pores. The only way of preventing the problems associated with the growth of bacteria and other organisms is to kill them with some form of chemical. The chemical which is used to do this is called a biocide.

Suspended Solids If we look closely at sea water we can see with the naked eye a whole variety of suspended solids. In areas of clear water we may see small fishes, shrimps, jelly fish and other life forms. In areas of muddy water we can see very fine particles of sand and clay. All of these items are classed as suspended solids. Bearing in mind what we have already said about the properties of reservoir rocks I’m sure you can visualise what would happen if we tried to inject water containing suspended solids into the rock.

Test Yourself 3 Explain what the consequences would be of injecting water containing sand or clay particles into a reservoir rock.

You will find the answer to Test Yourself 3 on Page 54.

To prevent the problem which you saw in the answer to Test Yourself 3, the suspended solids must be removed. This is done by filtering the sea water to ensure that all but the very tiniest particles of suspended solids are taken out.

Dissolved Gases Sea water contains dissolved gases, the main one of which is air. The air is made up mainly of oxygen and nitrogen. It is the oxygen from the air dissolved in the sea water which provides fish and other living organisms with the means to live.

The combination of sea water and oxygen is, however, very corrosive. It will cause corrosion problems in the pipes and flowlines used to inject water into the reservoir. Corroded equipment will require expensive replacement or repair. In addition, the products of corrosion, i.e. rust, may enter the reservoir as small particles and block the pores in the reservoir rock. It is necessary then to remove the oxygen from the sea water. This can be done by mechanically deaerating the water, injecting oxygen scavenging chemicals, or both.

Test Yourself 4 We have said that it is necessary to remove oxygen from injection water. This in itself could create further problems. What problems are these? You will find the answer to Test Yourself 4 on Page 55.

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Dissolved Solids

Other Chemical Treatment

A large number of chemical compounds are dissolved in sea water. We only have to taste sea water to decide that one of the dissolved solids is common salt. It is this salt, sodium chloride, which gives sea water its distinctive taste.

In addition to the biocide and scale inhibitor which may be added to the sea water, it may be necessary to inject other chemicals. We will look at some of these in more detail in a later section but let me just mention one or two at this point.

Elements such as potassium, calcium and magnesium are also present as compounds which are dissolved in the water. These compounds are called dissolved solids.

Oxygen scavengers — I mentioned earlier that the removal of oxygen can be done by mechanically deaerating the water or by injecting oxygen scavenging chemicals. These scavengers are chemicals which will react with oxygen to remove it. In theory, all the oxygen could be removed in this way but it would be a very expensive operation. Mechanical deaeration by itself usually leaves a small amount of dissolved oxygen in the water and the oxygen scavenger is used to remove this residual amount.

If we allow sea water to evaporate, the dissolved solids will come out of solution as small crystals. As the crystals leave the liquid, they are said to have been precipitated from the water. In a sea water injection system the precipitated crystals will form a deposit known as scale. This scale could block pipework and, if it formed in the reservoir, could block the pores in the reservoir rock. To prevent the formation of scale, a chemical known as a scale inhibitor is injected into the sea water.

You will see where each of these chemicals is injected into the water when we look at a typical system in Section 4.

Corrosion inhibitors — Chemicals which will retard the effects of various types of corrosion may be injected into the water. Defoamers — The operation of mechanical deaeration equipment may be improved by injecting chemicals to prevent foaming of the water. Polyelectrolytes — These chemicals are coagulants. They cause fine particles in the water to stick together, forming larger particles. These are then more easily filtered from the water.

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Summary of Section 2 In this short section we have looked at the make up of sea water which renders it unsuitable for injection in its raw state. You saw that: • Bacteria can cause slimes to form which can foul the system • Bacteria can cause the formation of hydrogen sulphide, a corrosive and toxic gas • Suspended solids can plug the pores of the reservoir

• Dissolved solids can create scale

• Dissolved gases, particularly oxygen, can cause corrosion

In order to overcome these problems you saw that: • Bacteria and other marine organisms are controlled by dosing the water with a biocide • Suspended solids are removed by filtering the water

In the next section we will be looking at the equipment used to treat sea water. Before you move on, have a go at the following Test Yourself question.

Test Yourself 5 Which of the problems associated with the use of sea water as injection water is tackled by each of the following treatments?

• Dissolved solids are treated by injecting a scale inhibiting chemical

1. Passing the water through filters.

• Dissolved gases are removed by deaeration or oxygen scavenging

3. Passing the water through mechanical deaerators.

You also saw that other chemicals may be injected into the water to prevent foaming, inhibit corrosion, and assist in filtration.

4. Injecting scale inhibitors.

2. Injecting oxygen scavengers.

5. Injecting polyelectrolytes. 6. Injecting biocides. You will find the answer to Test Yourself 5 on Page 55.

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Injection Water Treatment

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Section 3 - Sea Water Treatment Equipment We have seen that we need to do a number of things to the sea water before we can inject it into the reservoir. They are: • kill off all the bacteria and other marine organisms so that they cannot create slimes or corrosive products • remove the suspended solids so that the pores of the reservoir rock do not get blocked • remove the oxygen so that the sea water does not corrode the pipework and equipment • ensure that the sea water does not produce scale • treat the water with other chemicals to prevent foaming, inhibit corrosion and so on In this section, we are going to look at some of the equipment used to do these jobs. We will look at bacteria control first, as this is usually the first treatment which the sea water receives.

Bacteria Control I stated earlier that we use a biocide to kill the bacteria present in the sea water. A simple domestic bleach was advertised in the 1980’s as being capable of killing all known germs dead. It sounds ideal for our purpose. Take the time to look at the small print on a bottle of strong domestic bleach. You will find that it contains sodium hypochlorite. This substance is a biocide — a chemical which is capable of killing bacteria. Sodium hypochlorite is a very powerful biocide and is the most common one used in sea water treatment. The component of the bleach which does the job is chlorine. This is a gas, however, which would be difficult to handle. Hypochlorite is a convenient way of storing chlorine in liquid form. Occasionally the sodium hypochlorite may be purchased and delivered to the injection water treatment plant as bulk chemical. However, it can be made from sea water on-site and this is often much more convenient.

Let us take a look now at how this is done.

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solution. It can be used as a biocide in this form. Sodium hypochlorite generators may contain as few as two chlorination cells in the smaller units to over twenty cells in the case of large units. One of the disadvantages of the chlorination cell is the fact that hydrogen is produced. This is an inevitable by-product of this process. Hydrogen gas is of course very explosive, so it must be safely removed. This is done by routing the sea water plus the products of electrolysis to a degassing tank. In the tank, the hydrogen is liberated and excess air is introduced to dilute it. The hydrogen can then be safely vented. We will look at where the biocide is used in the next section. Let’s now move on to look at the equipment used to remove the suspended solids.

Filtration Equipment Figure 7 shows a simplified illustration of a sodium hypochlorite generator, where the process takes place. It consists of a length of pipe with a titanium coated steel tube suspended in the centre. Electrical connections are made to the pipe and tube. The combination of outer pipe and inner length of tube is called a chlorination cell.

Sea water flows through the space between the pipe and the tube. The chemical reaction takes place as a high voltage direct current flows from the pipe walls, through the sea water, to the central tube and back to the power source. As the electricity passes through the sea water electrolysis takes place. The sodium hypochlorite which is generated remains in the sea water as a dilute

The next treatment which the sea water has to undergo is that of removing the suspended solids. This is done using a filtration system. Filtration is the process whereby solids are removed from liquids by means of a permeable barrier which will allow the passage of liquid but will strain out solid particles. The equipment used to do this are called filters.

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The efficiency of a filter is measured by the diameter of the largest solid particle which will pass through it. This is usually expressed in microns. A micron is a measurement of length equal to one millionth of a metre. Most sea water treatment plants are fitted with filter systems which are designed to remove: • all suspended solids which have a diameter of more than 5 microns • 95% of all suspended solids which have a diameter of 5 microns or less There are a range of filter types in common use. We will look at some of those which you may find in water injection systems. Before we take a look at the different types of filter, I want to point out two features of these pieces of equipment. First take a look at Figure 8 which shows three illustrations of a glass of water and a funnel shaped mesh screen. The water contains solid particles of three different sizes. The mesh screen will only filter out the largest size of particle.

If we pour the mixture across the mesh screen once it will filter out the largest particles. The medium and small particles will fall through the screen. But look what happens if we pour the medium and small particles across the screen for a second time. On the second pass through the screen the larger particles assist the screen to remove the medium sized particles.

If we then pass the water through the screen for a third time the medium sized particles assist the screen in removing most of the tiny particles. If we kept passing the water through the screen we would eventually be able to remove nearly all of them. Before moving on, remember that the particles which have already been filtered increase the efficiency of the filter in removing other particles.

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The second point to remember is that, as filtration progresses, the filter itself will eventually become plugged with solids. When this happens the filter must be cleaned. As you will see, a common way of cleaning a filter is to flush away the filtered particles with water. This process is called backwashing. Let’s now look at the four main types of filter used for filtering sea water and see how they work. They are: • basket filters • sand filters • dual media filters • cartridge filters

Basket Filters Basket filters use a sheet of fine wire mesh to remove the solids from the sea water. They are most often used for coarse filtration, and are usually found at the very start of the filtration process. The sea water is pumped through the filter and the suspended solids are trapped on the front face of the mesh. As we have already seen, the solids build up on the surface of the mesh and create a more efficient filtering medium than the mesh itself. Take time to study the illustration in Figure 9, which shows a basket filter in normal operation.

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In this mode, the water enters the outer casing through the water inlet on the inside of the filter basket. As the water flows across the basket the solids are deposited on the inside of the mesh screen and removed from the water. The filtered water leaves via the filtered water outlet. Whilst filtering, the backwash valve is in the closed position. The pressure differential transmitter (PDT), measures the differential pressure between the water inlet and the filtered water outlet. As the solids build up on the wire mesh screen of the basket the differential pressure will rise. When this pressure reaches a preset level, the PDT will activate a filter backwash through a backwash controller. Figure 10 shows the flow of water through a basket filter when it is in the filter backwash mode.

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When the differential pressure switch activates a filter backwash, the backwash valve opens and the cleaning head motor starts to rotate the cleaning head. With the backwash valve open, filtered water from the outside of the filter basket flows:

In our example: • the support plate is a stainless steel plate which is drilled with holes or slots which are smaller than the support material



• back through the basket

• the support material consists of layers of ceramic balls, graded according to their diameter



• into the cleaning head



• through the backwash valve to drain



The filtered solids are washed off the front face of the wire mesh screen into the cleaning head. When the cleaning head has rotated through 360˚ the whole of the filtering surface will have been cleaned. The main advantage of this type of filter is that it can be backwashed whilst it is still on line and filtering water.

Sand Filters As the name implies, this type of filter uses grains of sand as the filtering medium. Figure 11 shows a simplified cross section through a sand filter.

• the top layer is garnet sand

Garnet sand is a special type of sand which will not chip or flake easily. Once again, as suspended solids are filtered from the water and collect on the sand, the differential pressure across the filter will increase. When this pressure reaches a predetermined value, a backwash sequence is activated. Water is pumped in the reverse direction through the filter to remove the filtered solid patricles. The major drawback of this type of filter is that only the top few inches of the filter bed are effectively used. This means that the rate of filtration is quite low. Therefore many filters are required for the large volumes of water used in water injection systems.

The water to be filtered enters the filter at the top and filtered water leaves from the bottom.

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Dual Media Filters Figure 12 shows a dual media filter which uses anthracite and garnet sand as the filter media.

The larger grains of anthracite have larger gaps between them and filter out large particles from the sea water. The smaller particles will pass through the grains of anthracite to the layer of garnet sand. The sand will then filter out the smaller particles. This technique increases the efficiency of the filter and reduces the frequency of backwashing.

Backwashing is carried out as for the sand filter. In Figure 13, I have illustrated a dual media filter with some of the ancillary valves and equipment.

In the illustration you can see that the layer of anthracite lies on top of the layer of garnet sand. The particles of anthracite are larger than the grains of sand but, because they are lighter, they lie on top of the sand. By having a layer of large particles on top of a layer of small particles we create two different levels of filtration within the same filter.

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Study the illustration carefully and identify:

• Turbidity Analyser (TA)



• Pressure Differential Transmitter (PDT)



• Backwash Sequence Controller

Turbidity Analyser The turbidity analyser (TA) is an instrument which measures the turbidity of the water. Turbidity is a measure of clearness. By measuring the level of turbidity we have an indication of whether or not the filter is performing properly. Most turbidity analysers shine a light through a stream of water, which is picked up by a receiver. The level of light transmitted is measured. The level of light received is also measured. The difference between the two is a measure of the turbidity of the water. It should be noted that turbidity is only an indication that a filter is operating correctly. Two or three large particles per m3 (large enough to clog the reservoir) may register the same level of turbidity as 150 to 200 very fine particles per m3 (small enough not to affect the reservoir).

Pressure Differential Transmitter The pressure differential transmitter (PDT) measures the pressure difference between the inlet and outlet of the filter. As the filter becomes blocked up with filtered particles, the differential pressure will rise. When this differential pressure reaches a pre-set limit, the PDT sends a signal to the backwash sequence controller. The backwash sequence controller will activate a backwash cycle, which is a series of timed events designed to clean the filter. We will look at the backwash cycle in a dual media filter in Section 4 of this unit.

Cartridge Filters The cartridge filter is one of the easiest filters to install, operate and maintain. It is also one of the most popular filters in everyday life.

• the paper bag in a vacuum cleaner. When it is full, the cleaner bag (cartridge) is removed and replaced with a new one. In our examples, the cartridge is removed and replaced with a new one. This results in an ongoing cost and prevents the cartridge filter being used more widely. Because of this lack of economy, cartridge filters are normally used where: •

a back-up is required to the normal filtering method, e.g. if an upstream filter fails, the cartridge filter will remove the particles to the required standard

•

a polishing stage of filtration is required, e.g. the cartridge filter is used to remove small amounts of very tiny particles to polish water which has already been filtered by other means

Two types of cartridge filter found in common usage are: •

the filter in the engine lube-oil system of your car. When the filter becomes blocked, the paper cartridge is removed and replaced with a new one.

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Cartridge filters are reliable because they:

• guarantee a specified level of filtration

• very seldom fail to filter to the required standard

• are easy to install and monitor

Figure 14 shows a cartridge filter arrangement. We can see that normal flow through the filter is from top to bottom. The water to be filtered:

• passes through valve V1



• passes through the filter cartridges



• exits the filter via valve V2

Study the illustration carefully and identify the cover plate, and the cartridges.

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The cover plate allows the operator to gain access to the filter, to remove the old cartridges and replace them with new ones. Because the cartridges are renewed, there is no backwash requirement on a cartridge filter. The cartridges used for water filtration are most often a series of cotton or synthetic material layers wrapped around a slotted stainless steel tube. The type of material, and the number of layers, will decide the particle size which the filter will remove. Cartridges may be installed, which will filter out all particles greater than, say, 5 microns in diameter, regardless of the state of the water entering the filter.

So much for filters for the time being. Before moving on to the oxygen removal part of this section, have a go at the following Test Yourself question.

Oxygen Removal In Section 2 we established that sea water contained dissolved oxygen and we determined that, if the oxygen is not removed:

Test Yourself 6 Are the following statements true or false ?

• it will cause corrosion problems in the pipes and flowlines used to inject the sea water into the reservoir

True

False

a) A non-permeable barrier is used to remove suspended solids in a filter.

o

o

b) A seven micron particle will pass through a ten micron filter.

o

o

c) A common way to clean a filter is to backwash it.

o

o

d) A basket filter uses garnet sand as the filter medium.

o

o

The process of removing oxygen from the water is called deaeration. This can be accomplished by:

e) In a sand filter the flow is : through the o o support plate, through the support material and then through the sand.

• increasing the temperature of the water

f) A cartridge filter has a removable cover plate which allows access to the unit to replace filter elements.

o

You will find the answers to Test Yourself 6 on Page 55.

o

•

it will support bacteria which produces slimes, causing blocking of the small pores in the reservoir rock and rendering the rock impermeable

• decreasing the concentration of oxygen in the gas mixture in contact with the water • reducing the total pressure in the system

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You will see in Section 4 that the injection water is used as cooling water in various process systems. This means that the water itself is heated, thus assisting in the oxygen removal. Decreasing the concentration of oxygen in the gas mixture in contact with the water, can be achieved by a process known as gas stripping. Let’s have a look at this now.

Gas Stripping The process of stripping dissolved oxygen from water is conducted in large stripping towers. Inside the tower the water is spread out to increase the surface area, and natural gas is passed over the surface of the water. This reduces the concentration of oxygen which is in the atmosphere in contact with the water.

Figure 15 shows a simplified view of a gas stripper tower.

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The flow of liquid and gas through the gas stripper is counter current. As the liquid falls downwards through the column, the stripping gas flows upwards. In the illustration, the pre-heated sea water:

• enters the stripper at the top of the column

• passes through a water distribution pipe which is often called a sparge pipe

• flows downwards through a packed section



• accumulates at the bottom of the stripper

The packed section contains packing rings. These are designed to increase the surface area of the water. A popular type of packing ring used in stripping columns is the pall ring which may be made from plastic or stainless steel. One of these rings is shown alongside the stripping tower in Figure 15. Other features of the stripper tower which you can see in Figure 15 include: • the distribution plate which ensures that the sea water is evenly distributed across the packing

• leaves the stripper from the bottom of the column

• the demister pad which ensures that the gas leaving the stripper is free from liquid droplets and mists

The stripping gas is often hydrocarbon gas from the oil and gas process. The flow through the tower is:

• the oxygen scavenger inlet which is used to introduce oxygen scavenging chemical to the tower

• into the stripper through the stripping gas inlet pipe

• through a gas distribution sparge pipe



• upwards through the packed section

• out through an gas outlet located at the top of the tower

• the vortex breaker on the deaerated water outlet which prevents gas from being sucked into the pump with the water Look again at Figure 15 and try to visualise what is going on in the gas stripper. When you have done that we can have a look at the other type of deaeration, i.e. vacuum deaeration.

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Vacuum Deaeration You should note that the term vacuum is used to denote any pressure below atmospheric pressure. We also speak of a deep vacuum, which is a vacuum well below atmospheric pressure. A shallow vacuum on the other hand, is a vacuum which is only slightly below atmospheric pressure.

Figure 16 is a simple illustration of a two stage vacuum deaerator. You will probably notice that it looks similar to the gas stripping deaerator. However, it does have a number of different features. The first thing we should note is that there are two separate sections of packing. The upper section operates under a shallow vacuum. The lower section operates under a deeper vacuum. This, then, is a two stage vacuum deaerator. Single stage and three stage deaerators have been used but two stages are the most common. In this vessel, the sea water from the process heat exchangers enters the deaerator through a sparge pipe and flows downwards over the upper section of packing. Oxygen and water vapour are sucked out of the vessel via an overhead vapour line, creating a shallow vacuum in the vessel.

Figure 16 31

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When the water reaches the bottom of the upper section of packing, it flows through a set of seal chimneys. These operate in a similar manner to the ‘U' bend under a sink. The water has to flow upwards and over a weir before it can reach the lower section of packing. The pressure difference between the two sections is maintained by the height of the chimney weirs. A simplified sketch of a seal chimney is shown alongside the deaerator in Figure 16. After the sea water leaves the seal chimneys it is re-distributed and flows downwards over the lower section of packing. Oxygen and water vapour are again sucked out of the vessel via a vapour line which goes to the deep vacuum section. The deaerated water leaves the lower section of packing, falls into the bottom of the column, and leaves via the deaerated water outlet. Vacuum deaerators are very efficient. Most two stage vacuum deaerators can achieve an oxygen concentration as low as 0.1 to 0.15 ppm (parts per million) in the deaerated sea water outlet. We have seen how a vacuum deaerator works. Now let’s take at look at two items of equipment which may be used to create the vacuum in the deaerator.

Figure 17 is a simplified drawing of a vacuum pump. (An actual pump would look very different from this. However, the illustration is intended to explain the principles of operation.) If you look at the first part of Figure 17 you will see that I have drawn a shaft, fitted with four vanes, rotating inside an empty casing. I have positioned the shaft and vanes central to the casing. In the second drawing of Figure 17, I have shown a water inlet and outlet which introduces service water into the casing at the near end and removes it from the far end. The spinning action of the shaft and the vanes imparts a centrifugal force to the water. The water is thrown against the inside wall of the casing due to this force. The water forced against the casing in this way is the liquid ring which gives the compressor its name. The central part of the casing is filled with air.

The most common method of creating this vacuum is by the use of a liquid ring compressor. These are also called vacuum pumps, which is the term I shall use.

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Now take a look at Figure 18 and see if you can detect the difference in the position of the shaft and vanes.

You will notice that I have drawn the shaft and vanes offset from the centre of the casing. Service water is again introduced into the casing and again forced outwards to form the liquid ring.

The differing sizes of these void spaces means that the pressure in each will change as the shaft and vanes rotate. The pressure will:

You should notice that:



• decrease when the void space increases



• increase when the void space decreases

• although the shaft and vanes are offset the water still forms a uniform liquid ring against the casing • the offset shaft and the uniform liquid ring combine to form different sized void spaces

If we control the flow of air into the unit so that it enters a void space at the low pressure side and leaves at the high pressure side, we have created a liquid ring compressor. The compressor will in fact suck the air into the low pressure side and create a vacuum. In real vacuum pumps, slide valves control the air inlet and outlet. Vacuum pumps are only capable of creating a limited amount of vacuum. Therefore, in order to create the deeper vacuum required in the second stage of deaeration, the vacuum pump is augmented by an ejector. This piece of equipment is also called an eductor or venturi. Figure 19, on the next page, shows how such a unit works.

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Test Yourself 7 Do the following statements apply to : gas stripping deaerators, vacuum deaerators, both types of deaerator or neither of them? a) Water enters the column at the top and is distributed through a sparge pipe. b) A vortex breaker ensures that gas is not drawn out of the vessel with the deaerated water. c) The upper packing support plate accommodates seal chimneys. d) The concentration of the oxygen in the gas mixture in contact with the water is reduced. e) Pall rings may be used as packaging. The discharge side of the ejector is connected to the suction of the vacuum pump. A line connects the nozzle of the unit to the deep vacuum section of the deaerator. As the vacuum pump draws atmospheric air through the nozzle of the ejector the air speeds up. This creates an area of very low pressure at the nozzle which in turn pulls a deep vacuum on the deaerator.

f) Renewable cartridges help to maintain the water quality. You should by now have a good idea of how oxygen is removed from the injection water. Check your understanding now by having a go at Test Yourself 7.

g) A liquid ring compressor is connected to the upper outlet. h) Water flows through a distribution plate located above the packing. You will find the answers to Test Yourself 7 on Page 56.

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Summary of Section 3 In this section we have been looking at the equipment and processes which are used to treat sea water. This equipment ensures that the water is rendered suitable for injection into a reservoir. We started by considering the equipment used to make sodium hypochlorite from sea water. You saw that the process is one of electrolysis using a chlorination cell. From there we went on to look at the different types of filter which could be used to reduce the suspended solids content of the injection water. The types which we looked at were:

You saw that injection water filters are typically designed to remove all suspended solids which have a diameter of more than 5 microns. In addition they will remove 95% of suspended solids which have a diameter of 5 microns or less. I pointed out that equipment is provided to backwash the filters when they become plugged with solids.



• basket



• sand

In the final part of the section we had a look at the equipment used to remove dissolved oxygen from the sea water. I said that the process of removing oxygen can be accomplished by:



• dual media





• cartridge

• decreasing the concentration of the oxygen in the gas mixture which is in contact with the water

• increasing the temperature of the water

The temperature is increased because the water is used as a cooling medium in heat exchangers. The remaining two methods use a deaeration tower. In the first method, gas stripping reduces the concentration of oxygen. In the second, a vacuum is created in the tower by a vacuum pump augmented by an ejector.

In the next section we will see how this equipment is used in a typical sea water injection system. Before you move on to that, however, try the following Test Yourself question to check your understanding of Section 3.

• reducing the total pressure in the system

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Test Yourself 8 To which piece of injection water treatment equipment or system do the following components belong ? If the piece of equipment is a filter, state which type. You should indicate if the components belong to more than one piece of equipment or system. sparge pipe ......................................................

backwash controller..................................................

degassing tank..................................................

cleaning head motor..................................................

support material ...............................................

demister pad.............................................................

vortex breaker...................................................

vacuum pump............................................................

ejector................................................................ seal chimney..................................................... cartridges........................................................... You will find the answers to Test Yourself 8 on Page 56.

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Injection Water Treatment

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Section 4 - A Typical Injection WaterTreatment System In this, the final section of our study of injection water treatment, we are going to look at a typical system which you might find on an offshore oil production platform. However, you should remember that each water treatment system is unique to itself. Each one will differ, in types of equipment used, process layout and so on. The sea water treatment system which I will describe is not meant to represent any particular facility. It is simply a hypothetical system which will give you a feel for the full sea water treatment process.

Look first at Figure 20. This is a simple block diagram which shows the flow through our system. From the diagram you can see that the sea water is pumped:

Familiarise yourself with the basic process before moving on.

• from the sea, via the sea water transfer pumps, through the coarse filters, to the sea water reservoir •

from the sea water reservoir, via the sea water supply pumps, through the fine filters, crude oil coolers and the vacuum deaerator to the surge vessel

• from the surge vessel, via the booster pumps and injection pumps, to the injection wells and then into the reservoir

Figure 20 37

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We are going to follow the flow through the system in more detail now. In order to make the task simpler, I will guide you through it section by section.

Sea Water Intake and Coarse Filters Take a look at Figure 21 which shows the sea water transfer pumps and the coarse filter system. You will see that I have only shown one pump and one filter. In practice there would probably be three of each but I have eliminated the rest for simplicity. The sea water transfer pumps are positioned inside caissons. These are long pipes which are open to the sea and extend below the platform. The pumps are selected and started manually by the operator. At each pump suction there is fitted a simple filter to prevent larger marine organisms from entering the system. It is also at this point that the first treatment, the introduction of sodium hypochlorite, takes place.

from NaOCI generator

The sea water leaving the sea water transfer pumps is analysed by an instrument called an analyzer indicator controller (AQIC). This monitors the sea water to ensure that there is a sufficient concentration of sodium hypochlorite (NaOCI) to kill all of the bacteria. If the NaOCI concentration is too low the controller opens valve AV1 further to allow more NaOCI to flow into the suction of the pumps.

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The sea water then flows to the coarse filters where the first stage of filtration takes place. The coarse filters are basket filters which can be backwashed whilst they are on line. A pressure differential switch high (PDSH1) monitors the differential pressure across the filters. If the differential pressure exceeds a pre-set limit, for example 1 barg differential pressure, PDSH1 sends a signal to the backwash sequence controller. The controller starts a backwash cycle which backwashes each filter in turn. We will look at a backwash sequence controller in more detail when we come to the fine filters. The filtered water now flows to the sea water reservoir. You will notice however that a portion of it is taken as a side stream to be used as feed for the NaOCI generator. Let’s leave the main flow for a time and see how the NaOCI generator fits into the overall system.

Chlorination Facilities Figure 22 shows the main pipelines and controls to be found on the NaOCI generator.

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Follow the line which enters the Figure at the top left. You will see that the feed from the coarse filters enters the NaOCI generator via:

• a flow switch low-low (FSLL1)



• an emergency valve (XV1)



• a pressure control valve (PCV1)

The flow switch is required to prevent the NaOCI generator from being damaged due to a low flow of sea water through the system. If this switch is activated, the local control system will close the emergency valve, shut off the electrical supply and send an alarm to the operator to warn him of the problem. The pressure control valve (PCV1) maintains a constant pressure on the chlorination cells during normal operation. The NaOCI generator is a skid mounted unit which is contained within its own housing. If there is a leak of water within the housing, an operator could be in severe danger of electrocution. To prevent this danger from arising, the area under the chlorination cells is monitored by a level switch. This switch is designated LSHH1 on the drawing. It is activated by a rising water level. If the level switch is activated, the local control system will again close XV1 , shut off the electrical supply and send an alarm to the operator to warn him of the problem.

The NaOCI, sea water and hydrogen mixture leaving the chlorination cells enters a hydrogen removal tank. You will remember from Section 3 that the hydrogen removal tank is kept constantly purged with air from an air blower. The hydrogen is diluted with the air which is then vented from the hydrogen removal tank to the atmosphere. To minimise the danger of an explosion, a flame arrestor is fitted in the vent line to prevent the backward movement of a flame should the vent ever ignite. If the supply of air from the blowers was to fail, a dangerous concentration of hydrogen would rapidly build up in the hydrogen removal tank. To guard against this, a pressure switch (PSLL1) is installed in the discharge line from the air blowers. The switch will sense a lack of air and activate a shutdown of the chlorination facilities via the control system. The liquid in the hydrogen removal tank is, of course, a mixture of NaOCI and sea water. The liquid level, is maintained by a controller (LIC1) operating valve LV1. The sodium hypochlorite and sea water mixture, in the line after LV1, goes either :

A dosing pump increases the pressure of the NaOCI and sea water so that it can be injected into a higher pressure area. You should note from Figure 21 that the injection point to the main sea water line is downstream of the take-off point to the NaOCI generator. If the injection point was upstream of this take off, then NaOCI would be re-circulated back to the generator resulting in high and dangerous concentrations of NaOCI. Another line from the hydrogen removal tank takes the NaOCI / sea water mix to the transfer pump caissons. This is a simple gravity feed via the valve AV1 to the pump suctions. Look back to Figure 21 and identify this injection point. You will note that there is a signal to the chlorination control system from a chlorine analyser at the inlet to the sea water reservoir. If there is insufficient NaOCI at this point, then AIC2 will signal the NaOCI generator to increase the amount of NaOCI being produced and injected into the discharge of the coarse filters. Before moving on to the next part of our system, try Test Yourself 9, on the next page.

• into the main sea water line just downstream of the coarse filters • to the shock dosing connections. ( We will be looking at these later )

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Sea Water Reservoir

Test Yourself 9 State the function of the following equipment in the sea water intake, coarse filters and chlorination facilities. a) The analyser indicator controller at the discharge from the transfer pumps. b) The differential pressure switch connected across the coarse filters. c) The low flow switch at the inlet to the NaOCI generator. d) The air blower in the chlorination facilities.

You will find the answers to Test Yourself 9 on Page 57.

The sea water from the coarse filters (which is now chlorinated) flows towards the sea water reservoir. Take a look now at Figure 23, on page 42, which shows this part of the system. The main sea water line is a continuation of the one leaving the coarse filters which you saw in Figure 21. Follow the flow through this part of the system starting at the bottom left hand side of Figure 23. You can see that the sea water flows towards the sea water reservoir via a level control valve and an analyser indicator controller. These are designated LV2 and AIC2 respectively. LV2 maintains the correct level in the reservoir and is operated by a signal from a level controller which is marked LIC2 in the drawing.

The pre-service flush line, allows injection water to be circulated through the fine filters and back to the reservoir prior to starting the system. You will see this when you move on to the next part of this section. However the shock dosing connection requires a little more explanation at this point. As a general rule the NaOCI is injected at a reasonably steady rate during operations. NaOCI is extremely effective but, after a number of weeks, a few of the bacteria may become resistant to the effects of NaOCI. When this occurs the regular dosage rate becomes insufficient and a shock dose, i.e. a large amount, of concentrated NaOCI is injected into the system over a short period of time. This usually kills off any resistant strains of bacteria. This shock dose can be injected at the connection upstream or, as you can see in the Figure, downstream of the reservoir.

As we have already seen, AIC2 controls the amount of NaOCI injected into the sea water as it leaves the coarse filters.

The sea water reservoir has two outlets. They are:

• an outlet to the sea water supply pumps

Note that, just before entering the reservoir, two lines join the main sea water line. They are:



• an outlet to the fine filter backwash pumps



• a pre-service flush line from the fine filters



• a shock dosing connection for NaOCI injection

The outlet to the sea water supply pumps is the main outlet from the sea water reservoir. The pumps transfer the water through fine filters.

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NaOCI shock dosing

The backwash pumps provide water to clean the fine filters. We will look at these in more detail when we move on to the fine filter system shortly. You will notice that there are four connections to the discharge line of the sea water supply pumps. These come from a chemical dosing skid and are:



• a biocide connection



• a scale Inhibitor connection



• a ferric chloride connection



• a polyelectrolyte connection

Let’s take the time to have a look at why we need these chemicals before moving on to the fine filter section.

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I pointed out earlier that a shock dose of concentrated NaOCI usually kills off any resistant strains of bacteria. However, if this shock dose of NaOCI fails to do the trick, then a dose of another biocide is used. The biocide connection is one of the points where such an alternative biocide would be injected.

If you look again at Figure 23 you will see that the main flow line passes from the sea water supply pumps to the fine filters. We are now going to look at this system but before we do, trace the lines again and identify the equipment associated with the sea water reservoir part of the overall system.

In Section 1, I stated that the sea water may start to precipitate dissolved solids as scale if it was not treated. The scale inhibitor is injected at this point to ensure that there is no scale precipitation later in the process.

Fine Filtration

You will remember from Section 2, that polyelectrolytes are coagulants. They could also be termed flocculants. The word flocculant comes from “flocking” or “gathering together”. These chemicals assist in gathering together tiny particles of material and converting them into larger particles. These larger particles are easier to filter out.

Figure 24, on the next page, shows one of the fine filters and its associated pipework. The main flowline from the sea water supply pumps continues into this drawing at the top left hand side. In our example the fine filters are dual media filters. Figure 24 is quite complicated. However, if you follow the lines carefully, together with the explanation, I don’t think you should have any problems with it. Once again I have only shown one filter with its associated controls. In fact there would probably be a number of fine filters working in parallel.

Ferric chloride does a similar type of job. It produces small electrically charged particles which attract other particles to them. The combined particles are large enough to be removed by the filters. Both of these substances, then, are aids to filtration. They are injected into the water at this point, which is upstream of the fine filters.

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Before looking at the valve and control arrangements in Figure 24, locate the two turbidity analyzers. One measures the turbidity in the main sea water line before the filter. The other monitors the turbidity of the water leaving the filter. Think about the location of the two analysers, then attempt the following Test Yourself question.

Test Yourself 10 What is the significance of having two turbidity analysers in the system?

You will find the answer to Test Yourself 10 on Page 57.

The main flow is shown entering the system near the top left hand part of the drawing. Follow this bold line now. As you can see, the flow passes through the filter and continues towards the deaerator. This line then is very straightforward. The rest of the lines are part of the filter control system. Let’s spend some time identifying the lines and controls, to see what their function is. The total flow of sea water to the fine filters passes through flow element 2 (FE2) which measures the water flowrate at this point. A signal from FE2 is fed to flow relay 2 (FY2). FY2 determines how much water should be flowing through each filter. It then sends a signal to a flow indicator controller located on each of the fine filters. In this case it is designated FIC3. This signal tells the FIC how much water should be flowing through its own filter. It is called a set point signal. The actual amount of water passing this filter is measured by the second flow element FE3. A signal from this element is also fed to FIC3. The controller compares the actual flow rate with the flow rate determined by the flow relay. If there is a discrepancy between the two it sends a correcting signal to the flow control valve FV3. The valve responds by opening or closing to maintain the correct flow.

The filtered sea water leaves the fine filter via valve XV3. This valve operates as part of the backwash sequence which we will look at shortly. The water then joins the water discharged from the other filters before being fed to the deaerator via the crude oil coolers. Before continuing, go over the last few paragraphs again, together with the drawing. Make sure that you understand the way in which the total flow is distributed between the filters. Look now at two other devices which are fitted to the fine filter. They are: •

pressure safety valve (PSV1). This is a valve which will open and release excess pressure from the filter should the pressure exceed a predetermined maximum value.

•

pressure differential switch high (PDSH2). This compares the pressures upstream and downstream of the filter. If the differential pressure exceeds a preset limit, PDSH2 will automatically activate a backwash sequence, through a controller.

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We can now see how the backwash is carried out. Let’s start from the point where the filter is operating normally.

This opens lines to dispose of the water in the filter via a dirty water sewer. It causes the water level in the filter to fall to just above the filter bed.

At this point you may be looking for FIC4 on Figure 24. In fact it appears on the previous Figure we were looking at, Figure 23.

The flow of water would then be along the following path:

After five minutes has elapsed the backwash sequence controller will:

Through valve FV3 - through the filter - through flow element FES - through valve XV3 - across the turbidity analyzer - on towards the deaerator.

5. Close valve XV7.

Go back to Figure 23 for a moment and look at the second line leaving the sea water reservoir. This line goes to the backwash pump then through a strainer. The strainer ensures that reasonably clean water is available for backwashing.

As the water is filtered, the differential pressure, as measured by PDSH2, would start to increase. When it reaches, say, 1 barg, the backwash sequence would be activated. The sequence of events from now would be as follows:

7. Start the air scour blower.

The backwash sequence controller will send a backwash sequence in progress alarm to the operator, and then: 1. Send a zero flow signal to FIC3 to ensure that FIC3 closes FV3. 2. Close valve XV3. The fine filter is now isolated from the main sea water line. The backwash sequence controller will now: 3. Open valve XV6.

6. Open valve XV9.

The air scour blower blows air through the filter. This violently agitates the filter bed to knock off any tiny particles which may be stuck to the grains of garnet sand. After ten minutes has elapsed the backwash sequence controller will: 8. Stop the air scour blower. 9. Close valve XV9. 10. Start the sea water filter backwash pump. 11. Open valve XV10 to direct the water back through the filter and to disposal via XV6. 12. Send a flow signal to FIC4 to regulate the amount of water being pumped.

After the strainer, the water flows past a flow indicator controller (PIC4) and through a flow control valve (FV4). This system controls the amount of water to be pumped to the filter as backwash water, the actual amounts being determined by the backwash sequence controller. Now back to Figure 24 and the sequence of events. After twenty minutes has elapsed the backwash sequence controller will: 13. Reduce the flow signal to FIC4. 14. Open valve XV5. 15. Close valve XV6. The flow is reduced to allow the filter bed to settle down as the filter is re-filled. The water will now flow back to the sea water reservoir instead of to the dirty water sewer.

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After a further five minutes, the backwash sequence controller will: 16. Open valve XV8. 17. Open valve XV4. 18. Close valve XV5.

The fine filter is now back in service. A backwash sequence completed signal will be sent to the operator. He will then check the outlet turbidity meter to ensure that the filter is operating efficiently.

There is just one line which I haven’t mentioned up to now which appears in Figure 24. That is the spill back line at the top right hand of the drawing. Disregard that for the moment. We will come back to it shortly.

The backwash sequence which I have just described takes approximately sixty minutes to complete.

At this point you may want to follow the sequence a couple more times to ensure that you can visualise what is going on. When you have done that, have a go at the following Test Yourself question.

19. Close valve XV10. 20. Increase the flow signal to FIC4 to maximum. The filter is now in a pre-service flush sequence. The water is flowing through the filter in the normal way, then back to the sea water reservoir. After a further fifteen minutes, the backwash sequence controller will: 21. Close valve XV4. 22. Close valve XV8. 23. Remove the flow signal to FIC4. 24. Stop the sea water filter backwash pump. 25. Remove the zero flow signal to FIC3, which opens FV3. 26. Open valve XV3.

Test Yourself 11 The following steps, which are out of order, are the first part of a typical backwash sequence. Place the steps in the correct order. Start with inlet valve closes and finish with filter is now being backwashed. a) b) c) d) e) f) g)

inlet valve closes. lower valve to dirty water sewer closes. air scour blower starts. sea water backwash pump starts. valves open to dirty water sewer. air scour blower stops. signal sent to controller on discharge of backwash pump to regulate flow of backwash water.

h) i) j) k) l)

controller closes outlet valve from filter. valve in line from air scour blower opens. valve in line from backwash pump opens. valve in line from air scour blower closes. filter is now being backwashed.

You will find the answers to Test Yourself 11 on Page 57.

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Deaeration The deaerator column in our system is a vacuum deaerator. Figure 25 shows the main components of the system and the associated controls. Take a few minutes to study this Figure before moving on. First of all try to establish the main flow of sea water through the deaerator column to the surge vessel and on towards the booster pumps.

Figure 25 48

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The sea water leaving the fine filters first flows through a turbidity analyser then a pressure indicator controller (PIC3). The turbidity analyser measures the turbidity of the combined flow of sea water from all of the fine filters. The PIC works together with a pressure control valve, PV3, to prevent excess pressure building up in the line to the deaerator. The sea water is then passed across crude oil coolers. These coolers are not really part of the injection water treatment system. They simply use the cold sea water as a cooling medium in heat exchangers to reduce the temperature of oil. However the sea water itself is heated up in the exchanger and , as you saw Section 3, heating up the water will assist in the deaeration process. The sea water then flows through a level control valve (LV3) which is activated by a level indicator controller (LIC3). LIC3 controls the level of water in the bottom of the deaerator column. If LV3 closes too far, the pressure on the filter side of LV3 will rise. This will activate PIC3 which will open PV3. Water from the filters will be spilled back to the sea water reservoir. This is the line that joins the pre-service flush line from the fine filters, which you saw in Figure 24.

The spill-back line : • protects the sea water supply pumps from damage if the flow to the deaerator column is stopped

After being injected with anti-foam agent, the sea water passes into the dearator column where it gives up its dissolved oxygen. It then accumulates at the bottom of the column, the water level there being controlled by LIC3 and LV3.

• allows the fine filters to be brought on line without flowing sea water to the deaerator column

At this point it can be treated with:

• a non-hypochlorite biocide

• allows the filtered sea water to be diverted back to the sea water reservoir if the fine filters ever go off specification



• an oxygen scavenger

Back to the flow through the system After passing through LV3 the filtered sea water is injected with an anti-foam chemical. The anti-foam chemical is injected at this point because, as the sea water passes across LV3, it may be subjected to a large pressure drop. The pressure drop would cause a lot of the dissolved air to come out of solution. The changing conditions may cause the water to foam. If foam is produced, the deaerator column cannot operate correctly. A lot of water will be lost with the vapour and a lot of oxygen will pass through the column with the water. The anti-foam chemical breaks down the bubbles in the foam.

You should be able to remember how a vacuum deaerator works. If you need to refresh your memory, however, I suggest that you go back to the relevant part of Section 3. The deaerated water leaves the column and flows to a surge vessel. This is in effect a storage tank from which booster pumps can take their suction. A balance line connecting the surge vessel to the deaerator column ensures that the water level in each remains the same. You will notice a line entering the surge vessel at the top, from the booster and injection pumps. This is part of the pump protection equipment which we will look at shortly. Now let's take a look at the remaining instruments and equipment which service the deaerator column and surge vessel.

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The first thing to notice is that there are two pressure relief valves on the column. PSV4 is located on the top of the column and PSV5 is located on the outlet line at the bottom of the column. Both PSV's are set to open at the maximum working pressure of the column (say, 1 barg). If the deaerator column ever completely filled with water, the pressure at the bottom of the column could exceed its maximum working pressure. A pressure alarm high (PAH4) is positioned just above the top layer of packaging and is set to initiate an alarm if the pressure exceeds, typically, 35 millibar. This could occur if: • the level of water reached more than 0.35 m above PAH4 • the level of water was below PAH4 and the internal gas pressure was greater than 35 millibar. PAH4 would activate and alert the operator to the fact that a pressure problem has occurred in the deaerator column. In both instances the alarm would activate before the maximum pressure at the bottom of the column was reached.

The only lines that we haven't identified on Figure 25 are the ones the ejector and vacuum pump. We can do that now. The top line to the vacuum pump is connected to the deaerator above the top packing. The other line is connected above the lower packing. It leads to the ejector and , from there, joins the upper line before the vacuum pump. From the pump the line goes to an air / water separator. From here, the air is vented to the atmosphere and the water is disposed of via the dirty water sewer. Note the location of a hydrocarbon detector (HD). If the crude oil coolers leak, crude could enter the sea water and cause severe problems with the deaerator column and other equipment. HD will detect the presence of hydrocarbons in the air leaving the air / water separator and activate an alarm to warn the operator that there is a problem.

Water Injection Pumps If we move on to Figure 26, on the next page, we can see that the main flow from the surge vessel is through the booster pumps and the injection pumps to the sea water injection wells via a header. The booster pumps, as their name suggests, boost or increase the pressure of the treated sea water. The pressure is increased from that of the surge vessel to the pressure required at the suction of the injection pumps.

We are almost at the end of the system now. All that remains is to have to look at the injection pumps and injection wells.

A pressure relief valve also protects the surge vessel from over pressure. On the drawing it is designated PSV6.

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The combined discharge of the booster pumps is fitted with:

• an oxygen analyser (AE Os)



• a chlorine analyser (AE CI;)



• a pH analyser (AE pH)

The oxygen analyser ensures that the oxygen content of the water is still below the maximum permitted level. The chlorine analyser ensures that there is a sufficient amount of Na0CI in the injection water. The pH analyser measures the acidity or alkalinity of the injection water. If you remember, we injected the scale inhibitor into the discharge of the sea water supply pumps just before the water entered the fine filters. The level of pH is an indication of the tendency of the sea water to form scale deposits. As a general rule, if the water has a high pH (above 7.0) it is more prone to forming scale. The level of pH is monitored at this point and the rate at which the scale inhibitor is injected is adjusted accordingly. The injection pumps will increase the water pressure to that required for injection into the reservoir. This could be well over 200 barg in many cases. The discharge lines of both the booster pumps and injection pumps are fitted with minimum flow controllers. They are FIC5 and FIC6 respectively.

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All centrifugal pumps require protection to prevent them from pumping at too low a flow rate. The minimum flow controllers measure the flow from the pumps. If this flow falls below a predetermined level, the controllers open their respective control valves (FV5 and FV6) and allow the water to recycle back to the surge vessel. When sufficient water is flowing through the pumps, FV5 and FV6 are fully closed.

The treated sea water then flows across:

The treated sea water leaves the injection pumps and enters a common water injection header. The flow lines to the individual water injection wells are then taken from this header.

and down into the reservoir.

Water Injection Wells There may be as few as two or three water injection wells in small oil reservoirs, or as many as twenty or more in larger ones. In Figure 27, I have shown a schematic drawing of a typical well. The flow of treated sea water is measured by a flow element (FE7) which is upstream of a choke valve. A choke is a specially designed valve which allows the flow of high pressure injection water to be controlled. In our example the choke is controlled manually and would be adjusted by the operator to give the required flow rate, as measured by FE7.

wing valve (XV20) upper master gate valve (XV21) lower master gate valve (LMV7) sub-surface safety valve (XV22)

The injection well is completed in an almost identical way to the oil and gas producing wells. Our open learning packages on Oilwell Drilling Technology and Oilwell Production Technology explain the construction of these wells in great detail.

You have almost completed this unit on injection water treatment but before going through the summary, have a go at the final Test Yourself question.

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Test Yourself 12 Make a simple block diagram of a water injection system similar to the one at the start of Section 4. In addition, your diagram should show: 1.

The chemical injection points.

2.

Where the water is taken from for the filter backwashing.

3.

Where water analysers may be located.

Summary of Section 4 In this section we have looked at the operation of a typical sea water treatment system which uses:

We looked at the operation of the coarse filters and fine filters and saw how they are backwashed.



• sodium hypochlorite as a biocide



• basket filters as coarse filters

We saw how the vacuum deaerator is operated and controlled, and how:



• dual media filters as fine filters

• vacuum deaeration to remove oxygen from the sea water We considered how the sodium hypochlorite is generated and injected into the sea water during treatment. We also saw how a different biocide may be used to shock dose the system to prevent resistant bacteria from surviving the sodium hypochlorite injection.

You will find the answer to Test Yourself 12 on Page 58.

We saw how scale inhibitor is injected into the sea water and how the efficiency of the scale inhibitor was measured by a pH Analyser.

• anti-foam agent is injected into the deaerator feed to prevent foaming • oxygen scavenger is injected into the system to remove the last few traces of oxygen We then looked at the booster pumps and the injection pumps and saw how they were operated and controlled. Finally we looked at a typical water injection well and located the various valves and controls. Now go back to the training targets on page 4 and make sure that you have met those targets.

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Check Yourself 1

Check Yourself 2

Check Yourself 3

a)

One fifth is 20%. So the porosity is 20%.

1.

Gas cap drive.

2.

Dissolved gas drive.

The particles of sand or clay would block the pores of the reservoir rock, reducing permeability and hence productivity.

b)

A cap rock prevents oil migration so its permeability is likely to be zero.

3.

Dissolved gas drive.

4.

Water drive.

5.

Gas cap drive.

c)

Sponge



Sandstone

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Check Yourself 4

Check Yourself 5

Check Yourself 6

Removing oxygen creates conditions which are ideal for the growth of anaerobic bacteria.

1.

Removal of suspended solids.

a)

FALSE - a permeable barrier is used.

2.

Removal of dissolved gases (i.e. oxygen, which makes the water corrosive).

b)

TRUE.

3.

Removal of dissolved gases.

c)

TRUE.

4.

Reduces scale deposition caused by dissolved solids being precipitated.

d)

FALSE - a basket filter uses a fine mesh

5.

Assists in removal of suspended solids.

6.

Preventing growth of bacteria.

e)

FALSE - the flow is in the opposite

f)

screen.

direction.

TRUE.

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Check Yourself 7 a)

Both.

b)

Both (air is a gas).

c)

Vacuum deaerator.

d)

Gas stripping deaerator.

e)

Both.

f)

Neither.

g)

Vacuum deaerator.

h)

Both

Check Yourself 8 sparge pipe...................... deaerators degassing tank................ NaOCI generator support material............... filters (sand & dual media) vortex breaker.................. deaerators ejector.............................. vacuum deaerator seal chimney................... vacuum deaerator cartridges......................... cartridge filter backwash controller......... filters (basket, sand, dual media) cleaning head motor........ basket filter demister pad.................... deaerators vacuum pump.................. vacuum deaerator

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Check Yourself 9

Check Yourself 10

a)

To monitor sea water, in order to ensure a sufficient concentration of NaOCI to kill bacteria.

By checking the water turbidity upstream and downstream of the filter, the efficiency of the filter can be monitored.

b)

To monitor differential pressure. When this pressure exceeds a pre-set maximum, the differential pressure switch initiates a backwash, through a backwash sequence controller.

c)

To protect the NaOCI generator from being damaged due to a low flow of sea water.

d)

To purge the hydrogen removal tank with air.

Check Yourself 11 The correct sequence would be (a)

—

(h)

—

(e)

—

(b)

(i)

—

(c)

—

(f)

—

(k)

(d)

—

(j)

—

(g)

—

(I)

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Check Yourself 12

58