N2 Operations & Calculation Of N2

JET Manual 17 Introduction to Nitrogen Operations Version 1.0

JET Manual 17 Introduction to Nitrogen Operations InTouch Content ID# Version: Release Date: Owner:

4221751 1.0 February 10, 2007 Well Services Training & Development, IPC

Schlumberger private

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Copyright © 2007 Schlumberger, Unpublished work. All rights reserved. This work contains the confidential and proprietary trade secrets of Schlumberger and may not be copied or stored in an information retrieval system, transferred, used, distributed, translated or retransmitted in any form or by any means, electronic or mechanical, in whole or in part, without the express written permission of the copyright owner.

Trademarks & service marks “Schlumberger,” the Schlumberger logotype, and other words or symbols used to identify the products and services described herein are either trademarks, trade names, or service marks of Schlumberger and its licensors, or are the property of their respective owners. These marks may not be copied, imitated or used, in whole or in part, without the express prior written permission of Schlumberger. In addition, covers, page headers, custom graphics, icons, and other design elements may be service marks, trademarks, and/or trade dress of Schlumberger, and may not be copied, imitated, or used, in whole or in part, without the express prior written permission of Schlumberger. An asterisk (*) is used throughout this document to designate a mark of Schlumberger. A complete list of Schlumberger marks may be viewed at the Schlumberger Oilfield Services Marks page: http://www.hub.slb.com/index.cfm?id=id32083 Other company, product, and service names are the properties of their respective owners.



Table of Contents 1.0  Introduction

1.1 Scope  1.2 Overview of training  1.3 Nitrogen manufacturing (air separation) 1.3.1 Nitrogen versus carbon dioxide

2.0  Safety Considerations 3.0  Job Information and Design 3.1 3.2

Job information Job design

4.0  Equipment Selection

4.1 Rate capabilities 4.2 Pressure capabilities 4.3 Storage capacities 4.4 Nonconventional equipment 4.4.1 Types of nonconventional equipment 4.4.2 PSA units 4.4.3 Nitrogen membrane units

5.0  Preparation for the Job 5.1 5.2 5.3 5.4 5.5

Equipment  Product availability Maintenance status Personnel training  Journey assessment and approval form

6.1 6.2 6.3

Specific training on cryogenic equipment Understanding pumping nitrogen vs. liquids Specific safety gear for handling LN 2

7.1 7.2 7.3 7.4 7.5

Properties of liquid and gaseous nitrogen Conversion factors Safety aspects Effects of pressure and temperature on GN 2 Compression and expansion of GN 2

6.0  Personnel Qualifications 

7.0  Characteristics of Liquid and Gaseous Nitrogen

JET 17 - Introduction to Nitrogen Operations  | 

5 5 5 6 6 9 11 11 11 13 13 13 14 14 14 15 16 17 17 18 19 19 19 21 21 21 21 23 23 23 23 24 25 iii

7.6

Volume factors for GN 2

8.1 8.2 8.3

Section 100: Introduction to Nitrogen Section 200: Displacement with Nitrogen  Section 300: Technical Information

8.0  Using the Nitrogen Engineering Handbook

9.0  Job Calculations

9.1 Displacements 9.1.1 Tubing displacement 9.1.2 Tubing and casing displacement 9.2 Well flowback using nitrified fluids 9.3 Fluid removal from the well 9.4 Sand cleanouts 9.5 Foam fracturing 9.6 Total nitrogen requirements

10.0  References 11.0  Check Your Understanding

iv  |  Table of Contents

26 31 31 31 32 33 33 34 35 36 37 39 41 44 47 49

1.0  Introduction There are numerous wells that do not have the necessary bottomhole pressure to produce the injected fluids out of the formation and back to the surface when treated. In cases like this, the well needs assistance in removing the fluids after the treatment is completed. One of the methods used to help remove the fluids from the well is to inject gaseous nitrogen with the fluids at the time of the treatment. This process is generally referred to as artificial energizing. Some nitrogen treatments are a mixture of gas and liquid to clean sand out of the wellbore. Others are structured to control the ratio of gaseous nitrogen to liquid, as in the case of a foam fracture. Executing a nitrogen treatment requires an understanding of equipment, the properties of liquid nitrogen and gaseous nitrogen, and downhole conditions. All of these play a role in the successful completion of the treatment. The objective of this job execution training manual (JET) is to explain the characteristics of nitrogen and the calculations necessary to design a job. The information included enables the field specialist on location to understand the downhole dynamics and present a professional image to the client. Nitrogen, in its most common form, is approximately 78% of the air you breathe. The nitrogen that is handled and pumped is 99.999% pure after being separated from the other components of the atmosphere. Gaseous nitrogen is often referred to as GN2, and liquid nitrogen as LN2.

1.1 Scope This JET is intended to train field specialists. When you have completed this JET manual, you should have the necessary information for the following: • required well information for successful job execution • equipment selection • understanding of the fluid being pumped • understanding of the reaction of the gas to temperature and pressure • use of available data for design and execution of the job • calculations • job preparation.

1.2 Overview of training This training takes the student from receiving the initial client request through executing the job.

Note: It is essential to understand the relationship between the initial information gathered from the client and the success of the final job execution.

JET 17 - Introduction to Nitrogen Operations  |  

1.3 Nitrogen manufacturing (air separation) Liquid nitrogen is one product of air separation. Figure 1-1 shows an air separation plant.

The containers constructed to hold cryogenic liquids can best be described as a thermos bottle. For additional information on cryogenic containers, how they are used, and how they are constructed, please see JET 11: Nitrogen Transport and Storage Equipment (InTouch Content ID# 4221680).

1.3.1 Nitrogen versus carbon dioxide Nitrogen and carbon dioxide (CO2) are the most commonly used as gases to stimulate oil and gas wells. Figure 1-2 shows a nitrogen and CO2 transport.

Figure 1-1. Air Separation Plant

Air separation is the process of compressing and condensing air that we breathe until the various components separate by specific weight as liquids and are extracted through the use of a separation tower. The typical results of air separation, in percents by volume, are: • nitrogen

78%

• oxygen

21%

• argon

0.93%

• carbon dioxide (CO2)

0.038%

• helium

0.0005%

• hydrogen

0.00005%

Because the various fluids are extremely cold after separation, they must be stored in cryogenically constructed vessels.

  |  Introduction

Figure 1-2. Loading a Transport

Although at first glance it seems as if both gases function the same, there are differences on how they react with well fluids and how they function in the well. Table 1-1 explains the differences.

Table 1-1. Physical Differences Between Nitrogen and CO2

Characteristics

Nitrogen

CO2

Weight

6.74 lbm/galUS

8.51 lbm/galUS

Liquid temperature

-320 degF

-69 degF

Type pump required

Cryogenic

Conventional

Critical temperature

-232 degF

87.8 degF

Solubility in oil

Moderate

High

Solubility in water

Moderate

Moderate

Hydrostatic head with gas

Lighter than water

Heavier than nitrified fluids

Pumped in the well as:

Gas†

Liquid††

† Because of low solubility in liquids and critical temperature, nitrogen is always present as a gas in the well or commingled with fluids. †† CO2 is pumped as a fluid, has a relatively high critical temperature, and, because it is highly soluble in water and oil, will act more like a fluid than a gas in the formation well. As the fluid commingled with C0 2 comes back up the wellbore, the gas comes out of solution and functions more like a free gas to assist in clean up.

JET 17 - Introduction to Nitrogen Operations  |  

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  |  Introduction

2.0  Safety Considerations The Well Services Field Safety Handbook is the authority for any questions regarding safety and location rig-up. The applicable standards for the majority of nitrogen treatments are Standard 5 and Standard 11. Management at the district level should be contacted if there is any question about safety on location. If time allows and further support is needed, Operations Support in Sugar Land should be contacted through InTouch.

JET 17 - Introduction to Nitrogen Operations  | 

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10  |  Safety Considerations

3.0  Job Information and Design In many cases, Schlumberger engineers and the client agree to the job design well in advance of the actual treatment. In these cases, many of the details have been defined before you receive the job. However, sometimes certain parameters of the well may have changed. These changes could dramatically affect the successful execution of the treatment.

Note: When the client calls to set a time for the treatment, it is imperative that all the well data be reviewed and confirmed.

3.1 Job information When dealing with a gas such as nitrogen (see Fig. 3-1), the space that a given volume of gas (measured in standard cubic feet, scf) occupies can change substantially with pressure and temperature; see Table 3-1. If well conditions have changed since the treatment design was finalized, the nitrogen pump rate could change significantly. Similarly, any changes in tubing or casing sizes can affect the volume of nitrogen required or the friction pressures that are produced while pumping.

Figure 3-1. Nitrogen Equipment Table 3-1. Nitrogen Required to Fill Barrel of Space

Pressure (psi)

Quantity of Nitrogen (scf)

100

35

1,500

525

3,000

1,025

3.2 Job design The job design is one of the most important aspects of a successful treatment. The combination of well data and job design allows the following: • appropriate pumper selection • appropriate size and number of transports (see Fig. 1-2) • planning for additional location equipment. For example, you may need a tree saver, shown in Fig. 3-2. • comparison of wellhead pressure limits to anticipated treatment pressures.

JET 17 - Introduction to Nitrogen Operations  |  11

Figure 3-2. Tree Saver

In the early days of nitrogen pumping, the treatment design was done by hand. Today, jobs are designed by the computer and can be adjusted during the treatment as required if conditions change at the wellsite. It is important not only to understand how to use the software to design a nitrogen treatment, but also to have a working understanding of what is taking place downhole. Visualizing the dynamic situation allows the field specialist to make better decisions during the job and provide the client with a quality treatment.

12  |  Job Information and Design

JET 17 - Introduction to Nitrogen Operations  |  12

4.0  Equipment Selection Equipment selection is important for any type of treatment being performed. A variety of factors must be considered when selecting equipment for the job. • road worthiness—Check the standard equipment maintenance form (STEM 1, available at InTouch Content ID# 2024129, attachment Inspec(ALA).xls).

that the equipment has sufficient pumping capacity to meet the requirements of the treatment as designed. For more information, see JET 29, Nitrogen Pumping Equipment, InTouch Content ID# 4221766.

• mechanical status • maximum rate capability • maximum pressure • pumping unit nitrogen storage • loose equipment necessary to complete the connect to the well, for example, a ball injector.

Figure 4-1. Cold Ends

In many cases, the equipment has been selected by the dispatcher or field service manager (FSM) before the field specialist is assigned. Nevertheless, it is very important for the field specialist to review the design, equipment selection, and loading tickets before leaving the district.

4.2 Pressure capabilities

4.1 Rate capabilities

Some cold ends have a maximum working pressure of greater than 10,000 psi. The unit will still be limited to 10,000 psi because of the limits on the vaporizer.

The rate capabilities of a nitrogen pumping unit are controlled by two things: the size of the vaporizer and the diameter and stroke of the cold ends (see Fig. 4-1).

Usually, the pressure limit of a nitrogen pumping unit is controlled by the maximum pressure limit of the vaporizer. Most of the units in the field today are rated for a maximum working pressure of 10,000 psi because of the working pressure of the vaporizer.

Usually the vaporizer is sized to handle the pumping capacity of the largest cold end that the pump (warm end) can drive. A pump that has been equipped with smaller cold ends (1 5/8 in) for use with coiled tubing can be refitted with larger cold ends (2 7/8 in) for a higher rate treatment. It is important to ensure JET 17 - Introduction to Nitrogen Operations  |  13

Note: It is company policy that all nitrogen units be limited to 10,000 psi. Anyone wanting an exception to the Schlumberger pressure limit policy must obtain it through the Operations Support in Sugar Land, Texas.

4.3 Storage capacities Currently, a wide variety of storage units are available. The suppliers of storage equipment have expanded the types and sizes of equipment that is available for use in the field. Not only has the size of the storage units gotten bigger, but the ability to unload at higher rates has also increased. When reviewing storage requirements for a treatment, always consider not only the amount of liquid required but the rate at which it must be delivered as well. See Section 10.6, Total nitrogen requirements, of this document to ensure that enough liquid nitrogen is available to perform the job. For further information on storage, see JET 11, Nitrogen Transport and Storage Equipment, InTouch Content ID# 4221680.

4.4 Nonconventional equipment The conventional equipment used for pumping nitrogen pumps the liquid with a positive displacement pump and then moves it to a vaporizer (heat exchanger), where the liquid is transformed from a liquid into a gas. The gas is then discharged from the unit. Because this process entails the handling of cryogenic liquids, the unit is more complex than a unit handling gas alone, and the cryogenic components make it expensive. The conventional cryogenic pumping unit 14  |  Equipment Selection

also requires that a source of cryogenic liquid be available. In many parts of the world, liquid nitrogen—if it is available at all—is very expensive, and liquid is always lost to evaporation when it is stored. There are alternatives to the standard cryogenic pumping method that do not require the use of liquid nitrogen. This equipment separates the nitrogen from the other gases in the atmosphere by filtration. The discharged nitrogen from the filtration unit is in a gaseous state and uses downstream air compressors to boost the gaseous nitrogen to a pressure sufficient for injection into the well.

Note: This nonconventional equipment is seldom encountered. It is included here so that you will know it does exist and recognize it if you should encounter it.

4.4.1 Types of nonconventional equipment The two most common types of nonconventional nitrogen pumping units are • pressure swing adsorption (PSA) unit (see Fig. 4-2) • membrane unit (see Fig. 4-3).

The advantages of PSA or membrane units are as follows: • No cryogenic equipment is required for pumping. • No special equipment is needed to handle liquid nitrogen.

• Two air compressors are required (one for input to the membrane and one for discharge to the well). • Both systems have limited applications because of the low maximum discharge pressure.

4.4.2 PSA units

Figure 4-2. PSA Unit

Figure 4-3. Membrane Unit

• No losses of product from evaporation occur.

The PSA unit is made of two large receivers filled with carbon molecular sieve (CMS) capable of producing gaseous nitrogen. The two receivers are alternately filled with air from the atmosphere. Each receiver goes through the following process that allows gaseous nitrogen to be released to a surge tank. The process is a physical process, not a chemical process. 1. The CMS is porous, somewhat like pumice or a sponge. The holes in the CMS are small enough to allow the larger nitrogen molecules to remain free in the sieve tank, while the oxygen and other gaseous molecules can go through them. After the separation process goes on for about 60 sec, the retained nitrogen molecules are released out of the vessel at low pressure to a surge tank. In the surge tank, the gaseous nitrogen is collected for use in the well.

• No onsite storage of product is necessary. • The typical safety concerns of handling LN2 are absent.

The disadvantages of PSA/membrane systems are as follows: • The system has a limited maximum well injection pressure of 2,500 to 3,000 psi, depending on the system. • The membrane and PSA sieve material must be replaced periodically.

JET 17 - Introduction to Nitrogen Operations  |  15

2. When the nitrogen has been drawn out of the sieve tank, the pressure in it is released quickly, allowing the other trapped gas molecules to escape back to the atmosphere, cleansing the CMS for the next cycle. The process continues to cycle between tanks to feed gaseous nitrogen to the surge tank.

The holes in the wall of the hollow fiber are small enough that CO, CO2, and O2 will exit through them under pressure. These same holes are too small for GN2 to go through; therefore, it is trapped.

3. Gaseous nitrogen from the surge tank then feeds the suction side of another compressor, which prepares the gaseous nitrogen for injection into the well. Discharge pressures for typical PSA systems are 2,500 to 3,000 psi. Units with higher pressure capabilities have been built, but only on special request. Because of the size of a PSA unit, the maintenance required, and the low maximum pressure, it is not usually the preferred equipment.

4.4.3 Nitrogen membrane units The use of membrane technology is somewhat similar to that of the PSA unit in that it uses the size of the nitrogen molecule to separate it from the other components of air. Unlike the PSA system, however, the membrane system provides continuous flow. There is no switching back and forth between vessels containing CMS. Membrane canisters, shown in Fig. 4-4, are a series of hollow fibers that have holes in the walls. Compressed air is forced through these hollow fibers. Usually, the air is heated in the membrane to excite the molecules in the hollow fiber and increase the chance that they will permeate the holes.

16  |  Equipment Selection

Figure 4-4. Membrane Canister

5.0  Preparation for the Job Several factors can lead to the safe and successful completion of the treatment and the satisfaction of the client. The field specialist should be prepared to complete the treatment as designed, while considering reasonable extenuating circumstances. Some unexpected circumstances or conditions that could affect the outcome of the treatment are • multiple cooldowns caused by well problems • change in nitrogen flow rate • change in wellhead pressure • change in condition of location • change in road conditions • change in total nitrogen required ○ if the bottomhole injection pressure changes ○ if sand continues to flow into the well bore during cleanout • change in bottomhole injection pressure • change in friction pressures.

Any of these or other factors that are not listed can impact the treatment and ultimately the client’s satisfaction. While field specialists do not have complete freedom to cover all eventualities, they can plan ahead and think through possible options. These actions convey an air of experience and professionalism to the customer and can help prepare for unplanned occurrences on the job. When preparing for a job, the field specialist will usually work from a treatment design that the client has agreed to. It is the field specialist’s responsibility to perform the treatment with the

proper equipment and personnel to provide the client with the highest possible satisfaction according to the treatment design. The following items should be carefully considered in preparation for the treatment.

5.1 Equipment When determining the type of nitrogen unit to be used on a given treatment, consider the vaporizer capacity as well as the cold-end pumping capacity. Always ensure that the cold-end pumping capacity and the vaporizer capacity are compatible and sufficient for the requirements of the job. Also, ensure that the cold-end pressure rating is sufficient to handle the anticipated pressure requirements at the well. When selecting the pumping equipment, use the following guidelines: • Ideally, the expected treatment pump rate should be between 50 and 75% of the maximum pumping capacity of the unit. • When multiple pumpers are required, select equipment that will provide approximately 25% excess capacity. See Fig. 5-1 for an example of multiple pumps. • Do not use a high-rate unit (5,000 scf/ min or higher) for low-rate pumping requirements. That is, avoid using a unit outfitted with higher-volume cold ends. The temperature of the discharge gas is difficult to control, and the returned liquid to the tank on a long pumping job will make it difficult to keep prime.

JET 17 - Introduction to Nitrogen Operations  |  17

Figure 5-1. Multiple Pumps

• If possible, select the pumping unit that can carry all the liquid nitrogen required for the job. Reducing the number of units on location eliminates the need for transferring liquid and reduces liquid losses. Also, reducing the number of units and personnel on location reduces the capital investment required to perform the treatment.

5.2 Product availability In determining the amount of liquid nitrogen to be taken to location, some considerations may not be tied directly to the amount of product used on location. When loading for a job (see Fig. 5-2), it is usually better to fill the transporting equipment completely even though the quantity might be well in excess of the nitrogen required on location. When transferring from district storage, a certain amount of liquid nitrogen is required to cool down the transfer pump and associated pipe whether 1,000 or 5,000 galUS is moved. Thus, when in an active nitrogen district, it is far better to fill the unit completely each time the transfer equipment is cooled down. This practice reduces inventory losses and the cost of managing the inventory.

18  |  Preparation for the Job

Figure 5-2. Loading a Transport

The field specialist must also anticipate pumping situations on location. With experience in certain types of treatments, client techniques, or geographical locations, the field specialist quite often decides to bring additional liquid. The following are some reasons why additional liquid would be required on location: • The well could be a problem well that will require multiple, unanticipated cooldowns. • The type of treatment calls for multiple stages; therefore, multiple cooldowns will be required. • Transfers on location will cause losses. • It is not possible to pump all the liquid N2 out of the tanks, leaving some quantity in the bottom of the transport. • There may be a long standby time on location, with the accompanying liquid losses because the pumps must partially recirculate back to the supply tank. • The job could be a low pumping rate coil tubing job, where partial recirculation back to the supply tank increases losses.

Other situations may require nitrogen in addition to what is called for in the job design. For example, the distance between the supplier and the district or wellsite might be so great that there will be considerable losses along the way.

It is up to the field specialist to closely examine the potential for additional nitrogen and plan accordingly. It is much better to take too much nitrogen to the job than it is to run short in the middle of the job.

5.3 Maintenance status While a field specialist is not directly responsible for equipment maintenance, he or she should constantly monitor the condition of equipment that is used on assigned treatments. In many cases, the field specialist uses the same piece of equipment on future jobs, so it could save time and trouble to be aware of the condition of the equipment. It is a field specialist’s responsibility to ensure that the operator notifies maintenance through the posttrip process (STEM 1 form available at InTouch Content ID# 2024129, attachment Inspec(ALA).xls) of any problems with a given piece of equipment. It is also the field specialist’s responsibility to notify line management when a required piece of equipment may not be in acceptable condition to successfully complete the treatment.

Note: Remember that how the equipment and personnel perform directly reflects on the Schlumberger image.

5.4 Personnel training Equipment performance and personnel training are two of Schlumberger’s most valuable assets. It goes without saying that well-maintained, properly operating equipment greatly contributes to the success of any job and the client’s impression of the results. However, the best-maintained equipment

cannot perform its intended task unless the personnel operating it are adequately trained and qualified. When pumping nitrogen, it is not only the physical pumping of the product but also the understanding of how gaseous nitrogen reacts in the well that helps a treatment field specialist provide the client with expected results and increased production. To competently perform treatments that require the pumping and transport of nitrogen, at a minimum, the person running a unit, pumper, or transport must have training on that piece of equipment. He or she should also be trained according to the following JET manuals, in addition to this JET manual. • JET 11: Nitrogen Storage and Transport Equipment, InTouch Content ID# 4221680 • JET 29: Nitrogen Pumping, InTouch Content ID# 4221766

Any training that an operator or field specialist receives always references the Schlumberger Well Services (WS) Safety Standards. Anyone operating or supervising a nitrogen pumping job must be familiar with and understand WS Standard 5: Pressure Pumping and Location Safety, and WS Standard 11: Pumping Nitrogen.

5.5 Journey assessment and approval form Before departing from the district, the job field specialist must complete the Journey Assessment and Approval form (available from InTouch Content ID# 3498095). The evaluation of the risk must be completed and appropriate approval must be obtained. A copy of this form can be found at InTouch Content ID# 4195154.

JET 17 - Introduction to Nitrogen Operations  |  19

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20  |  Preparation for the Job

6.0  Personnel Qualifications It is essential that personnel on location performing the treatment have the necessary qualifications or be under the supervision of someone capable of training them on the equipment in question.

6.1 Specific training on cryogenic equipment Training related to nitrogen services and equipment can be found in the Learning Management System (LMS). For available courses, location, and timing, access the LMS Web site at https://lms.slb.com/SabaWeb. The WS Hub page, http://training.wellservices. oilfield.slb.com, also has information.

6.3 Specific safety gear for handling LN2 When operating a nitrogen pumper or a nitrogen transport, the operator must have available and use the safety equipment and personal protective equipment (PPE) listed in WS Safety Standard 11. It is the location supervisor’s responsibility to ensure that all of the necessary safety equipment is used during the treatment.

6.2 Understanding pumping nitrogen vs. liquids To operate a nitrogen pump properly, you must clearly understand that pumping liquid nitrogen is significantly different from pumping typical water-based fluids. If operators have not been properly trained in and clearly understand the differences in pumping a cryogenic fluid, they should not be pumping nitrogen without proper supervision. Many operators learn to operate a unit through OJT (on-the-job training). The person conducting the OJT must clearly demonstrate that he or she has the knowledge and experience necessary to train the new operator.

JET 17 - Introduction to Nitrogen Operations  |  21

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22  |  Personnel Qualifications

7.0  Characteristics of Liquid and Gaseous Nitrogen In performing nitrogen pumping services, the field specialist and operator must deal with both liquid and gaseous nitrogen. The liquid nitrogen is primarily handled during the storage, transport, and pumping. The gaseous nitrogen is handled after the liquid converts to gas after passing through the vaporizer. The piping and pumps must be able to handle cryogenic temperatures. After the nitrogen has passed through the vaporizer and is gaseous, the operator must understand and deal with the nitrogen as a gas. Understanding how the gas performs under pressure and temperature improves the potential success of the treatment, as well as the safety of all concerned.

7.1 Properties of liquid and gaseous nitrogen As described in the Matheson Gas Data Book, nitrogen at room temperature and atmospheric pressure is a colorless, odorless, nontoxic, nonflammable gas. The following provides some information about the characteristics of nitrogen. • chemical symbol: N2 • temperature of liquid: –320.4 degF (‑195.8 degC) • percent of atmosphere: 78% by volume • volume (scf) in 1 gal GN2: 93.05 scf at 1 atm (be sure to check the latest version of the Data Book for these numbers) • percent oxygen causing noticeable effects: 10 to 14% by volume. (Percent shown is remaining O2 caused by displacement by another gas. The normal percentage of O2 is 20.9%)

• exposure of the skin to LN2: can cause severe burns • exposure of mild steel to LN2: will cause distortion, cracking and ultimate failure of structural member • critical temperature: –232.6 degF • boiling point: –320 degF

7.2 Conversion factors The following provide some basic conversion facts about N2: • liquid volume per pound: 13.8 scf/lbm of LN2 • gaseous volume per pound: 93.05 scf at 1 atm • weight: 6.74 lbm/galUS • pressure gradient for any fluid: .052 x weight (lbm/galUS) of the fluid in question.

7.3 Safety aspects Because the nitrogen is injected into the well as a gas under pressure, it has very high potential energy. The illustration used in the safety film ”There are still no clowns” (this film can be ordered from http://www.orcamedia.com/slb. htm) is the best example of the kind of potential energy available from compressed gaseous nitrogen. If you think that you may have liquid nitrogen in the treating lines, it is important that you be sure of exactly what is occurring. A misdiagnosed situation could cause overreaction and concern on the client’s part. The critical temperature of liquid nitrogen is –232 degF. By definition, nitrogen at its critical

JET 17 - Introduction to Nitrogen Operations  |  23

temperature or above cannot be returned to a liquid solely by increasing the pressure. Therefore, the odds of actually having liquid nitrogen in the treating lines are quite low. However, the treating pipe can still be filled with very cold gas and eventually the temperature can be lowered to unacceptable levels. The treating pipe used in Schlumberger is only designed for use at temperatures as low as –20 degF. Usually, the most noticeable indication that the treating lines are exposed to low temperatures is a frost forming on the outside surface. Moisture in the atmosphere will collect on any surface colder than the atmosphere. As the temperature of the treating pipe goes below 32 degF, the collected moisture begins to freeze. As mentioned above, the field specialist and operator should not overreact, but take the required corrective actions.

All of these are mixtures of various base fluids plus additives. With none of these mixtures is it necessary to consider the effects of pressure or temperature, either at the surface or in the well. Gaseous nitrogen is different; the effects of pressure and temperature must be considered. The standard unit of measure for nitrogen is a standard cubic foot (scf), which is the volume the gas occupies at a temperature of 60 degF under 1 atm of pressure. The relative volume of a standard cubic foot of gas that is being introduced into the well or other fluid is a direct function of the injection pressure. The higher the pressure is, the smaller the space is that a given amount of nitrogen occupies.

step 03

The gaseous nitrogen is also influenced by the temperature at which it is pumped or the temperature of the environment into which it is pumped. The effects of the temperature relative to the space occupied is not as heavily influenced by temperature as pressure, but it is still a factor. Volume factors are selected at a temperature of 100 degF as a standard for calculations. Unless the gas temperature is discharged at a temperature higher than 100 degF, the 100-degF column is used in all calculations.

step 04

An example of the influence of pressure and temperature is shown in Table 7-1.

Step 01

Slow the liquid pump down to allow the vaporizer to catch up.

step 02

Increase fuel flow to the vaporizer. This increased flow creates more heat. Completely shut the pumps down until the frost is gone. Shut the pumps down until any problems with the vaporizer are corrected.

7.4 Effects of pressure and temperature on GN2 With the exception of nitrogen, all other services pumped by Schlumberger are introduced into the well as a fluid. Examples of some of the more common ones are acid, fracture fluids, and cement. Even CO2 is pumped into the well as a fluid.

24  |  Characteristics of Liquid and Gaseous Nitrogen

Table 7-1. Influence of Pressure and Temperature on N2

Pressure (psi)

Standard cubic feet of nitrogen required to fill one barrel of space at temperature 80 degF

100 degF

120 degF

1,500

549

526

505

5,000

1,544

1,482

1,430

8,000

2,076

2,006

1,947

Note: The pressures and volumes shown in Table 7-1 can be used as convenient estimates when having general discussions with the client with no handbook or computer available.

gaseous nitrogen occupies (see Fig. 7‑1) for an illustration. Each balloon contains 1 scf of nitrogen gas.

GN 2

For all calculations and treatments, the temperature will be assumed to be 100 degF, unless otherwise stated.

Note: Information related to volume factors can be found in the online I-Handbook (InTouch Content ID# 3874787) or from one of the nitrogen field handbooks, such as the Nitrogen Engineering Handbook (InTouch Content ID# 3016873).

7.5 Compression and expansion of GN2 Gaseous nitrogen has no particular shape when commingled with other liquids. It is a random collection of differently sized bubbles dispersed throughout the fluid to which it is added. The nitrogen injected into the fluid occupies a given amount of space (volume) within the fluid, based on the pressure to which it is exposed. To understand the relationship between the pressure and the volume a standard cubic foot (scf) of nitrogen occupies, imagine 1 scf of nitrogen as a balloon filled with GN2. As the nitrogen is discharged from the unit, it is exposed to the pressure in the treating line. The balloon expands or contracts according to the pressure that is exerted on it. This initial pressure will determine the relative volume the

GN 2

GN 2

1,500 psi 5,000 psi 8,000 psi External pressure on the outside surface of the balloons As the pressure increases, the volume (space) occupied by 1 scf of nitrogen decreases.

Figure 7-1. Volume of Nitrogen at Varying Temperatures

Straight gaseous nitrogen fills all of the space into which it is injected. This space includes the treating lines, wellhead, and all downhole tubing and casing goods. The pressure required to occupy the space determines the extent to which each standard cubic foot is compressed, and therefore the number of standard cubic feet it takes to fill a barrel of space. The higher the pressure is, the more standard cubic feet of nitrogen are required to occupy a barrel of space. The reverse is true as well. As gaseous nitrogen that is mixed with fluid is exposed to less pressure, the ratio of the nitrogen to the fluid increases. As a nitrified fluid comes up the hole, the gas expands, increasing the amount of nitrogen relative to fluid in any given barrel of space. The net effect is that the nitrified fluid occupying that barrel of space now weighs less and generates less hydrostatic head. This is a valuable function when kicking off a well.

JET 17 - Introduction to Nitrogen Operations  |  25

Note: Nitrogen gas generates hydrostatic head just as a liquid does. The main difference between nitrogen gas and a liquid is that the pressure generated by 1 scf of gaseous nitrogen increases as the well gets deeper. The density of the gas increases with every foot of depth because of the hydrostatic head of the gas column above it. Other fluids, such as water and acid, do not behave the same way. Because they are noncompressible fluids, their pressure gradient (psi/ft) does not change with depth.

7.6 Volume factors for GN2 In designing and performing nitrogen treatments, it is necessary to understand the relationship between the volume of nitrogen pumped and the volume of liquid pumped. The rate at which the nitrogen is pumped in relation to the rate of the liquid is vital in many designs involving nitrogen. A foam fracture is probably the best example of this. The relation between the gaseous nitrogen rate converted to barrels and the liquid rate in barrels is determined by using the nitrogen volume factor. For example, looking at the Nitrogen Engineering Handbook (see Fig. 7-2), if the discharge pressure is 100 psi and the gaseous temperature is 100 degF, it would take 35 balloons filled with one standard cubic foot of gaseous nitrogen each to fill one barrel of space. The number of scf/bbl, 35 in this case, is referred to as the nitrogen volume factor. See Fig. 7-3 for an illustration of the nitrogen barrel factor.

26  |  Characteristics of Liquid and Gaseous Nitrogen

Figure 7-2. Nitrogen Volume Factor for 100 psi Surface

2 3/8-in tubing 5.5-in casing

If the average pressure in this part of the tubing was 1,500 psi it would take 525 scf to fill this space

ft

Length of tubing that holds one barrel

End of tubing

Bottom of the well

Zone of interest

Figure 7-3. Nitrogen bbl Factor

JET 17 - Introduction to Nitrogen Operations  |  27

Now, increase the pressure on the balloon to 1,500 psi (see Fig. 7-4). It now requires 525 of these balloons to fill a barrel of space. If the nitrogen unit were pumping 525 scf/min, then effectively the nitrogen pump rate would be 1 bbl/min at the surface. As demonstrated in Fig. 7-2, the higher the pressure is, the more standard cubic feet are required to equal a barrel in volume. When the gaseous nitrogen is mixed with another fluid that is being injected into the well, the relative volume of the fluid must be considered when determining the total volume injected into the tubing or the rate at which the combination is pumped. As an example, if the nitrogen pumper were pumping 525 scf/min (standard cubic feet per minute) at 1,500 psi and 100 degF, and a fluid pump were injecting 1 bbl/min of acid, the total injection rate at the surface would be 2 bbl/min. (see Fig. 7-5).

Figure 7-4. Nitrogen Volume Factor for 1,500 psi

28  |  Characteristics of Liquid and Gaseous Nitrogen

Note: It is very important to understand that in this example, the injection rate will decrease on the bottom because of the increased hydrostatic head resulting from the column of nitrogen and acid. In this example, the ratio of nitrogen to liquid is 50/50 on the surface. At bottomhole conditions, the ratio has changed to 34/66. It is very important to understand this concept. The ratio of nitrogen to liquid and how it is affected by pressure and temperature is the key to understanding what is going on downhole and to properly design a job.

525 scf GN 2 (one bbl/min at 1,500 psi)

1 bbl/min Acid Surface

Well pressure of 1,500 psi

2 3/8-in tubing

5.5-in casing

Bottomhole pressure is 3,000 psi

Bottom of the well

Combined rate is 1.5 bbl/min (1/2 bbl/min N 2 and 1 bbl/min acid)

Zone of interest

Figure 7-5. Combined Rates

One of the best examples of the importance of keeping the effect of temperature and pressure on the nitrogen in mind is when maintaining the ratio of nitrogen to liquid during a foam fracturing job. If the ratio is not adjusted for variables such as sand, temperature, and pressure, the quality of the foam changes and the quality of the job could be affected.

Note: Remember that volume of the sand being added to the fluid must be taken into consideration when determining the ratio of fluid to nitrogen.

JET 17 - Introduction to Nitrogen Operations  |  29

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30  |  Characteristics of Liquid and Gaseous Nitrogen

8.0  Using the Nitrogen Engineering Handbook Nitrogen conversion information tables are available in hardcopy and electronically. A variety of nitrogen engineering handbooks are in use in the field. The information in these is essentially the same. Some are formatted differently, depending on the version being used. Some can be accessed through InTouch. The two most important documents that are available in InTouch are the i-Handbook (InTouch Contend ID# 3874787) and the Nitrogen Engineering Handbook InTouch content ID# 3016873. The Nitrogen Engineering Handbook is used for data in the calculations in this JET. Each section of the handbook has examples demonstrating the use of the information in that section.

8.1 Section 100: Introduction to Nitrogen This section is a general introduction to nitrogen with calculation examples for various types of nitrogen jobs. Some of the information found in this JET can also be found in the handbook. This section of the handbook also provides conversion data. The matrix in Table 9-1 provides necessary conversion factors for converting nitrogen to various units of measure.

8.2 Section 200: Displacement with Nitrogen The displacement tables provide a quick reference for determining the nitrogen gas displacement volume needed for various pressures, depth, and sizes of well tubular goods. In many cases, you must interpolate to get the exact nitrogen volume.

Table 8-1. Conversion Data for Nitrogen

Weight (lbm)

Weight (US ton)

Volume (SCF)

Liquid Volume (galUS)

Liquid Volume (ft 3 )

Liquid Volume (L)

Liquid Volume (m3 )

1 lb

1.0000

0.0005000

13.80

0.14830

0.019820

0.56130

0.000561

1 ton

2000.0000

10000000

27600.00

296.60000

39.650000

1122.60000

1.122612

1 scf gas

0.0724

0.0000362

1.00

0.01075

0.001436

0.04068

0.0000507

1 gal liquid

6.7430

0.0033720

93.05

1.00000

0.133700

3.78500

0.003785

1 ft3 liquid

50.4500

0.0252200

696.10

7.48100

1.000000

28.32000

0.0353107

1 l liquid

1.7820

0.0008908

24.58

0.26420

0.035310

1.00000

0.001

1 m3 liquid

1781.5600

0.8907530

19708.00

264.20080

0.028320

1000.00000

1

JET 17 - Introduction to Nitrogen Operations  |  31

Section 200 provides the following information: • displacement techniques and calculations • displacement estimating tables ○ given nitrogen wellhead pressure and depth or bottomhole pressure ○ given fluid density in wellbore and depth ○ U-tubing down tubing ○ U-tubing down casing

Note: The tables in the Nitrogen Engineering Handbook are very helpful in performing calculations. It is important to understand the features of the tables.

The following must be understood to properly use these tables. • The wellhead pressure given assumes that the well is completely displaced with GN2. • The bhp (bottomhole pressure) is actually the bhp when the displacement is complete and there is a column of GN2. It can be used as the static bottomhole pressure of the well.

Section 300 provides the following information: • nitrogen volume factors (BN2, which means the amount of nitrogen required to fill a barrel at a given pressure) • nitrogen requirements for foamed fluids • gaseous nitrogen column–bottomhole pressure • hydrostatic pressure of a nitrified fluid • bottomhole or wellhead pressure of an energized fluid • average density of a nitrified mud • nitrogen-liquid flowback tables • miscellaneous technical information • pipeline capacities.

The following features of the tables in this section must be understood. • For the best selection of the standard cubic foot GN2/bbl volume factor, the most representative pressure and temperature of the well should be used. • The tables provide the following data, among many others: ○ amount of nitrogen required per barrel per minute fluid rate to produce a given foam quality

• The tables can be used to determine the hydrostatic head of gaseous nitrogen.

○ bottomhole pressure due to a column of gaseous nitrogen

8.3 Section 300: Technical Information

○ hydrostatic pressures produced by various nitrified fluid based on the ratio of nitrogen to fluid

This section provides information that allows the user to determine the quantity of gaseous nitrogen to occupy a barrel of space, the hydrostatic head of a column of nitrogen, and the hydrostatic head of a column of nitrified fluid.

32  |  Using the Nitrogen Engineering Handbook

○ The amount of nitrogen required to flow back a given density of fluid with a known bottomhole producing pressure ○ The amount of nitrogen required to fill a pipeline at various pressures.

9.0  Job Calculations Calculations for nitrogen treatments are much like those in other designs. The overall approach to calculating acid and fracture treatments is the same in that the designer must know surface and downhole pressures and rates. The primary difference is that the nitrogen, when injected into the well as a gas, does not occupy the same space at all combinations of pressure and temperature. Because of the compressibility of the gaseous nitrogen, the designer must be constantly aware that the volume of the nitrogen relative to the other fluids being pumped is changing with pressure. The illustration in Fig. 8-3 is a very good example of how the volume of a standard cubic foot of nitrogen changes with pressure. Remember the following factors when doing calculations involving nitrogen. • The pressure at which the nitrogen leaves the unit determines the initial space a standard cubic foot of nitrogen occupies, in units of barrels. • As pressure increases, more gaseous nitrogen is required to fill a barrel in volume. • At a given pressure, an increase in nitrogen rate in scf/min also increases the nitrogen rate in bbl/min. • If the pressure drops, the nitrogen rate in bbl/min increases. • If the pressure rises, the nitrogen rate in bbl/min decreases.

• If the pressure increases and the nitrogen and liquid rate is set, the quality (ratio) of a foam decreases. • If the pressure decreases and the nitrogen and liquid rate is set, the quality (ratio) of a foam increases.

Temperature swings of the nitrogen can technically affect the expansion and contraction of a standard cubic foot. Unless the temperature variation is more than ±30 degrees from 100 degrees to significantly affect the volume factor.

Note: As a standard, the nitrogen is pumped at 100 degF.

9.1 Displacements The displacement of the well is a very common service provided by nitrogen pumping. It is very common to use nitrogen as the displacement fluid when the final hydrostatic pressure at the perforations is critical. To understand the calculations of a displacement, the main point to realize is that pressures used in the final determination of the nitrogen required are those present when the well is full of nitrogen gas. Remember that the average pressure is determined based on the well having a column on nitrogen gas. If it is necessary to remove all fluids from the well, tubing and annular space, displacement tables providing the needed information

JET 17 - Introduction to Nitrogen Operations  |  33

can be found in Section 200 of the Nitrogen Engineering Handbook. To make a displacement calculation, the approach is similar to calculating the displacement of the tubing with fluid. Additional steps convert the tubing volume in barrels to standard cubic feet of nitrogen at an average pressure. If required, additional nitrogen volume can be calculated for the annular space between the tubing and the casing. The following demonstrates the calculations. See Fig. 9-1 for a schematic of the well used in the following examples.

• casing: 5 ½ in • wellhead pressure (WHP) with 9.5 lbm/galUS fluid = 500 psi • top of perforation: 8,500 ft • Tsurface = 80 degF (geothermal gradient of 1 degF/100 ft)

9.1.1 Tubing displacement Question: What volume of nitrogen (how many tanks) must be at the wellsite to completely displace the tubing?

Step 01 tubing:

Calculate volume to displace the

= (8,000 ft x 0.0058 bbl/ft) = 46.4 bbl (tubing volume)

3,161-psi wellhead pressure with tubing filled with GN2

Step 02 2 7/8-in tubing

Calculate the average pressure between the BHP and the WHP when the well is filled with nitrogen (WHPN from the Nitrogen Handbook, pg 315.05, Gaseous Nitrogen Column-Bottomhole pressures):

5.5-in casing

46.4 bbl

WHP = (Nitrogen Handbook value) = 3,600 psi

Pressure at end of tubing - 4452 psi 8,500 ft BHSP= 4699 psi

Pavg = (4,452 psi BHP + 3,600 psi WHP)/2 = 4,000 psi 8,000 ft Producing zone

12.2 bbl

Figure 9-1. Displacement of Nitrogen Under Pressure

Well Data • tubing: 2 7/8 in (6.5 lb/ft) • end of tubing: 8,000 ft • produced oil weight: 9.5 lbm/galUS fluid

34  |  Job Calculations

BHP = pressure of column of oil + WHP = (8,000 ft x 9.5 ppg x 0.052) + 500 psi = 4,452 psi BHP

Step 03

Calculate the average temperature between the surface temperature and the bottomhole static temperature (BHST): BHST = 80 + 8,000 x 1 degF/100 ft = 160 degF Tavg = (80 degF + 160 degF)/2 = 120 degF

Step 04

Now the nitrogen volume factor can be determined from the Nitrogen Engineering Handbook (pg 305.03). Using the Pavg = 4,000 psi and the Tavg = 120 degF, then the nitrogen will be 1,236 scf/bbl.

Step 05

Total gaseous volume of nitrogen can be determined by applying the nitrogen volume factor to the tubing volume displaced: GN2 = NVF x Vtubing = 1,236 scf/bbl x 46.4 bbl = 57,350 scf GN2

Step 06

Add 10,000 scf for the cool-down (rule of thumb) and then 10% for losses: = (57,350 scf + 10,000 scf) x 1.1 = 74,085 scf

Step 07 nitrogen:

Convert gaseous nitrogen to liquid

= GN2 / 93.12 scf/gal = 74,085 scf/93.12 scf/gal = 800 gal LN2

step 01 Well volume = (tubing volume 8000 ft + casing volume to 8500 ft). Volume to displace the tubing: = 8000 ft x 0.0058 bbl/ft = 46.4 bbl Volume of casing = (8500 ft - 8000 ft) x 0.0244 bbl/ft = 12.2 bbl Well Volume = 12.2 bbl + 46.4 bbl = 58.6 bbl

step 02

Calculate the average pressure between the BHP and the WHP when the well is filled with nitrogen (WHPN), using the Nitrogen handbook 315.05 (Gaseous Nitrogen Column-Bottom hole pressures): BHP = pressure column of oil + WHP = (8,500 ft x 9.5 ppg x 0.052)+ 500 psi = 4,700 psi BHP WHPN (Nitrogen handbook p 315.05) = 3,750 psi WHP

Step 08

Determine how many tanks of 2,000 gal each are needed at the wellsite:

Pavg = (4,700 psi + 3,750 psi)/2 = 4,225 psi

1 LN2 tank

step 03

9.1.2 Tubing and casing displacement Question: What volume of nitrogen (how many tanks) are needed at the wellsite to completely displace the well down to the perforations?

to

Calculate the average temperature between the surface temperature and the BHST: TSurface = 80 degF BHST = 80+ 8500 x 1 (0 degF/100 ft) = 165 degF Tavg = (80 degF + 165 degF)/2 = 122 degF

step 04 Now the nitrogen volume factor

(NVF) can be determined from the Nitrogen Engineering Handbook (305.03). With the Pavg (4225 psi) and the Tavg (1220F), therefore the NVF = 1,280 scf/bbl. JET 17 - Introduction to Nitrogen Operations  |  35

step 05

The total gaseous volume of nitrogen can be determined by applying the NVF to the tubing volume displaced: GN2 = NVF x Vtubing = 1,280 scf/bbl x 58.6 bbl = 75,000 scf

Step 06 Add 10,000 scf for the cooldown (rule of thumb) and 10% for losses: = (75,000 scf + 10,000 scf) x 1.1 = 93,500 scf

Step 07 nitrogen:

Convert gaseous nitrogen to liquid

LN2 = 93,500/93/12 = 1,000 gal LN2

step 08

Determine how many 2,000 gal tanks are needed at the wellsite. One will suffice.

amounts to lighten the column of fluid in the well enough that the bottomhole producing pressure unloads the well. An extension of the concept is applied to the fluid that is actually injected into the well. The theory is that enough of the nitrogen commingled with the fluid remains with the fluid when it returns to the wellbore to keep the hydrostatic head below the bottomhole producing pressure. Fluid flowback is addressed in the Nitrogen Engineering Handbook. The handbook is careful to explain that liquid flowback is not an exact science. The handbook states “The volumes given are very idealistic and are given for various weight fluids for ideal conditions. Field experience has shown that for effective cleanup the design will probably increase the SCF shown in the table and charts.” Factors that should be taken into consideration when designing a flowback treatment are • the size of the tubing or casing in the well • the velocity at which the fluids in the tubing flow

9.2 Well flowback using nitrified fluids

• porosity of the formation

Wells often do not have sufficient bottomhole producing pressure to keep the well unloaded and producing. Some of these wells have the necessary producing pressure to produce hydrocarbons but not water-based fluids. The hydrostatic pressure of the column of water is too great.

• type of fluids produced by the zone.

Another common situation is that when fluids with high hydrostatic heads are used to kill the well or are injected into the formation, it is necessary to retrieve the fluids. The technique for solving either problem is to introduce gaseous nitrogen to reduce the hydrostatic head of the wellbore fluid. This procedure is commonly referred to as well flowback using nitrified fluids. The technique is simple: nitrogen is introduced in sufficient 36  |  Job Calculations

The following is an example of the calculations to design a flowback treatment. See Fig. 9-2 for a schematic of the well. Well Data • well depth: 7,000 ft • bottomhole static pressure: 3,000 psi • tubing: 2 3/8 in • fluid injected S.G.: 1.1 (9.17 lbm/galUS) • casing: 4 1/2 in • produced oil weight: 7.0 lbm/galUS

Step 01

Determine the bottomhole producing pressure if it is not available from the client. ebhpp

x

=

BHSP x 50% for oil wells

=

3,000 psi x 0.5

=

1,500 psi

2 3/8 in Tubing

The nitrogen required per barrel of fluid injected is 800 scf. As mentioned in the Handbook, the required nitrogen is a conservative estimate. Experience in a given field should always be factored in when finalizing the design. As it turns out, this well will not flow with a column of oil in the wellbore. If the client wanted to continue with the unloading of the well when oil came to the surface, nitrogen would still be required to reduce the hydrostatic head of the column of oil.

4.5 in Casing

9.3 Fluid removal from the well Fluid removal from the well can involve a variation of techniques used in displacements or well flowback, depending on what is to be accomplished.

Producing zone

bhsp 3,000 psi

bhhp 1,500 psi

Figure 9-2. Well Schematic for Flowback Example

Step 02

Using the table on page 215.03 of the Handbook, determine the nitrogen necessary for flowback of the 1.1 S.G fluid. Using the left-hand column of the same table, find the depth of 7000 ft. Follow across to the column that corresponds to bhpp of 1,500 psi.

When the objective is to remove all fluids from the wellbore, the method used for calculations would be very similar to that used for a displacement. If the treatment is more of a well kickoff using coiled tubing in the hole, then the calculations and design are similar to those used in fluid flowback treatments. In the example shown here, fluid removal will be defined as removing all fluids from the well. Depending on the well setup and the desires of the client, the fluids can be removed by introducing the displacement nitrogen through the tubing or through the casing. Functionally, either way works. Economically, in terms of time and money, it is cheaper for the client to introduce nitrogen down the tubing and wellbore fluids out the casing side. In the nitrogen displacement calculations, the nitrogen volume factor was selected from the Handbook based on the calculated average pressure in the tubing. The same applies if a volume factor is selected for a casing displacement. The major difference is that the JET 17 - Introduction to Nitrogen Operations  |  37

annular volume in barrels, between the tubing and casing, is generally larger than the volume in barrels in the tubing. Therefore, the quantity of nitrogen required to displace the annular space is greater than that required to displace the tubular space. After the nitrogen has reached the bottom and is returning to the surface through the tubing or casing, depending on the injection point, pumping can soon cease. When the pressure at the interface of the nitrogen and the fluid being removed is equal to or greater than the that of the remaining fluid column, the well will effectively unload the remaining fluid. Therefore, the nitrogen volumes given in the handbook as required never equals the combined volume of the tubing and casing.

3,585-psi wellhead pressure with a column of Surface 2 3/8-in tubing

Tubing to 10,000 ft 4,680 psi at end of tubing

The following calculation demonstrates the difference in nitrogen volumes required when choosing to go down the tubing or casing. See Fig. 9-3 for a schematic of the well. From the Handbook, page 300.09, determine the well pressure to support a column of 9 lbm/galUS mud with a column of nitrogen. Well Data • well depth:

10,000 ft

• tubing:

2 3/8 in

• casing

5 1/2 in

• wellbore fluid:

mud

• fluid weight:

9 lbm/galUS

• hydrostatic gradient of mud:

0.486 psi/ft

• pressure at 10,000 ft

4,680 psi

Using the depth of 10,000 ft in the left column, follow the table across until a pressure of 4,680 psi is found. It may be necessary to interpolate.

38  |  Job Calculations

5.5-in casing

Zone of interest

Figure 9-3. Well Schematic for Fluid Removal Example

The results will be a wellhead pressure of 3,585 psi to displace the tubing with nitrogen and provide a pressure at the end of the tubing of 4,680 psi. For purposes of determining the appropriate nitrogen volume factor the average pressure is as follows: = 3,585 psi + 4,680 psi = 8,265 psi / 2 = 4,132 psi average pressure From page 300.04, the interpolated nitrogen volume factor is 1,299 scf/bbl (at 4,132 psi and 100 degF)

Tubing capacity to the end of the tubing is 0.00387 x 10,000 = 38.7 bbl The casing capacity to the end of the tubing = 0.0189 x 10,000 ft = 189 bbl As a comparison, calculate the nitrogen required to unload the well pumping via the casing or the tubing. Casing volume (scf): = 1,299 scf/bbl x 189 bbl = 245,511 scf Tubing volume (scf): = 1,229 scf/bbl x 38.7 bbl = 50,273 scf Total volume of the well in scf = 295,784 scf The same information can be obtained from the handbook on pages 200.20 and 200.21. It is obvious from the information above that it would require approximately 5 times as much nitrogen by going down the casing than by going down the tubing. This is the nitrogen required to turn the corner and start back up the well only. Additional nitrogen should be pumped to ensure the remainer of the fluid is removed. In using the tables in the handbook, it is found that the total nitrogen calculated to unload the well is substantially less than the totals calculated here. As mentioned in the text, after the nitrogen has turned the corner at the end of the tubing, the column of fluid is no longer heavy enough to control expansion of the nitrogen gas. At this stage, the gas expands in

either the tubing or casing and unloads the well without further pumping. In comparing the numbers, approximately 20% for the tubing volume or 30% for the casing volume is the additional nitrogen required once the corner has been turned. Therefore, if going down the casing, add 20% of the tubing volume to the casing volume to unload the well. If going down the tubing, add 30% of the casing volume.

9.4 Sand cleanouts A common problem in wells is the production of formation sand into the wellbore. Ultimately, the sand production covers the perforations and production from the well stops. The client generally uses a combination of coiled tubing and nitrified or foamed fluids to clean the sand out of the wellbore. The most common approach is to pump a light foam that can carry small amounts of sand from the bottom of the well to the surface. The foams that are used in well cleanouts are generally a low quality (low ratio of gas to liquid) and are regulated by the field specialist in response to indicators on location. The factors that the field specialist monitors while cleaning out a well are as follows: • maximum wellhead pressure when the foam reach bottom • weight indicator on the coil tubing • pressure on the return side of the well going to the disposal pit • velocity of fluids flowing to the pit • too high a nitrogen-to-fluid ratio (high quality) that limit the foams’ ability to carry sand to the surface

JET 17 - Introduction to Nitrogen Operations  |  39

• any condition allowing sand to fall down the hole and stick the coiled tubing or tubing packer • too low a nitrogen-to-fluid ratio (low quality) such that insufficient velocity is available to move sand up the hole • the downward speed of the coiled tubing. It is important to ensure that the cleanout fluid is not overburdened with too much sand, but it is also important to make the best sand cleanout as is practical.

From the list of factors to monitor, it is apparent that the initial approach to sand cleanout design can be set, but from that point on the field specialist must adjust the design to conditions at hand in close coordination with the coiled tubing crew. To start, the field specialist must ensure that the following are addressed. • All of the applicable well bore information is available. • The rig-up allows nitrogen and fluid to be pumped down the coiled tubing. • The flow and pressure of the returned well fluids are controllable. • The tank or pit to which the returned well fluids is directed is, first of all, present, and if present, is suitable for the type and quantities expected. • Pump-in and return pressures are monitored at all times.

The generic approach for sand removal can be described as follows:

step 01 Establish pumping communication

through the coiled tubing up and out of the line, returning fluids to the pit. If the hole cannot withstand a full column of fluid, a lightly nitrified fluid should be pumped until circulation is clearly established.

40  |  Job Calculations

step 02 If necessary, tag the top of the sand with the coiled tubing. Generally, the customer has already done this with a slick line unit. If so, the coiled tubing should be used to verify the location of the sand.

step 03 Start the nitrogen and fluid

combination with the nitrogen rate set between 200 and 300 scf/min. Continue pumping until the nitrogen/fluid combination arrives at the surface on the return side.

step 04 When a steady fluid flow is

established, lower the coiled tubing into the top of the sand bridge. The jetting action of the combination of nitrogen and fluid out of the coiled tubing will agitate the sand and mix it in the foam. The sand-laden foam will travel back to the surface and to the return tank or pit (Fig. 9-4). When the first of the sand and foam have reached the return pit, nitrogen-fluid ratio adjustments and coiled tubing injection rates can be modified to give the most efficient sand cleanout rate. This decision is made based on the well conditions and the desires of the client.When making adjustments, consider the following: • A decreased nitrogen pump rate may provide a low-quality foam at bottomhole conditions and reduce its capability to carry sand. • Too much nitrogen also affects the foam quality and may reduce the sand-carrying capability.

Allowing the coiled tubing to travel into the sand plug too quickly can create slugging of the sand, which can cause problems as the slugs travel to the surface.

1 3/4-in coiled tubing

Return tank

Fluid and nitrogen

Surface 2 7/8-in tubing

5.5-in casing

Tubing to 6,000 ft Sand bridge in the casing Zone of interest

Figure 9-4. Sand Cleanout

9.5 Foam fracturing Schlumberger performs fracturing to introduce high-permeability sand into a formation. The objective is to create a highly conductive path for the hydrocarbons from the recesses of the formation to the wellbore.

bubbles are trapped in a lattice work of fluid shells. See Fig. 9-5 for an example.

A number of different types of fluids are used to convey the sand down the well and into the formation. The fluid described here as an example is referred to as foam fracturing. The fluid is a combination of gaseous nitrogen and one of the variations of ClearFRAC*. The ratio of nitrogen to water is generally between 70 to 80%. The resulting combination is best described as a foam, where the nitrogen gas Figure 9-5. Foam JET 17 - Introduction to Nitrogen Operations  |  41

The foam’s ability to convey the sand relies on the sand being trapped in the fluid in the interstitial sites of the foam. The control of the foam quality is critical to its ability to penetrate the formation and carry the sand. Figure 9-6 illustrates sand in foam. Clear FRAC water phase

(add generic well head) Surface 2 7/8-in tubing 5.5-in casing

Sand grains Foam fracturing from surface to the perforations during the PAD

Nitrogen gas

Figure 9-6. Sand in Foam

Within Schlumberger, software is available to calculate all the information necessary to perform a foam fracture. In reality, FracCADE (InTouch Content ID# 38566337) will be used to design the fracturing treatment. However, the field specialist must understand the calculations in this example so that the design provided by FracCADE has meaning and if necessary, appropriate steps can be taken during a treatment. The following is an example of the calculations for a foam fracture treatment. See Fig. 9-7 for a schematic of the well.

42  |  Job Calculations

Tubing to 8,000 ft

8,600 ft

70% quality foam

Bottomhole fracturing pressure = 4,988 psi

Figure 9-7. Foam Fracturing Example

Well Data: • tubing depth:

8,000 ft

• hydrostatic gradient of mud: 0.447 • tubing: 2 7/8 in • BHSP: 4,844 psi

= 1,481 scf/bbl x 10.5 bbl/min = 15,550 scf/min This can also be calculated by using information from the handbook on page 310.05: = 3,464 scf/bbl of liquid x 4.5 bbl/m fluid = 15,588 scf/min

• casing: 5 1/2 in • static wellhead pressure: 1,000 psi • wellbore fluid: brine • fracture gradient: 0.58 psi/ft

The combination of the two pump rates at the surface will provide a 70% quality foam at the bottomhole injection pressure of 4,988 psi.

• fluid weight: 8.75 lbm/galUs

Nitrogen

10.5 bbl/min

15,550 scf/min 70%

• backside fluid: 9 lbm/galUS mud

Liquid

4.5 bbl/min

4.5 bbl/min 30%

• center of perforation: 8,600 ft • desired foam quality: 70% • max sand concentration: 4 lbm/galUS • pump rate: 15 bbl/min.

Note: FracCADE calculates all of the necessary information to design a foam fracture. This calculation is presented to allow the student to understand the dynamics in the well during the treatment.

To determine the wellhead treating pressure, the hydrostatic pressure of the column of foam is needed. Likewise, the friction pressure of the foam at 15 bbl/min down 2 7/8-in tubing will also be needed. From page 320.12 of the Handbook, the hydrostatic pressure 70% foam at 8,600 ft is 1,100 psi. From the Fracturing Engineering Manual - CleanFRAC Service (InTouch Content ID# 3012433) and the Fracturing Engineering Manual - Foam Fracturing (InTouch Content ID# 3015576) the friction pressure

With the information that the desired bottom hole foam quality is 70% (nitrogen to liquid), we will start the calculations at bottomhole conditions.

= 70% foam at 15 bbl/min down 8,000 ft of 2 7/8 in tubing = 5,600 psi

Bottomhole injection rate is 15 bbl/min total:

The wellhead treating pressure, once the foam starts into the formation is

Nitrogen

10.5 bbl/min 70%

Liquid

4.5 bbl/min 30%

From the handbook, page 305.07, the nitrogen volume factor is 1,481 scf/bbl at 5,000 psi and 100 degF.

= 4,988 bottom hole injection - 1,110 psi hydrostatic of the foam = 3,939 psi = 3,939 psi + foam friction pressure = 3,939 psi + 5,600 psi = 9,539 psi

JET 17 - Introduction to Nitrogen Operations  |  43

Note: The foam quality at the surface during the PAD stage of the treatment is 61%. It is important to understand that as sand is added, it occupies liquid space in each barrel. If the liquid or nitrogen rates are not adjusted to compensate for the sand added, the quality of the foam will continue to change. Schlumberger policy calls for adherence to the FracCADE design by adjusting the nitrogen and fluid rates to maintain the foam fracture rate at the perforations. Options in FracCADE, when chosen, calculate the necessary changes in rate.

9.6 Total nitrogen requirements Regardless of the type of nitrogen treatment being performed, having the right amount of liquid nitrogen on location is essential. It is not always obvious to a field specialist new to nitrogen treatments where all of the sources of nitrogen losses might be. Typically provisions should be made for at least the following conditions. • pumped nitrogen—The most obvious and generally the largest requirement for liquid nitrogen is that amount actually pumped during the treatment. The field specialist should always doublecheck the calculations for the amount of liquid to be used in performing the treatment ok. • cool-down nitrogen—The amount of nitrogen used to cool down the nitrogen pumping unit is another bulk requirement that must be carefully considered. In jobs where the unit is cooled down once, the impact of the total volumes required may not be critical. During services associated with coiled tubing, it is very possible that

44  |  Job Calculations

cooldown might be required multiple times. In this instance, the volume required for cooldowns might be a significant part of the total volume pumped. • nitrogen left in tanks—Nitrogen treatments will not be significantly different from standard treatments in that there will always be some liquid left in the bottom of all tanks and transports. It will be the same with nitrogen transports and pumpers. Allowances should be made for leaving approximately 10 in of liquid in all nitrogen storage containers on location. If high-rate nitrogen pumping is required, it may be necessary to make provisions for 10 to 15 in of liquid in the pumper tanks. High-rate pumping requires good net positive suction head on the input line to the boost centrifugal. If this suction head is not available, there is the possibility of cavitating and losing prime on the cold ends. • miscellaneous losses—During all nitrogen treatments, not all liquid nitrogen going through the pumps is going downhole. Depending on how much pressure is being applied to the suction of the cold ends and the downhole nitrogen pump rate, there is always some liquid nitrogen returned to the tank through the return to tank line off the cold ends. The liquid that is returned is constantly picking up heat from the atmosphere through the walls of the return piping. On short pumping jobs, the hot liquid returned to the tank has negligible effects. On long pumping jobs, such as pumping down coiled tubing all day or for multiple days, the loss of liquid nitrogen to heating needs to be factored in. • cool-down ○ transports ○ pumpers ○ transfer piping ○ multiple cool downs, if required.

• LN2 loss ○ bleedoff from tanks to condition LN2 ○ evaporation loss from tanks if the tanks are on location for a long period of time (off shore) • pressure test ○ nitrogen required to pressure test lines and equipment ○ extra nitrogen if multiple pressure tests required • treatment nitrogen ○ nitrogen required for the fluids injected into the formation ○ nitrogen required for performance of the job if fluids are not injected into the formation (sand cleanout) • nitrogen for displacement of well tubulars • nitrogen required in bottom of tanks to prevent the loss of prime on the cryogenic pumps.

These are the most common requirements that need to be considered when finalizing the total nitrogen required on location.

JET 17 - Introduction to Nitrogen Operations  |  45

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46  |  Job Calculations

10.0  References Well Services (WS) Safety Standard 5, InTouch Content ID# 3313681 WS Safety Standard 9, InTouch Content ID# 3283959 WS Safety Standard 11, InTouch Content ID# 3283975 FracCADE, InTouch Content ID# 3278776 JET 29: Nitrogen Pumping Equipment, InTouch Content ID# 4221766 Journey Assessment Form, InTouch Content ID# 4195154 Online I-Handbook, InTouch ID# 3874787 Nitrogen Engineering Handbook, InTouch Content ID# 3016873 JET 11: Nitrogen Transport and Storage Equipment, InTouch Content ID# 4221680

JET 17 - Introduction to Nitrogen Operations  |  47

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48  |  References

11.0  Check Your Understanding 1.

One pound of liquid nitrogen has

6.

a. 93.05 scf

a. 15%

b. 13.8 scf

b. 78%

c. 6.74 scf 2.

A gallon of liquid nitrogen, compared with a gallon of CO2, weighs _________ .

c. 75% 7.

a. more b. less

The volume occupied by a standard cubic foot of gaseous nitrogen _________ . a. increases as the pressure increases

b. the bottom c. anywhere 8.

b. stays the same regardless of the pressure

The vaporizer on a nitrogen unit has a pressure limit of _________ . a. 12,500 psi b. 10,000 psi c. 15,000 psi

5.

At one atmosphere, the number of standard cubic feet in one gallon of liquid nitrogen is _________ . a. 100 scf b. 93.11 scf c. 93.05 scf

A benefit of a PSA nitrogen unit is that _________ . a. it produces gaseous nitrogen without cryogenics

c. decreases as the pressure increases 4.

When you fill a tank during a pumping operation, enter the tank through _________ . a. the top

c. about the same 3.

Gaseous nitrogen is what percentage of the air people breathe?

b. no cryogenic pump is required c. both a and b 9.

The liquid nitrogen can be conditioned by _________ . a. applying more pressure to the top of the tank b. bleeding pressure off the tank and removing the heat c. filling the tank with fresh fluid

10. When hand calculating a displacement with gaseous nitrogen, what pressure must be known? a. the bottomhole pressure b. the wellhead pressure c. the average pressure in the tubing JET 17- Introduction to Nitrogen Operations  |  49

11. In the design of a 70-30 foam frac, what does the 70% represent?

16. The addition of sand to the foam frac fluid will

a. the water phase

a. not affect the fluid ratio

b. the nitrogen phase

b. take up liquid space in the foam

c. a combination of both

c. take up nitrogen space in the foam

12. The temperature of liquid nitrogen is a. –183 degF b. –232.6 degF c. –320 degF 13. The nitrogen volume factor is _________ . a. the amount of nitrogen required on location b. the amount of nitrogen needed for cooldown c. the number of standard cubic feet of gaseous nitrogen to occupy a barrel of volume at a given pressure 14. To determine the hydrostatic head produced by a column of gaseous nitrogen, do which of the following? a. Multiply the depth by the hydrostatic gradient for nitrogen at the discharge temperature. b. Divide the surface injection pressure by 2. c. Use the hydrostatic tables in the Schlumberger Nitrogen Handbook. 15. Provisions should be made for additional nitrogen because of a. cooldown b. nitrogen left in the tank c. both a and b

50  |  Check Your Understanding

17. As a general rule, select a pumper that is capable of a. approximately 70% to 75% of the required rate b. approximately 85% of the required rate c. The percentage is not important. 18. A standard cubic foot of nitrogen is measured at a. 1 atm b. 1 atm and 60 degF c. 1 1/2 atm and 60 degF 19. Frost will be seen on the outside of the treating line only when there is liquid nitrogen inside. a. true b. false c. depends on the pressure 20. When you arrive on location, you must double-check the well information because _________ . a. wellbore conditions and equipment can change b. you must make sure the supervisor and client are working with the same information c. both a and b