Mi-lost Circulation Manual.doc

TECHNICAL REPORT TITLE:

LOST CIRCULATION MANUAL

REPORT FOR: AUTHOR: APPROVED BY: REPORT NO: DATE:

Operations Norway Knut Taugbøl Charlie Svoboda TR2000-02 LOST CIRCULATION MANUAL March 22, 2000

SUMMARY

This report covers all aspects around prevention and curing of lost circulation of mud during drilling operations. The report is mainly focusing on drilling offshore Norway, but the recommendations will also be applicable elsewhere

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

1.

INTRODUCTION...................................................................................................................................3

2.

CAUSES OF LOST CIRCULATION....................................................................................................4 2.1 NATURAL LOSSES............................................................................................................................. 4 2.2 INDUCED LOSSES.............................................................................................................................. 4 2.3 TYPICAL LOSS ZONES OFFSHORE NORWAY..........................................................................................5

3. PREVENTION OF LOST CIRCULATION.............................................................................................7 3.1 WELL DESIGN................................................................................................................................... 7 3.2 DRILLING FLUID SELECTION.............................................................................................................. 8 3.3 DRILLING PROCEDURES..................................................................................................................... 8 4. QUANTIFICATION AND ANALYSIS OF LOSSES.............................................................................11 5. CURING LOST CIRCULATION PROBLEMS....................................................................................13 5.1 THE THEORY BEHIND BRIDGING AND SEALING OF FRACTURES AND PORE OPENINGS...........................13 5.2 THE USE OF REINFORCING PLUGS..................................................................................................... 15 6. CURING LOSSES WHEN DRILLING WITH WATER BASED MUD.............................................16 6.1 SEEPAGE LOSSES............................................................................................................................. 16 6.2 PARTIAL LOSSES............................................................................................................................. 16 6.3 TOTAL LOSS OF RETURNS................................................................................................................ 17 6.4 REINFORCING PLUG......................................................................................................................... 17 6.5 LOSSES IN PAY ZONE....................................................................................................................... 17 6.6 LCM COMPATABILITY WITH DRILLING EQUIPMENT...........................................................................17 7. CURING LOSSES WHEN DRILLING WITH OIL-/SYNTHETIC BASED MUD.........................19 7.1 SEEPAGE LOSSES............................................................................................................................. 19 7.2 PARTIAL LOSSES............................................................................................................................. 19 7.3 TOTAL LOSSES OF RETURNS............................................................................................................. 20 7.4 REINFORCING PLUG......................................................................................................................... 20 7.5 LOSSES IN PAY ZONE....................................................................................................................... 20 7.6 LCM COMPATABILITY WITH DRILLING EQUIPMENT...........................................................................21 8. REINFORCING PLUGS..........................................................................................................................22 8.1 FORM-A-PLUG................................................................................................................................ 22 8.2 REVERSE GUNK SQUEEZE................................................................................................................. 24 8.3 VERSAPAC...................................................................................................................................... 25 8.4 GUNK SQUEEZE............................................................................................................................... 26 9. FLOW CHART.........................................................................................................................................28

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

1. INTRODUCTION The term Lost Circulation describes the partial, or complete loss of whole drilling fluid from the hole into the formation. Lost circulation occurs when the openings in the formation are large enough to permit the entry of whole mud. The hydrostatic pressure in the drilling fluid column must also be greater than the formation pressure strength. In practice, if the formation openings are greater than about 200 microns, there is a potential for losses of whole mud to occur The openings can occur naturally in the formation or they can be induced by the drilling process. The size of these openings can be increased mechanically by a number of drilling practices that can cause losses. In this report methods for prevention and curing of lost circulation will be described. Lost circulation can cause dramatic well control situations, as well as it can be very costly, both due to the high cost of the lost drilling fluid and due to the high cost related to non productive time for the drilling rig. Lost circulation is a bigger problem when drilling with invert muds than with water based muds for several reasons. One is the higher cost of inverts, another is the fact that curing of lost circulation in invert traditionally is more problematic than curing losses when drilling with water based mud. New LCM pills specially designed for invert emulsion fluids based on material selection and bridging theory will be introduced. The report is mainly focusing on drilling offshore Norway, but the recommendations will also be applicable elsewhere.

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2. Causes of Lost Circulation Loss zones can broadly be categorized into two types: natural or drilling induced causes. The occurrence of each type varies around the world and is dependent on the drilling operation and the geology. Within the North Sea most losses are due to induced fractures. However there are areas where drilling into fractured chalk formations is a problem.

2.1 Natural Losses (a) Losses Through Pores/Matrixes For whole mud to be lost, formation openings must be about three times larger than the particle size found in the mud. Thus, a formation must typically have a permeability in excess of 10 to 25 darcy or a pore size of 200 microns for a well formulated mud to be lost. Therefore this type of loss is confined to coarse sands and gravels. (b) Formations Containing Permeable Natural Fractures These losses can occur at high rates, even at overbalances as low as 3-4 bar (50psi). Permeable natural fractures can be found in most types of formations and cause some of the most troublesome lost circulation incidents. Within the North Sea fracture apertures as wide as 3mm have been recorded for instance in the Tor formation. (c) Cavernous Formations Over geological periods of time, limestones, dolomites and salts are easily dissolvable by groundwater. This can lead to the creation of fractures and caverns ranging from pin size to tunnel size. These formations are not normally found in the North Sea.

2.2 Induced losses Losses to induced fractures occur when the wellbore pressure exceeds the fracture gradient, causing the formation to break down. These losses may occur due to excessive ECD or during pressure surges. Fracture gradients cannot be extrapolated linearly with depth, but alter with change in rock type. Thus certain formations are more susceptible to induced fractures than others. Drilling into pressure depleted reservoir zones have shown to be specially troublesome in many Norwegian fields. The pore pressure depletion from producing the reservoir will cause a reduction in the formation strength. In some formations the fracture gradient can be reduced to a value below the initial pore pressure for the reservoir. Thus, the well must be drilled with mud weight below the initial pore pressure which can cause dramatic well control incidents if reservoir zones with virgin pressure is encountered. Induced fractures are also often a result of poor drilling practice. Inadequate hole cleaning will cause an increase in mud density and ECD which can cause the formation to break down. The hole being packed off with cuttings during drilling or reaming can give extremely high pressures to act on the formation. Pack off is a frequently seen cause for lost circulation. Surge pressures from running in with drill pipe or casing too fast can also

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break down the formation as well as pump surge from increasing the pump speed too rapidly.

2.3 Typical loss zones offshore Norway Lost circulation can be a problem anywhere caused by poor drilling practice. However, there are areas and formations where lost circulation is more likely to happen and where it can be very difficult to prevent it from happening. The nature of losses can also differ from one formation to another and thereby also the way it should be resolved. An investigation among the different operators in Norway has shown that there are differences in the way losses should be approached in different areas. Drilling into natural fractures will often require different treatment resolving circulation than from induced fractures. In Norway induced fractures are the most common cause, but there are areas in the southern part of the North Sea were natural fractures occur. In this section some typical loss zones offshore Norway will be discussed. In the Tampen area where some of the biggest oilfields offshore Norway like Snorre, Gullfaks and Statfjord are located, the major problem is drilling into pressure depleted reservoirs. Pressure depletion will cause a reduction in fracture pressure. In a layered reservoir like the Snorre the pressure depletion is difficult to predict for the different zones and there is often big variations in pressure which makes it difficult to drill the whole reservoir section without either losing circulation or taking an influx. The reservoir consists of sand and shale layers and the losses often occur at the shift in lithology. The fractures are caused by to high ECD and the way to prevent it from happening in this field has been by focusing on well design in order to limit ECD and to split the reservoir into different hole sections which then can be drilled with different mud weights. However, losses still occur due to the big uncertainties in the pore- and fracture pressure predictions. The best way to cure losses has shown to be by pumping the Form-a-Plug. Conventional LCM mixtures containing blends of mica, nutplug, fyber, kwickseal etc. has shown not to be effective in the synthetic mud used at this field. However, very recently a pill containing Gseal and calcium carbonate together with , Mica, nutplug and fyber was able to heal losses. Thus, by changing the pill contents a better effect of particular LCM can be achieved. In the Statfjord field there are some coal layers in the Brent group that often causes losses, in addition to the problems seen in the Statfjord group caused by pressure depletion as in the Snorre field. Also on Statfjord, pumping of conventional LCM pills have shown little success. In a recently drilled well at this field addition of bridging particles to the mud, Gseal and calcium carbonate, proved to be effective in preventing losses to occur and actually increasing the strength of the formation by plugging of micro fractures. Thus, adding bridging particles like G-seal to the mud before drilling into zones were fractures are likely to be induced, can be a good preventive method. Fluid invasion over time is thought to be the reason for sloughing shale, causing pack off and lost circulation in the shales at the Valhall field. Analysis of the shale has shown that it contains natural micro fractures which are believed to be the cause of the problems. A blend of G-seal and calcium carbonate added to the mud during drilling of these sections has given successful results, blocking the micro fractures and thereby preventing this from happening. 425446178.doc

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In the southern part of the North Sea, such as the Ekofisk, Eldfisk and Valhall fields, where we have the chalk reservoirs, there are problems with natural fractures. These fractured areas are often located in the cap rock just above the chalk reservoir. These fractures cause a sudden total loss of circulation when drilled into it and can normally not be cured by LCM pills. In this type of losses a plug which sets up immediately is the preferred choice. The best solution in this area has proven to be the reverse gunk squeeze. If high possibility for encountering such a zone, the gunk is often made up in forehand ready to be pumped as soon as losses start. Another approach used at Valhall has been to drill through the fractured formation with a drill-in liner without returns to surface. Drilling through faults is another area were problems are suspected to occur. Faults can often be difficult to detect on the seismic and is therefore often difficult to omit. A study conducted by Saga for the Snorre field however, showed that there were no increased probability of loosing circulation when drilling through faults unless the formation were pressure depleted after producing from the reservoir. In such a case the faults seemed to increase the probability of losses. Many fields are not mentioned in this discussion. However, most of the field in the North Sea will have some similarities to one of the above mentioned areas. When finding the best cure for lost circulation in a specific field, the cause of losses should be analyzed and the way to treat the problem chosen thereafter. More discussion on treatments is given in the coming sections.

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3. PREVENTION OF LOST CIRCULATION For all drilling operations, lost circulation contingencies and prevention procedures should be considered. Remedy of lost circulation will be most effective when it is planned before the well is spudded.

3.1 Well design Clearly the casing plan exerts the single greatest influence in avoiding lost circulation. In many cases induced fractures occur because the intermediate casing string was set too high and the mud weight needed to control deeper, high-pressure zones fractured an exposed low-pressure formation. As a general rule, there should be a minimum of open hole between the casing shoe and the expected loss zone. Good casing design using all available tools and information to identify potential problem zones is critical as is knowledge of fracture gradients and the existence of depleted zones. The fracture gradient of depleted zones is often unknown. Plans may also be needed for contingency casing strings. If the loss zone is hydrocarbon bearing, consideration needs to be given to possible bullheading operations. Casing design can also be used to reduce the ECD increment. For instance drilling a 12 ¼” hole with a certain mud weight gives lower ECD than drilling an 8 ½” hole with the same mud weight, since the annular clearance is greater between the drill pipe and the bore hole wall in a bigger hole, giving lower frictional pressure loss. Installing of liner instead of casing can reduce the ECD increment when drilling the subsequent hole section. For instance installing a 9 5/8” liner from 13 3/8” casing shoe instead of running it all the way up to seabed have shown to give a reduction in ECD increment of approximately 50 % when drilling on the Snorre field. The 9 5/8” liner can be tied back to seafloor after the 8 ½” section has been sealed off by a cemented liner if 9 5/8” casing is needed as production casing. Also the drill string configuration will have an effect on the ECD increment. For instance replacing 5 ½” drill pipe with 5” when drilling a 8 ½” hole can have great effect in reducing the ECD. Drilling with rotary stearable systems (such as Autotrack) can also be an important tool in reducing the possibilities for lost circulation since it improves hole cleaning and thereby the changes for pack-off etc. By using the software Virtual Hydraulics, which calulate ECD, hole cleaning performance etc. during drilling operations, in the planing of wells, the casing design and drill string configuration can be optimized to give minimum ECD during drilling of the problem zones. Virtual Hyrdaulics can also be used to optimize the drilling fluid rheology and pump rate to ensure minimum ECD’s at acceptable hole cleaning.

3.2 Drilling fluid selection

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Drilling fluid selection is very important. If lost circulation is likely, then preference should normally be given to water based fluids as opposed to oil- or synthetic-based fluids. This is not only for cost reasons; the logistics of supply are usually easier for water-based muds and they can be mixed more quickly. Lost circulation has also historically been easier to cure when drilling with water based fluid. For all drilling fluids it is important to optimize the rheology to ensure maximum hole cleaning at minimum ECD. When drilling through weak formation with high potential for losses, addition of LCM to the drilling fluid can be recommended. The size distribution of these LCMs should be tailord to match the expected fractures or pore sizes. Recent experience from the Statfjord field showed that addition of G-Seal and calcium carbonate to the mud system during drilling seemed to strengthen the formation and nearly eliminating losses. Experiences from former wells have been large problems with losses during drilling through the same formations. The theory is that the G-Seal is sealing off the fractures created due to elevated ECD before the fracture have time to grow and propagate. LCM can also be used to help prevent losses before running and cementing casing. A tight fluid loss will help to maintain maximum annular clearances and minimize ECD, swab and surge pressures.

3.3 Drilling procedures Drilling procedures have an important part to play in avoiding lost circulation. The drilling program should address the following points: Mud weight and ECD The lowest safe mud weight should be used to minimize hydrostatic pressures. Efficient solids control and clear dilution guidelines will help to minimize mud weight and ECD. Rheology should be optimized to minimize the equivalent circulating density (ECD). Increased ECD will increase the pressure on the formation and aggravate the lost circulation problem. The rheology should not be decreased to the point where barite sag might occur. If the mud density cannot be reduced without endangering the well or destabilizing the hole, it may be possible to reduce the ECD by altering the rheological properties, or by reducing the circulation rate while the potential loss zone is being drilled. "Seepage" type losses can frequently be cured by the simple action of reducing the pump rate and allowing the formation to heal and become sealed off by the development of a filter cake. Sometimes the pump rate can gradually be increased after the losses have stopped. Hole cleaning Drilling rates and hole cleaning capabilities should be consistent with the need to avoid overloading the annulus with cuttings. An increase in annular mud weight due to drilled cuttings can result in formation breakdown, particularly in surface holes. The effective increase in annular mud weight must be taken into account. In extreme cases the cuttings 425446178.doc

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will build up and bridge off the annulus, this results in an induced fracture in the formation and the total loss of returns. Calculation of effective mud weight (contribution from drilled solids only, not included ECD): First calculate "Circulation rate" of cuttings (m3/hr): Qc = ROP • (D • 0.0127)2 •  Where : ROP = Rate of penetration (m/hr) D = Hole diameter (inches)  = 3.14 Then calculate effective mud density (s.g.): SGE = SGM •(Q – Qc) + SGc•Qc Q Where :

SGM SGC Q

= = =

Mud density (s.g.) Cuttings density (s.g.) Circulation rate (pump output) (note: m3/hr)

NOTE: This formula assumes a steady penetration and that all the drilled cuttings are being removed. If hole cleaning is inadequate, the mud density increase can be substantially higher. Specially under oriented drilling (sliding) when the pipe is not rotating the hole cleaning efficiency can be drastically reduced. Good hole cleaning is the number one criteria for avoiding lost circulation problems. If hole cleaning is inadequate the well bore will be overloaded with cuttings which will increase the ECD and subsequently increase the probability of pack-off, especially during tripping.

Drilling practices Drilling practices should be designed to reduce transient pressure fluctuations, in particular surge pressures generated while running into the hole, or break-circulation pressures. Such pressures cause a temporary increase in bottom-hole pressure, increasing the risk of lost circulation. Swab and surge computer simulators (Virtual Hydraulics) should be used to help predict problem situations, especially while tripping or running casing. Staging into the hole and rotation of the drill string to help break gel strength should be considered before restarting circulation after a trip. When restarting circulation, the pump rate should always be increased slowly. The BHA should be designed for maximum annular clearance and minimum sticking potential consistent with other requirements. Use wiper trips if appropriate to check hole condition. Larger bit nozzles should be considered if lost circulation is expected. The internal diameters of downhole tools and motors should be checked. 425446178.doc

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Consider conditioning trips midway through long logging programs. Increased gelation may occur if the fluid is left static in the hole for extended periods. Take necessary precautions to avoid barite sag. Monitor the mud weight of returns after a trip to ensure that these are effective. Ensure that no unnecessary mud transfers or additions/dilutions are made when drilling into a potential loss zone. Ensure that the driller and mud loggers are informed prior to making any transfers or treatments that will affect pit volume. Review pit volume measurements and crew alertness of same.

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4. QUANTIFICATION AND ANALYSIS OF LOSSES Knowing why a loss is occurring helps in selecting the best LCM solution, the probable position of the loss zone and whether any changes to mud weight, mud properties or drilling practices are necessary. By discussing the loss with the mud logger, rig geologist and mud engineer, the cause of the loss can be determined. The mudloggers’ data chart can provide an accurate record of how and when the loss initiated. The rig geologist often has offset log data of the formation from adjacent wells, which can help identify the cause and potential location of the loss. The mud engineer can help in deciding how to alter any of the mud properties. It is important to confirm that losses are to the formation and not at surface. Before trying to cure the loss, carry out the following procedure: (1) Establish loss rate. (2) Check all solids control equipment to ensure no increase in background loss rate or that any equipment has just been switched on. (3) Check with mudlogger/mud engineer/pit room/driller that no mud has been dumped or transferred. Check riser slip joint for leaks. Characteristics of different types of loss zones (a) Losses Through Pores Occur in unconsolidated or high matrix permeability formations. Occur when solids content of mud is low. Loss starts gradually, building up to maximum rate with additional penetration. Loss rate not appreciably higher whilst tripping out. Loss rate not particularly sensitive to pump rate. Mud weight substantially below fracture gradient. (b) Losses Through Induced Fractures Mud weight approximately equal to fracture gradient. Formation may be impermeable and is not of high matrix permeability. Likely to occur on change in lithology from shale to sandstone. Loss starts suddenly, initial rate is maximum rate. Loss rate is considerably higher when tripping etc. Loss rate is very sensitive to pump rate. Loss rate not associated with drill break. Fluid is often produced back when pumps are switched off. (c) Losses Through Natural Fractures and Caverns. Mud weight can be substantially below expected fracture gradient. Formation is not of high matrix permeability. Loss starts suddenly. Loss rate may decline exponentially. Loss rate whilst tripping is of similar magnitude to when circulating. Loss rate moderately sensitive to pump rate. 425446178.doc

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With additional penetration, loss rate is highly variable at constant overbalance. Loss may be associated with drill break. Identifying the loss zone The on site geologist should ensure that the drilling team are kept aware of the location of potential loss zones. If a loss occurs while drilling, it is likely that the losses are on bottom. However, it should not be automatically assumed that the thief zone is on bottom. A poor cement bond around the casing shoe may have broken down, or a fracture could have been induced into a weak formation anywhere in the open hole. If losses are due to natural fractures or caverns the losses are likely to be at bottom. If losses occurs whilst tripping, wiping or increasing mud weight, the losses can be off bottom. Losses off bottom are often caused by induced fractures. There are several methods of locating the loss zone. The geologist should be able to identify the most likely formations. A temperature survey or other logging run can also be used to locate the zone.

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5. CURING LOST CIRCULATION PROBLEMS In order to cure or prevent lost circulation most effectively, proper planning must be done. If drilling through a certain formation is likely to give losses, treating the mud with LCM before entering the potential loss zone can be recommended. If total losses are expected when drilling into a naturally fracture zone (like the Tor formation on Ekofisk) or a pressure depleted zone (like the reservoir on Snorre) a reinforcing pill should be mixed ready prior to entering this zone and pumped immediately when the losses occur. However, in many cases, losses are not expected and the situation must be analyzed before treatment starts. The result of the quantification and analysis of losses will decide the type of curing method to be used. In addition the type of drilling fluid will decide which type of material to use in the plugs to be pumped. The first LCM to be pumped is often a pill containing different types of particles to bridge off the fracture. These materials are mixed directly into the mud, pumped down and will work immediately if successful. For severe losses the LCM pill will not always be able to seal off the fracture and the solution will be a reinforcing plug such as Form A-Plug or Gunk, or Cement. When drilling into known formations with natural fractures, the solution can be to go directly to a reinforcing plug if experience have shown that LCMs do not work in sealing the fracture.

5.1 The theory behind bridging and sealing of fractures and pore openings Selection of bridging material For a bridge to be effective it is very important that the bridging particles enters into the fracture. Bridging on the fracture opening will normally only be temporary and disrupted when the bit and drill string is passing the bridge. For the bridge to enter and seal inside the fracture it is important that the LCM pill contain the correct particle size distribution and that the particles are designed correctly for the drilling fluid used. Water based and oil/synthetic based drilling fluids will require different types of particles. In a water base mud the surface of the particles should be preferably hydrophilic whereas in an oil/synthetic based mud the particle surface should be lipophilic. Some type of material such as G-Seal have a neutral surface and can work in both types of fluid. The reason for the importance of the surface conditions of the particles is that if the wettability is not correct, the particles will not be transported effectively into the fracture opening and seal the fracture inside the fracture. In stead the particles will typically screen out of the solution and form a bridge on the outer surface of the fracture. The use of a cellulosic fiber product like KwickSeal in an oil/synthetic based mud is a typical example of this, where a temporary seal can be formed from screening out the material on the fracture surface, but where the seal very often is broken when drilling operation is resumed Fracture sealing in water versus oil/synthetic based systems. Although there should be no difference between water-and oil base muds in the pressure needed to initiate a hydraulic fracturing of the formation, there is a significant difference once the fractures are formed. Water based fluids typically have a higher “spurt” fluid loss

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causing an almost instantaneous filter cake to form that aids in formation sealing in permeable formations. When drilling with water based mud the cuttings are often partly dispersed giving particles of all different sizes in the system. These particles also act in building the filter cake. It is also often seen that the fracture heals by itself if the mud in the hole is left static for a period of time probably because of some swelling of shale surfaces. Oil base fluids do not display the same characteristics. Once the fracture is initiated the pressure necessary to propagate the fracture is much smaller by comparison. This is compounded by lack of a significant pressure drop across the filter cake of most oil base muds. This allows changes in wellbore pressures to be transmitted to the formation more readily and further propagate the fracture. When using oil base fluid, almost no cuttings dispersion take place and the filter cake is built mainly by the emulsion droplets in the mud, giving a very thin impermeable filter cake. To get an effective heal of the fractures then, particles of certain size must be added to act as the bridging agent within the fracture, giving the spurt loss to build an effective filter cake which can prevent the fracture from propagating. The size of the particles must be tailord in size so they give an effective seal, thus it need to contain both large particles to bridge off the fracture and smaller particles to seal off in between the bigger ones. Particle size and shape distribution In addition to have the right type of particles the particle size distribution is very important. To get a good strong sealing of the fracture big particles are needed for the initial sealing and smaller particles are needed for sealing in between the big ones. When drilling with water based mud the main part is to have big particles for the initial bridge since the drill solids often will have sizes to bridge off in between the big ones. The exception might be silicate mud which is so inhibitive that the cuttings stay almost intact not forming smaller drill solids. When using oil base fluid the particle size distribution must be more properly designed. In all pills the base should be almost the same but with addition of larger particles for larger fractures. Dynamic flow experiments, where fractures of 3000  size were sealed, also showed that there should be a blend of granular, flakes and fybers to make an effective seal. For smaller fractures and pore throats only granular particles will do the job. A typical formulation for curing seepage losses will then contain a blend of the granulars Calcium Carbonate and G-Seal. For larger losses, Ven Fyber and Mica may need to be added as well.

5.2 The use of reinforcing plugs The use of a reinforcing plug such as Form-a-Plug, gunk or reverse gunk is normally considered as the next option for curing losses when the LCM pills have not been successful. The plug will set up down hole and thereby regain strength to the formation. The plug can in some cases be the only necessary treatment to allow continued drilling of the section. However, often it is also necessary to reduce mud weight and/or pump rate to reduce ECD when drilling further since the plug will not give any additional strength to the

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formation. In many cases it can be the necessary base to be able to run and cement casing before drilling ahead a new section with reduced mud weight. A reinforcing plug needs to be spotted in the right place to be effective, i.e. across the loss zone. Location of the loss zone can sometimes be difficult. Pumping of LCM pills ahead can in such cases give some indications of where the losses are if some partial improvement is seen when pumping these pills. Pumping of reinforcing pills always imply some danger of plugging pipes or flow lines if the pumping is not done by following the specific procedures. This often results in some hesitation on the rig to start mixing and pumping these systems. The result is then often that they continue pumping the standard LCM even if it has not given any effect and the result is that valuable time is lost. Especially when drilling with oil-/synthetic based muds there are seldom any improvements over time. Thus, if the LCM pill did not work in the first place the next step should be a reinforcing plug.

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6. CURING LOSSES WHEN DRILLING WITH WATER BASED MUD 6.1 Seepage losses Losses up to approximately 3 m3/hr (20 bbls/hr) are normally considered as seepage losses. Seepage type losses are frequently cured by the simple action of reducing or stopping the pump rate and allowing the formation to heal and become sealed off by the development of a filter cake. The pump rate can be gradually increased after the losses have stopped. If losses do not heal by themselves and the amount of loss can not be tolerated by economical or other reasons LCM pill for curing the loss must be pumped. Recommended pill to be used: Mix II Fine Calcium Carbonate (C) G-seal

20 kg/m3 20 kg/m3 40 kg/m3

If seepage losses are expected during drilling through a certain formation, treating the whole mud system with LCM before entering the loss zone can be recommended. Such a treatment will depend on pore/fracture size distribution in the actual formation but a general recommendation will be to treat the system with: Mix II Fine Calcium Carbonate (M)

15 kg/m3 15 kg/m3

6.2 Partial losses Losses from 3 m3/hr up to total loss of all returning fluid are often referred to as partial losses. The situation will usually require treatment. However, as a first action the bit should be pulled off bottom, the pumped turned off and the formation given the chance to heal by itself. If losses stop, drilling can be resumed, if possible with reduced mud weight and/or pump rate. If losses do not heal by it self the following LCM pill is recommended: Mica M: Mix II Med Calcium Carbonate (C) Nut plug M G-seal

25 kg/m3 25 kg/m3 25 kg/m3 25 kg/m3 50 kg/m3

If this pill does not heal the losses, the pill described below for curing total losses should be pumped.

6.3 Total loss of returns The priority will usually be well control so the annulus must be filled from the top with either drilling fluid, water or other lightweight liquid. Unless the fracture is induced, losses can normally not be stopped by pumping conventional LCM pills and the alternative then is

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a reinforcing plug or cement. However, a pill of LCM is often the first choice since it gives quick response if it works and it is easy to do. Recommended pill to pump should contain as much LCM as possible. A standard formulation would be: Mica M: Mix II Med Mix II Coarse Nut plug F G-seal

40 kg/m3 30 kg/m3 30 kg/m3 50 kg/m3 50 kg/m3

If this pill does not heal the fracture a reinforcing plug or cement should be set across the loss zone.

6.4 Reinforcing plug If the above mentioned pills are not able to stop the losses, a reinforcing pill should be pumped. If total losses are expected, always have a pill ready mixed prior to entering the zone. When using a pill like this, it is essential to know where the loss zone is located since the pill need to be spotted across the loss zone to be effective. For water based mud Form-a-Plug is the recommended plug to set. Form-a-plug need to be static when it is setting up, thus if there is large static losses this pill will not likely cure the losses. In such a case a gunk, which set up immediately when in contact with the mud down hole should be pumped. Formulations and procedures for these pills are given in section 8.

6.5 Losses in pay zone Formation damage from the LCM is normally considered as a problem only in wells that are open hole completed, since for wells that are completed with cemented and perforated liners the perforations will go through the possible damaged zone. However, also in wells with open hole completions curing the losses are most often the primary goal and the LCM is chosen from that perspective using the most effective LCM. If there are a desire of removing the LCM after completion the particles of the pills should mainly be calcium carbonate which can be acidized. As a choice for reinforcing plug the Form-a-Plug should be used since it also can be removed by acid.

6.6 LCM compatability with drilling equipment Before an LCM pill is pumped it should be clarified by the operator that the particles chosen can pass the BHA without plugging the tools. For most tools used in the North Sea in normal drilling operations, the above-mentioned pills will pass the MWD, motors and bits used. Coarse particles like Nutplug M/C will cause the biggest problems. These coarse materials can be replaced by finer material if plugging problems are suspected. Particular LCM can also cause problems for the slurryfication units used for cuttings reinjection. It is therefore advisable to take out the LCM when it hit the shakers and send it to share as special waste material in such occasions.

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7. CURING LOSSES WHEN DRILLING WITH OIL-/SYNTHETIC BASED MUD 7.1 Seepage losses Losses up to approximately 3 m3/hr (20 bbls/hr) are normally considered as seepage losses. Seepage type losses are frequently cured by the simple action of reducing or stopping the pump and/or reducing the mud weight. If losses do not heal by themselves and the amount of loss can not be tolerated by economical or other reasons LCM pill for curing the loss must be pumped. Recommended pill to be used: Ven Fyber: G-Seal: Calcium carbonate (C)

20 kg/m3 40 kg/m3 20 kg/m3

If seepage losses are expected during drilling through a certain formation, treating the whole mud system with LCM before entering the loss zone can be recommended. Such a treatment will depend on pore/fracture size distribution in the actual formation but a general recommendation will be to treat the system with: G-Seal Calcium Carbonate (M) Calcium Carbonate (C)

10 kg/m3 10 kg/m3 15 kg/m3

7.2 Partial losses Losses from 3 m3/hr up to total loss of all returning fluid are often referred to as partial losses. The situation will usually require treatment. However, as a first action the bit should be pulled off bottom, the pumped turned off and the formation given the change to heal by itself. If losses stop, drilling can be resumed, if possible with reduced mud weight and/or pump rate. If losses do not heal by it self the following LCM pill is recommended: Mica M: Ven Fyber G-Seal Calcium Carbonate (C)

30 kg/m3 40 kg/m3 50 kg/m3 30 kg/m3

If this pill does not heal the losses a pill with some coarser particles is recommended and the pill given below for curing total losses should be pumped. If that one also fails, one of the reinforcing plugs or cement is recommended.

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

7.3 Total losses of returns The priority will usually be well control so the annulus must be filled from the top with either drilling fluid, water or other lightweight liquid. Unless the fracture is induced losses can normally not be stopped by pumping conventional LCM pills and the alternative then is a reinforcing plug or cement. However, a pill of LCM is often the first choice since it gives quick response if it works and it is easy to do. Recommended pill to pump should contain as much LCM as possible. A standard formulation would be: Mica M: Ven Fyber Mix II Coarse G-Seal Nut plug M

40 kg/m3 30 kg/m3 30 kg/m3 50 kg/m3 50 kg/m3

If this pill does not heal the fracture a reinforcing plug or cement should be set across the loss zone.

7.4 Reinforcing plug If the above mentioned pills are not able to stop the losses, a reinforcing pill should be pumped. If total losses are expected, always have a pill ready mixed prior to entering the zone. When using a pill like this, it is essential to know where the loss zone is located since the pill need to be spotted across the loss zone to be effective. For oil based mud Versapac oil based LCM plug is the recommended plug to set. Versapac will be activated by a combination of shear and temperature and can be used for temperatures of 60-100 oC. It can not be used in synthetic mud for environmental reasons. For synthetic mud and as a secondary option for oil based mud, Form-a-plug should be used. Form-a-plug need to be static when it is setting up, thus if there is large static losses this pill will not likely cure the losses. In such a case a reverse gunk, which sets up immediately when in contact with the mud down hole should be pumped. Formulations and procedures for these pills are given in section 8.

7.5 Losses in pay zone Formation damage from the LCM is normally considered as a problem only in wells that are open hole completed, since for wells that are completed with cemented and perforated liners the perforations will go through the possible damaged zone. However, also in wells with open hole completions curing the losses are most often the primary goal and the LCM is chosen from that perspective using the most effective LCM. If there are a desire of removing the LCM after completion the particles of the pills should mainly be calcium carbonate which can be acidized. As a choice for reinforcing plug the Form-a-Plug should be used since it also can be removed by acid.

7.6 LCM compatability with drilling equipment Before an LCM pill is pumped it should be clarified by the operator that the particles chosen can pass the BHA without plugging the tools. For most tools used in the North Sea in normal drilling operations, the above-mentioned pills will pass the MWD, motors and 425446178.doc

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

bits used. Coarse particles like Nutplug M/C will cause the biggest problems. These coarse materials can be replaced by finer material if plugging problems are suspected. Particular LCM can also cause problems for the slurryfication units used for cuttings reinjection. It is therefore advisable to take out the LCM when it hit the shakers and send it to share as special waste material in such occasions.

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

8. REINFORCING PLUGS This section gives the different options of reinforcing plugs with operational procedures to be used in severe loss situations.

8.1 Form-A-Plug Form-a-plug can be used to cure severe losses of both water- and oil/synthetic based mud. The formulations below have shown to set up after approximately 2.5 hours when tested in the lab. This should be sufficient for most operations. When pumped down there will take some time before the plug heats up to the down hole temperature, thus the plug should be left for 6 hrs to set up. The setting time at different temperatures will be depended on the concentration of Form-a-plug and Form-a-plug accelerator in the water and by the portion between the two chemicals. The desired weight of the plug can be adjusted by adding barite. Barite will not affect the setting time. The amount of barite needed can be calculated by the weight of the unweighted formulation. The following amount of chemicals should be added to make 1 m3 of unweighted Form-aplug: Temperature (oC)

4

38

55

65

70

80

85

95

110

Drill water (m3) Form a plug (kg) Form a plug acc. (kg) CaCl2 (kg) Weight of formulation (SG)

0.90 148 50

0.88 163 82

0.87 165 83

0.89 146 73

0.88 148 74

0.89 137 69

0.90 128 64

0.88 172 43

0.88 183 41

25 1.12

1.13

1.13

1.12

1.12

1.11

1.10

1.10

1.10

A retarder can also be added if the setting time needs to be prolonged. Mixing instructions The cross-linking mechanism is a chemical reaction. Therefore it is important to carefully control the ingredients and mixing conditions in order to ensure that the reaction proceeds as expected. 1.

Ensure that the pit and mixing lines are flushed clean and free from salt, brine and/or any materials which could affect the salinity or pH of the pill.

2.

Add the appropriate volume of drill water to the pit; pH should be 6-8.

3.

Add Retarder as fast as practicable via the mix hopper (If retarder is required).

4.

Add the appropriate quantity of Form-a-Plug via the mix hopper at 3-4 minutes per. 25 kg sx. (Depending on the performance of the mixing equipment).

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

5.

Shear to yield the polymers.

6.

Add barite to desired density.

7.

Keep the fluid agitated but not mixing (generates heat).

8.

Immediately before pumping add the appropriate quantity of accelerator. This should be added through the hopper as quickly as possible, but only at a rate that will allow even distribution throughout the mix.

9.

Displace the FORM-A-PLUG to the well.

10.

Pull above Form-a-Plug pill. Close annular preventor and squeeze down annulus and drill pipe to ensure pill has entered the formation.

Viscous spacers (minimum 1,5 m3) are recommended ahead and behind the FORM-APLUG if the drilling fluid has a high salinity. The spacer can either be water based, or oil based (synthetic/mineral oil), but should not contain any salt. It should be weighted to the same density as the drilling fluid and the FORM-A-PLUG. Operational comments and guidelines Once the accelerator has been added, the fluid should be regarded as cement. It will cross link and set. The final composition should be pumped immediately down hole. Form-A-Plug can be pumped through BHA. Any fluid left static in the string will set-up inside. The fluid should be adequately displaced and spotted as required. Similarly, if the mud pumps were to fail prematurely, an alternative means should be used to pump the fluid into place i.e. have the cement unit on stand-by as contingency. Once displaced, the string should be pulled clear above the estimated top of plug. Time spent waiting on plug , circulating or squeezing, should be performed with the string out of the plug. Avoid ballooning effect of the plug. Any movement of the pill will increase the setting time. It is recommended that the plug should be left in place for a minimum 6 hours before drilling out/dressing the plug. If possible a sample of the pill should be left static in a heating oven at the same temperature as the lost circulation point. This will confirm the setting time.

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

Surplus FORM-A-PLUG left in the pit should be dumped while it is still fluid. Pit, lines and pumps should be thoroughly flushed with sea/drill water. Treat the fluid with care Addition of lime or calcium chloride to the system cause rapid setting. Low pH will delay (or in extreme cases prevent) setting up the FORM-A-PLUG. Keep acid out of the fluid. High pH will cause rapid setting. The set-up rate increases with temperature, therefore avoid situations such as prolonged shearing of the fluid, which could cause heating.

8.2 Reverse gunk squeeze Reverse gunk squeeze can be used in severe loss situation when drilling with oil or synthetic based mud. When drilling into a possible total loss formation, a Gunk pill should be prepared in advance, to enable immediate response and minimize losses. The Gunk pill should be prepared in a separate batch mixing tank, well away from the mud pits. Once mixed, the Gunk should be continuously circulated. Formulation for 6 m3 of gunk 4 m3 Drill water 25 kg Lime 50 kg Lignosulphonate 3200 kg Organophilic clay Barite to reach the required mud weight Spotting technique 1.

RIH and place the bottom of the drill pipe +/- 1 joint above the thief zone. Pump 6 m3 gunk down with 1 m3 viscosified water ahead and behind as spacer.

2.

Displace with mud until water ahead reaches bottom of drill pipe.

3.

Close annular preventer.

4.

Squeeze gunk by using both pumps on the cement unit. No. 1; Pump down DP at 300 l/min. No. 2; Pump down annulus at 80 l/min.

5.

Squeeze total 6 m3 (until all water is cleared from the drill pipe).

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

6.

Pull drill pipe well above gunk.

7.

Wait on gunk (2-4 hrs).

Additional comments  Barite can normally be excluded from the formulation since in events of total losses the hydrostatic column imparted from the barite addition is not required. Additions of barite to the formulation take time and can be difficult to achieve.  Base oil may be applied to the annulus in the event of total losses, where a temporary reduction in hydrostatic can be tolerated until the gunk is in place.  Occasionally a second gunk may be required, should the first not work. Be aware that a deposit of gunk may form inside the drill pipe when pumping the gunk. Subsequent attempts, or gunking frequently, results in overdisplacement into the annulus.  Be sure of spotting techniques to squeeze the gunk, and minimise any gunk coming up into the annulus. This can result in higher ECD when re-attempting circulation, and or uncomfortable quantity of clay to segregate at the surface when returns are achieved at the shakers. Gunk at the shakers needs to be by-passed as excessive blinding will occur otherwise.  No attempts should be made to reverse circulate the gunk as it will plug the string.  The Gunk should ideally be prepared by use of a batch mixer or cementing tanks, isolated from the surface mud system. Plugging of mud lines is a serious potential hazard.  The gunk can be pumped through most BHAs.

8.3 Versapac Versapac is an oil based LCM plug to be used when severe losses occur when drilling with oil based mud. The formulation of the plug is very simple. It is made from the mud with addition of three products and it can be pumped down without the use of spacers. A standard VersaPac formulation consist of: Oil based mud 1 m3 Versapac 60 kg MI-X II 15 kg Ecotrol 15 kg Versapac is activated by a combination of shear and temperature. Versapac is a thermally activated gelling agent which will generate viscosity and develop gel structure as soon as

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

the temperature exceeds 60 oC. the melting point of Versapac is 120 oC, thus the temperature range for this plug is between 60-110 oC. Operational guidlines A typical volume to pump will be 3-4 m3. The chemicals should be added to the mud in a separate pit. This pill should then be pumped downhole, displaced by mud. It can be pumped through the BHA. Any fluid left in the string will set-up inside. The fluid should be adequately displaced and spotted as required. Similarly, if the mud pumps were to fail prematurely, an alternative means should be used to pump the fluid into place i.e. have the cement unit on stand-by as contingency. Once displaced, the string should be pulled clear above the estimated top of plug. Time spent waiting on plug , circulating or squeezing, should be performed with the string out of the plug. It is recommended that the plug should be left in place for a minimum 6 hours before drilling out/dressing the plug. If possible a sample of the pill should be left static in a heating oven at the same temperature as the lost circulation point. This will confirm the setting time. Contamination of the mud with the plug will not cause any dramatic effect to the mud. Only slight increase in the viscosity can be expected when drilling out the plug.

8.4 Gunk squeeze Gunk squeeze can be used in severe loss situation when drilling with water based mud. Since oil is used in the gunk it can only be used in zero discharge situations. Formulation for 10 m3 of gunk 8 m3 Base oil 4000 kg Bentonite The bentonite will not yield in the oil, but it will hydrate and swell into the formation when in contact with water based mud. Spotting technique 1.

RIH and place the bottom of the drill pipe +/- 1 joint above the thief zone. Pump the gunk down with 2 m3 oil ahead and 1 m3 behind as spacer.

2.

Displace with mud until oil ahead reaches bottom of drill pipe.

3.

Close annular preventer.

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

4.

Squeeze gunk by using both pumps on the cement unit. No. 1; Pump down DP at 300 l/min. No. 2; Pump down annulus at 80 l/min.

5.

Squeeze total 10 m3 (until all oil is cleared from the drill pipe).

6.

Pull drill pipe well above gunk.

7.

Wait on gunk (2-4 hrs).

 Occasionally a second gunk may be required, should the first not work. Be aware that a deposit of gunk may form inside the drill pipe when pumping the gunk. Subsequent attempts, or gunking frequently results in overdisplacement into the annulus.  Be sure of spotting techniques to squeeze the gunk, and minimise any gunk coming up into the annulus. This can result in higher ECD when re-attempting circulation, and or uncomfortable quantity of clay to segregate at the surface when returns are achieved at the shakers.  No attempts should be made to reverse circulate the gunk as it will plug the string.  The Gunk should ideally be prepared by use of a batch mixer or cementing tanks, isolated from the surface mud system. Plugging of mud lines is a serious potential hazard.  The gunk can be pumped through most BHAs.

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European Technical Center Stavanger, Norway Gamle Forusvei 43, N-4033 Forus, Norway, Phone No. 47-51 57 73 00, Fax No. 47-51 57 0605

9. FLOW CHART Loosing fluid while drilling

Stop drilling and observe levels

Losses on surface

Yes

No

Locate and stop losses

Well flowing?

Yes

No

Record shut in pressure. Kill the well

Measure rate of loss

Seepage losses, < 3m 3/hr

Oil/synthetic mud

Spot pill: Ven Fyber 20 kg/m3 G-Seal 40 kg/m3 CalcCarb (C) 20 kg/m3

Partial losses, > 3 m3/hr

Waterbased mud

Oil/synthetic mud

Spot pill: Mix II Fine 20 kg/m3 Calc.Carb.(M) 20 kg/m3 G-Seal 40 kg/m3

Total loss of returns

Waterbased mud

Spot pill: Mica M 30 kg/m3 Ven Fyber 40 kg/m3 G-Seal 50 kg/m3 CalcCarb (C) 30 kg/m3

Oil/synthetic mud

Waterbased mud

Spot pill: Mica M 25 kg/m3 Mix II Med. 25 kg/m3 Calc.Carb (C) 25 kg/m3 G-seal 50 kg/m3 Nut Plug M 25 kg/m3

No success No success

Spot plug: VersaPac or Form-a-plug or Reverse Gunk or Cement

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No success

Spot pill: Mica M 40 kg/m3 Ven Fyber 30 kg/m3 Mix II Coarse 30 kg/m3 G-Seal 50 kg/m3 Nut Plug M 50 kg/m3

Spot pill: Mica M 40 kg/m3 Mix II Med. 30 kg/m3 Mix II Coarse 30 kg/m3 G-Seal 50 kg/m3 Nut Plug M 50 kg/m3

No success

Spot plug: Form-a-plug or Gunk or Cement

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