drilling fluids

STANDARD DRILLING PROCEDURES MANUAL CHAPTER 6 PAGE 1DRILLING FLUIDS REVISION 2 08-05-03

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CHAPTER INDEX

BULLETIN ITEM PAGE

6.1 INTRODUCTION 3

6.2 PROGRAMMING AND REPORTING 36.2.1 Programming 36.2.2 Reporting 3

6.3 DESCRIPTION OF MUD SYSTEMS 46.3.1 Functions of Drilling Fluids 56.3.2 Minimum Bentonite and Barytes stocks 6

6.4 MUD PREPARATION AND TREATMENT 66.4.1 General 66.4.2 Bentonite Mud Systems 86.4.3 KCl Polymer Mud Systems 106.4.4 KCl Polymer/Gypsum Mud Systems 126.4.5 Silicte Mud Systems 136.4.6 TAME Mud Systems 156.4.7 Pseudo Oil Based Mud (POBM) 176.4.8 Low Toxicity Oil Based Mud (LTOBM) 19

6.5 SPECIAL PROCEDURES 206.5.1 Mixing Caustic Soda and caustic Potash 206.5.2 Acids 216.5.3 Drilling Detergent 216.5.4 Lost Circulation 216.5.5 Stuck pipe and Spotting Fluids 226.5.6 Use of Mud Lubricants in Water Base Mud in High Angle Wells 23

6.6 MUD ENGINEERING PRACTICES 246.6.1 Mud Testing Procedures 246.6.2 Reporting Guidelines 256.6.3 Quality Control 25

6.6.4 Pilot Testing 25


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6.6.5 Mud Laboratory Testing Equipment and Chemicals 26

6.7 SOLIDS CONTROL 286.7.1 General 286.7.2 Shale Shakers 296.7.3 Hydrocyclones 346.7.4 Centrifuges 376.7.5 Fluid Routing 386.7.6 Trouble Shooting in Solids Control Equipment 386.7.7 Solids Control Equipment Maintenance 39


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6.1 INTRODUCTION

This chapter on drilling fluids (muds) has been prepared with the objective ofproviding a practical instruction in mud mixing and treatment. The different mudsystems used in SPDC have been highlighted with a section on trouble shooting.The responsibility for programming, maintenance of the mud system andreporting are discussed. More detailed information on individual mud systemscan be found in the Drilling Fluids Manual EP88 - 2637 and various mudengineering companies' manuals.

Any variation from the recommended procedures should be approved by theSenior Drilling Engineer (SDE).

Note: All chemicals used on the rig must have been approved by the PCLaboratory. The DSV must ensure that a valid Safe Handling of Chemicals(SHOC) card for each chemical is available at the rig site.

6.2 PROGRAMMING AND REPORTING

6.2.1 PROGRAMMING

The preparation of the mud programme is the ultimate responsibility of theSenior Drilling Engineer (SDE), in consultation with the mud consultant. Adetailed programme, inclusive of mud type, properties, estimated consumption,costs and treatment should be requested from the contractor Mud Engineeringcompany and will form the basis of the SPDC mud programme. Expert adviceshould be requested from POX-CEM and the DWX-TEC mud focal point asrequired and should always be requested in the case of non-routine mud systemsor products.

6.2.2 REPORTING

The Drilling Supervisor (DSV) has overall responsibility for the mud systems.

The contractor Mud Engineer is responsible for the preparation and maintenanceof the mud system, within the guidelines of the drilling programme. He willreport to the DSV and make recommendations on treatment to the system. When


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his advice/recommendations imply a change in the programmed mud properties,such changes must be discussed with the SDE prior to being implemented.

The DSV has to ensure that the mud programme is carried out in accordance withSPDC standards and procedures.

The routine monitoring of pit levels and basic mud properties (e.g. mud density)is the responsibility of the drilling contractor personnel. They must be measuredevery fifteen (15 ) minutes or as directed by the DSV. Any significant changesin mud properties must be reported immediately to the DSV and the MudEngineer.

The DSV reports to the SDE/office Drilling Engineer who will consult and relayinformation and advice from POX-CEM and DWX-TEC drilling fluid focalpoints.

6.3 DESCRIPTION OF MUD SYSTEMS

Currently, water based mud (WBM) and pseudo oil based mud (POBM) are usedin land/swamp drilling operations, while in offshore drilling activities WBM andLow Toxicity Oil Based Mud (LTOBM) are used. The mud type selected for aparticular well section depends on the following considerations:

• well off-take area• closeness of well direction to the direction of maximum in-situ stress• angle of inclination• open hole exposure time• hole depth• shale reactivity• impairment potential• environmental restriction and waste disposal costs• estimated bottom hole temperature• degree of inhibition offered by the mud system.


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Given in Table 6.1 is the mud selection guide.

Table 6.1: Mud Selection Guide

WBM POBM/LTOBMAll inclinations, Hmin and Low Reactive Shales < 12,000 ftss ü< 45º inclination, Hmax or Moderate/High Reactive Shales, <12,000 ftss, OHT (open hole time) > 45 days

ü

< 45º inclination, Hmin/Hmax and Moderate/High ReactiveShales, < 12,000 ftss, OHT < 45 days

ü*

> 45º inclination, Hmax and Moderate/High Reactive Shales üAll inclinations, Hmin/Hmax or Low/Moderate/High ReactiveShales, > 12,000 ftss

ü

* On offshore wells, LTOBM should be used.

To meet the drilling requirements of the Niger Delta, availability of materials,environmental concerns and economic criteria, the mud systems are presentlyrestricted to the following:-

1. Spud water based muds.2. Non-dispersed polymer water based muds.3. KCl-Silicate-Polymer water based muds.4. Esters5. Pseudo Oil Based muds (POBM)/synthetic Based Muds.6. Mineral oil based muds - Low Toxicity Oil Based Mud (LTOBM).

6.3.1 FUNCTIONS OF DRILLING FLUIDS

Regardless of the type of drilling fluid, there are ten basic functions to beconsidered:

1. To transport drilled cuttings (or cavings) to the surface.2. To control sub-surface pressures.3. To cool and lubricate the bit and drill string.4. To minimise washouts and damage to the well bore.5. To suspend cuttings, weight material and other solids when circulation is

stopped.6. To transmit hydraulic power to the bit.7. To form low permeability filter cake at the borehole face.8. To provide mechanical stability in uncased sections of the well bore.


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9. To minimise torque, drag and pipe sticking problems.10. To assist in well logging operations.

6.3.2 MINIMUM BARYTES AND BENTONITE STOCKS

The following minimum stock levels of barytes and bentonite (Table 6.2) arebased on the requirement to increase the density gradient of the completecirculating system by 0.052 psi/ft (1.0 lbs/gal) in the event of a kick. Thestocking level of Barytes and Bentonite must never fall below these levels.Should stocks fall below these levels, the rig must cease drilling operations, pullto the shoe and circulate until such time as stocks are replenished.

Table 6.2: Minimum Bentonite and Barytes Stocks:

Chemical Hydrostatic Wells Over-pressured Wells

Barytes 100 Metric tonnes 200 Metric Tonnes

Bentonite 50 Metric Tonnes 75 Metric Tonnes

Due to the lag in actual supply times in SPDC, it is recommended that sufficientstock of all drilling chemicals, barytes and bentonite are maintained on the rig toallow uninterrupted drilling for 7 days.

6.4 MUD PREPARATION AND TREATMENT

6.4.1 GENERAL

All mud preparation and treatment should be carried out under the supervision ofthe contract Mud Engineer. Good communication is imperative for a successfuloperation to be achieved. It is recommended that:

1. The Mud Engineer is provided with a copy of the drilling programme.

2. No additions of chemicals should be made without the approval of the MudEngineer.


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3. The Mud Engineer should be present at tour changes to ensure that correctinstructions are given in writing to the oncoming crew.

a) Mud Properties

Mud properties should be closely monitored. Sudden large changes in properties,leading to a change in the chemical consumption pattern, are indicative of someform of interactive between the mud and formation. Remedial treatment ormodification of the programmed properties will be required.

The Mud Engineer should pilot-test suggested modifications of programmed mudproperties and advise the DSV of his preferred changes to the mud programme.Such changes should be relayed by the DSV to his Senior Drilling Engineer forapproval. The SDE should notify POX-CEM and the DWX-TEC drilling fluidsfocal point of such changes, so that they can monitor proceedings and note themfor future programming.

b) Housekeeping

The mud pit area, chemical storage facilities and mud laboratory should bemaintained in a clean orderly and safe manner.

1. Dangerous chemicals (caustic, lime, acids, etc.) should be roped off and storedseparately. A warning sign indicating that the chemicals are of a dangerousnature should be erected so that it is in clear view of all personnel working in thearea.

2. All equipment should be correctly stored when not in use.

3. All personnel should wear the required safety clothing and apparatus.

4. Pallets should never be stored more than 10 (ten) pieces per stack.

5. All broken sacks or damaged drums of chemicals should be handed over to thewaste management team. Dumping of chemical wastes in the swamp is not anacceptable method of disposal.


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c) Inventory Control

Sufficient stocks of chemicals should be maintained at all times to enable asmooth drilling operation to progress. Due to logistical and supply problems, it isrecommended that sufficient chemicals are kept on site to allow the presentsection to be drilled. Where this is not possible, a minimum of two weeks stocksshould be maintained. Adequate lead times should be allowed for the re-supplyof chemicals. If chemical stocks fall to a level such that the correct treatment ofthe mud system is not possible, then drilling must stop until such time assufficient chemicals are available.

Chemical stocks should be physically checked on a regular basis. A weeklyphysical stock check is mandatory.

6.4.2 BENTONITE MUD SYSTEMS

In a sea water (salt water) environment, bentonite yield is greatly reduced (lowviscosity) because of the presence of ions like Ca++, Mg++, Cl-, etc. For suchareas, the limitation is overcome by prehydrating the bentonite in fresh water.The prehydrated bentonite is blended with sea water. Higher rheology can beachieved by using HV polymers. Below the surface casing shoe, LV filterationcontrol materials can be added to adjust the water loss properties.As most of the source water is saline, prehydration is always desired to obtainmaximum yield.

Water for prehydrated bentonite must be fresh (salinity < 2000 mg/l). The watershould be free of offending ions like Mg++ and Ca++. If these ions are present,they should be treated out to below 100 mg/l with Soda Ash. A residual Ca++

level of below 100mg/l is desirable.

a) Composition and Preparation

1. Treat water with +/- 1/2 lbs/bbl Caustic Soda (for pH of +/- 9).

2. Add Soda Ash if required and add 30 - 40 lbs/bbl Bentonite.

3. Allow 4 - 6 hours for hydration.


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Note: If the mix water contains no Ca++ or Mg++, omit Soda Ash. SodaAsh addition is to reduce the Ca++ and Mg++ ions present in thewater. Allowing some hydration time initially, will deflocculate themud and allow more bentonite addition.

4. Dilute prehydrated Bentonite slurry to 20 - 25 lbs/bbl by adding equal volume ofsea water.

5. Add 1 – 1.5 lbs/bbl CMC-HV/PAC-R for increased viscosity.

Note: Lime could be used to flocculate the system.

b) Treatment

SPDC encourages the philosophy of whole new mud dilution. It is preferable tomix fresh mud in the mixing pit and slowly bleed it into the active system than todilute with water and add chemicals directly into the active system. Table 6.3gives a summary of the most common problems encountered while drilling witha Bentonite mud system together with suggested causes and treatment. Moredetails can be found in the Drilling Fluids Manual.

Table 6.3: The Effects of Contaminants on Bentonite Mud

Conta-minant

Density

Visc. FluidLoss

MBT pH Alkali-nity

Cl- Ca++ TreatmentRequired

Clays &shales

+ +++ + ++ - - Dilution. Treat with CausticSoda

Sand &Silt

+ +(PV) + + Solids removal. Dilution /addition of fresh mud.

Cement +++ ++ ++ ++ ++ Pre-treat with SodiumBicarbonate. Add filterationmaterials to restore fluid loss.Dump contaminated mud.

Anhydrite +++ ++ + Add Soda Ash. Restore fluidloss. Consider conversion toGYP. System.

Lignite -- - - Increase viscosity and treat forpH.

Surfacewater

- --- ++ - - - +++

++ Increase density and rheology.Reduce fluid loss.

+ : Parameter increases with contaminant- : Parameter decreases with contaminant.


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The number of + or - signs indicates the degree of sensitivity, with +++ or ---indicating the highest degree of sensitivity.

6.4.3 KCL/POLYMER MUD SYSTEMS

KCl/Polymer water based mud systems are inhibitive. The main application istherefore in drilling hydrated shales. The benefits are (I) inhibitive properties ofthe ‘K’ ions towards shale, (ii) the encapsulating of cuttings, (iii) coating ofborehole wall by polymers. Stable borehole and efficient cuttings removal lead tofast ROP. When compared with other WBM systems, KCl/Polymer systems arevery efficient, however, they are limited by their low tolerance to solids. Theconcentration of KCl is dependent on the level of inhibition required, but atypical concentration of 5-8% is common. This mud may not prevent theoccurrence of pore pressure penetration entirely. The stability of the system istime dependent. Therefore, prolonged open hole exposure time could lead toshale failure.

a) Composition and PreparationDilute prehydrated bentonite slurry (with sea water) to the required bentonite

concentration MBT. Treat with NaOH or KOH to a pH of about 10. Add required concentration of KCl. Treat with polmers to obtain desired characteristics. A typical formulation is as follows:

Bentonite - 5 – 10 Ibs/bblKOH/Caustis Soda - 0.23 – 0.30 Ibs/bblKCl - 25 – 30 Ibs/bblStarch - 4 Ibs/bblPac-R - 1 – 1.5 Ibs/bblPac-L - 0.5 – 1.0 Ibs/bbl*XCD Polymer - 0.25 – 0.5 Ibs/bblLubricant - 0.5 – 2.0 % v/vBarite (as required)

ContingencyLCMShale stabilizersPipe free agent* optional (for suspension)


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Note: Polymers should be added slowly for proper mixing to occur. If theyare added too quickly, 'Fish-Eyes' will occur in the mud.

b) Treatment

Whole new mud dilution is essential for effective treatment of KCl/Polymermuds. Fresh mud should be prepared in the reserve pits and slowly added to theactive system. Fresh whole mud should be prepared as above. However, nofurther Bentonite should be added.

KCl/Polymer muds are highly sensitive to drilled solids, with an associateddetrimental effect on both viscosity and gels. It is therefore very important thatall solids removal equipment is functioning in an optimal manner.

Table 6.4 gives a summary of the most common problems encountered whiledrilling with a KCl/Polymer mud system together with suggested causes andtreatment. More details can be found in the Drilling Fluids manual.

Table 6.4: The Effects of Contaminants on KCl/Polymer Muds

Conta-minant

Density

Visc. FluidLoss

MBT PH Alkali-nity

Cl- a+ K + andPolymer

TreatmentRequired

Clays &shales

+ ++ + ++ - - -- Add KCl and Polymer.Dump part of old mudand replace by fresh mud.Maintain MBT < 25.Improve solids removal.

Sand &Silt

+ +(PV) + + Solids removal.Dilution/addition of freshmud.

Cement + ++ ++ ++ -- Pre-treat with SodiumBicarbonate. Restorefluid loss and Polymercontent. Dumpcontaminated mud.

Anhydrite

+++ ++ ++ No treatment required.

Surfacewater

-- --- + - - - -- -- Increase density andrheology. Add Caustic orPotash to correct pH.Correct fluid loss.Restore excess polymer.


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+ : Parameter increases with contaminant.- : Parameter decreases with contaminant.

The number of + or - signs indicates the degree of sensitivity to a contaminantwith +++ or --- indicating the highest degree of sensitivity.

6.4.4 KCL/ GYPSUM /POLYMER MUD SYSTEMS

KCl/Gypsum/Polymer mud is an inhibitive system used for drilling hrdratableshales. The system combines the inhibitive properties of K+ and Ca++ tostabilise the formation. The rheology of the system is easy to control.

a) Composition and PreparationNote: Pre-hydrate Bentonite in fresh water. Volume increases can be made withsea water.

Bentonite - 0 – 5 Ibs/bblKOH - 0.2 – 0.35 Ibs/bblKCl - 25 – 30 Ibs/bbl (as reqd.)Gypsum - 4 – 6 Ibs/bblStarch - 4 Ibs/bblPac-R/L - 1.5 – 2 Ibs/bbl

ContingencyXCD - 0.5 – 0.75 Ibs/bblLube - 2 – 4 % v/vLCMShale stabilizerBarite for weight

b) Treatment

Whole new mud dilution is essential for effective treatment of muds. Dump sand trapthrough waste management system inorder to create space for new mud volume. Freshmud should be prepared in the reserve pits and slowly added to the active system inbatches. Fresh whole mud should be prepared as above. However, no furtherBentonite should be added if the formation is highly bentonitic (MBT < 2Ibs/bbl).


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Like all polymer muds, KCl/ Gypsum/ Polymer mud systems are highly sensitive todrilled solids, with an associated detrimental effect on both viscosity and gels. It istherefore very important that all solids removal equipment is functioning in an optimalmanner.

6.4.5 SILICATE MUD SYSTEM

Silicate mud system is a highly inhibitive and environmentally friendly water based mudsystems. It is particularly suitable for drilling reactive formations such as clays andShales, highly dispersible formation such as chalk and unconsolidated sands. This mudsystem utilises sodium or potassium silicate to provide primary inhibition whilemonovalent salts such as sodium and potassium chloride provide the secondaryinhibtion. Commercial silicate liquors have molecular ratios typically between 1.5 and3.3. Silicate liquor with molar ratio of 2.0 is preferred for mixing silicate mud systems.

Silicate mud provides shale inhibition by two mechanisms. Monosilicates polymeriseeasily to form negatively charged oligomers which can easily penetrate the shalemicropore structure as mud filtrate. In the first mechanism, as the oligomers (filtrate)with pH of +/- 12 penetrate the shale micropore fabric, it comes in contact with porefluids ( pH +/- 7). The pore fluids dilute the filtrate (lowers the pH ) resulting in theformation of silicate gels, which forms a membrane around the shale walls. In the second

mechanisms, the oligomers react with free polyvalent cations (Ca++ and Mg++ ) in theshale pores to form insoluble precipitates. The gelled and precipitated silicates thusstabilise the shales by providing physical barriers, which prevents further mud filtrateinvasion and pore pressure penetration. The wellbore and shale are thus ‘pressureisolated’.

b) Treatment

Silicate mud is highly tolerant to low gravity solids compared to most polymer fluids.This can disguise the build-up of ultra-fine solids in the system. This will eventually leadto the need for large volume dump and dilution to restore rheology and filtrationproperties. Therefore, dilution with whole new mud on a continuous basis is important.Dilution rates of 0.4 - 0.6bbl/ft for 17-1/2” hole and 0.3 – 0.5bbl/ft for 12-1/4” and 8-1/2” holes may be adhered to. The build up of low gravity solids should be avoided earlyenough by making maximum use of available solids control equipment.


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Depletion of silicate ion usually occurs while drilling with silicate mud system. This isattributed to silicates preferentially wetting steel tubulars, mud pits and contaminants inthe circulating system. Depletion occurs also due to contact of silicates with formation.Depletion levels are higher in anhydrite, dolomite and highly reactive shales, lower inlow reactive shales and lowest in clean sands. Acid gases can also deplete silicatesrapidly as the silicates leave solution due to declining pH. To re-establish therecommended silicate ion level for a given application, addition of fresh sodium silicateliquor need to be done at intervals.

Therefore in addition to the API mud check, SiO2 and Na2O concentration has to

be included in routine mud check. Refer to the contractors drilling fluids manualson the procedures for testing and estimating these ions.

a) Composition and Preparation

The silicate mud system may be formulated using freshwater, seawater,potassium chloride or sodium chloride brine base. The final formulation willdepend on the application. Moreover, most conventional lubricants are noteffective in Silicate mud. Therefore, polystyrene beads may be added in highangle wells. However, the following formulation and order of addition couldserve as a guide.

Water : 0.9 bblKCL : 30 lbs/bblSodium silicate : 640 lb/bblPAC R (or equivalent) : 0.5 lb/bblPAC L (or equivalent) : 1.8 lbs/bblStarch : 5 lbs/bblXC-Polymer (or equivalent) : 0.6 lb/bblBarite : as requiredLubricant : 5% v/v


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Table 6.4.5: The Effects of Contaminants on Silicate Mud.

Contaminant Symptoms Treatment required

CO2 Total SiO2 depletion

Serious gelling of system

Decreased pH

Increase in fluid loss

Raise pH with NaOH

Replace silicates

Dilute with new mud

Raise density

Replace polymers

Anhydrite Slight-high SiO2 depletion

Increased PV/YP


Reduced pH

Replace silicates

Dilute and add polymers as required

H2S High SiO2 depletion

Gelling of mud

Slight decrease in Ph


Raise pH with NaOH

Replace silicates

Dilute with new mud

Raise density

Add scavenger (e.g. zinc oxide)

Clay solids Slight SiO2 depletion

Slight increase in PV

Increase in MBT

Replace silicates

Dilute mud

Run solids control equipment

Cement High SiO2 depletion


Slight/no drop in PV/YP

Replace silicates

Add polymers

6.4.6 TAME MUD SYSTEM

Thermally activated mud emulsion (TAME) drilling fluid is an improvedKCl/polymer mud for the minimisation of mud related borehole instabilityproblems in shales. TAME drilling fluid is an inhibitive mud system, which iscapable of minimising fluid invasion into shale pores, thereby reducing the effectof the associated pore pressure penetration. The inhibitive property of the mud isderived from the presence of salt of potassium chloride (KCl) in the formulation.The swelling of reactive shale is minimised by the potassium ion in KCl salt.

The incorporation of alcohol alkoxylates commonly known in the industry aspolyalkylene glycols in the mud formulation is responsible for the reduction of


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hydraulic invasion into the shale pores. The penetration of mud pressure into theshale pores is impeded by TAME due to its possession of viscosified filtrate andits ability to cloud-out and block shale pores at the elevated temperature normallyexperienced downhole.

The Cloud Point Temperature (CPT) of a polyglycol is the temperature at whichit changes from water soluble to water insoluble. When CPT is reached, thepolyglycol starts to come out of solution and an otherwise clear liquid starts tobecome opaque and cloudy. Control of CPT is critical for the inhibitivecharacteristics of the fluid and is a function of salinity and type and concentrationof polyglycol.

TAME should be designed and formulated by ensuring that its down hole cloudpoint temperature (true CPT) is equal or above the bottom hole circulatingtemperature (BHCT) but below the bottom hole static temperature (BHST). TheBHCT data to be used in TAME design, are those taken with the measurementwhile drilling (MWD) tool.Surface CPT is the cloud point temperature taken at surface under atmosphericpressure. TVD is the true vertical depth in ft and mud weight should be in psi/ft.

a) Composition and Preparation:

1. Treat water with +/- 1/2 lbs/bbl Potassium hydroxide (KOH) for pH +/- 10. Iftotal hardness is above 400ppm, treat with Soda ash (Na2CO3).

2. Add KCl + NaOH to required concentration.

3. Add Polyglycol to required concentration.

4. Add 2-4lbs/bbl prehydrated Bentonite if necessary.

5. Add 2 lbs/bbl PAC-LV or equivalent.

6. Add 1.5 - 2.0 lbs/bbl PAC-HV or equivalent (optional).

7. Add 0.25-1.0 lbs/bbl XC polymer.


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b) Treatment

Whole new mud dilution is essential for effective treatment of TAME drillingfluid. Fresh mud should be prepared in the reserve pits and slowly added to theactive system in batches. Fresh whole mud should be prepared as above.However, no further Bentonite should be added.

TAME systems are somehow sensitive to drilled solids, with an associateddetrimental effect on both viscosity and gels. It is therefore very important thatall solids removal equipment are functioning in an optimal manner.

6.4.7 PSEUDO OIL BASED MUD (POBM)

The three types of pseudo oil based muds currently used in SPDC are Petrofree,Synteq and Ultidrill. Petrofree has vegetable ester as the continous phase whileSynteq and Ultidrill has isomerised olefins and linear alkyl olefins (LAO)respectively as the continous phase. These muds systems are claimed to benontoxic and biodegradable, therefore environmentally tolerable. Disposal of thecuttings generated while drilling with these POBMs should be in accordance withDPR guidelines and standards.

The olefins or ester, which is in contact with the formation, prevents shale fromits two agents of destabilisation (i.e. hydration and pore pressure penetration),since it is a known fact that shale demands water not oil. The emulsified CaCl2brine in the POBM osmotically dehydrates the shale penetrated and the drilledcuttings to ensure minimal mud maintenance and effective solids removal atsurface. These positive attributes of OBM provide the opportunity to drill shalesat a 0.024 - 0.12 SG lower mud weight than would be required with a waterbased mud. POBM gives excellent lubricity resulting in low torque and overpull.

To ensure a stable emulsion, a minimum electric stability of 500 volts should bemaintained during drilling operations.

The table below provides a guide for the oil:water ratio to use for a given muddensity. Note that these values could vary depending on the kinematic viscosityof base oil. POBM formulated from base oil with low kinematic viscosity (eg.iso-olefin, linear alkyl olefins ) can tolerate lower oil:water ratio than those from


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base oil with high kinematic viscosity (eg. esters). In general, for a given type ofPOBM, the lower the oil:water ratio, the lower the mud cost.

mud gradient range (psi/ft) oil:water ratio< 0.47 60:400.47 – 0.57 65:350.57 – 0.72 70:300.72 – 0.83 75:250.83 – 0.98 80:20> 0.98 90:10

POBM Consumption while drilling one unique advantage of POBMs is that theycan be recycled and re-used over a number of wells thereby bringing down theeffective mud cost. To maximise this benefit, downhole and surface losses needto be kept as low as operationally possible. The following steps should be takento minimise POBM losses:

Source Recommended action(s)

Starting POBM

volume

Confirm mud volume prepared/received using calibrated dipstick

Shakers Use appropriate screen mesh size, Ensure circulating rate is not greater than shakers

rated capacity

Centrifuge Build different weighted systems for build-up and drainhole sections to avoid

excessive loss of barytes and POBM while cutting down mud weight.

Surface Ensure all surface equipment, mud tank piping, valves, etc are leak-proof.

Downhole Appropriate mud weight (STABOR), Weight-up with calcium carbonate, add LCMs

if necessary.

Left in hole

when

cementing

Displace with large volume of spacer, e.g.

Dumped (mud

tank dead

volume)

Use small pumps to suck out mud that cannot be pumped out with rig pumps. Use

cylindrical or bevelled mud tanks where available.

End volume Use dipstick to confirm final mud volume at end of well.

Although oil muds have a greater tolerance to drill solids, a correct solids controlprogram will ultimately improve overall efficiency.


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6.4.8 LOW TOXICITY OIL BASED MUD (LTOBM)

The low toxicity oil based mud currently being used in SPDC is Enviromul. TheLTOBM has mineral oil as the continuous liquid phase. These muds arenonbiodegradable, therefore environmentally unfriendly.

The mineral oil, which is in contact with the formation, prevents shale fromswelling and pore pressure penetration. The LTOBM gives excellent lubricityresulting in low torque and overpull. To ensure a stable emulsion, a minimumelectric stability of 600 volts should be maintained during drilling operations.

Although oil muds have a greater tolerance to drill solids, a correct solids controlprogram will ultimately improve overall efficiency.

Table 6.5: Effects of Contaminants on OBM

Property Water Cement Acid Gases Soluble Salts Drilled Solids

H20 Ca(OH)2 H2S - CO2 Ca/MgCl2 Clay

Plastic viscosity > > > >

Yield point > > > >

Gel Strength 0/10 > > > >

Electrical stability < < < <

Filtrate loss > > > >

Calcium chloride < >

Sodium chloride <> <

Alkalinity > <

Density < >

Solids < > >

Oil

Water >

Treatment Adjustdensity. addlime, saltemulsifiers

Effects areminimal ifcement is notgreen. Add oilwetting agent.

Zn or Fescavengers. Addlime. Increasedensity if required.

Add fresh waterto dissolveinsoluble salt.

Dilute with oil.Add emulsifier.Improve solidscontrol.

Note: 1) > - means Increase2) < - means Decrease


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6.5 SPECIAL PROCEDURES

6.5.1 MIXING CAUSTIC SODA AND CAUSTIC POTASH:

Caustic Soda and other such products are highly corrosive and require specialhandling and mixing procedures. The proper safety equipment and clothing mustbe provided and used at all times.

a) Safety Equipment and Clothing

1. Long, heavy-duty rubber gloves.2. Full face shields or masks.3. Full length rubber aprons.

4. Chemical eye-wash, including one gallon of household vinegar.5. Shower and eye-wash facilities.6. Long sleeved coveralls.7. Safety boots (Rubber).

b) Mixing Equipment

Chemical barrel: This should be light-weight and easily movable with a topmounted, hand cranked paddle mixer and 1" bottom discharge valve to which ahose can be attached. A 55 gal drum can be easily modified for this purpose.

c) Recommended Mixing Procedure :

1. Half fill the drum with water.

2. Slowly add the chemical through the drum top while stirring (all chemicals mustbe supplied in drums, not sacks).

3. When thoroughly mixed, top up with water and bleed into an agitated pit or intothe mud stream.

6.5.2 ACIDS

The mixing of concentrated acids should be done by trained personnel wearingappropriate Personal Protective Equipment (PPE). Concentrated acid should be


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transferred using transfer pump only. Handling by untrained rig labour should beminimal and only under strict supervision complying with safety regulations. Asafety meeting should be held immediately prior to the handling of acids, with allparties made aware of the operations to be performed, potential hazards andremedial actions to be taken in case of spillage or leaks. Extra care and vigilanceis required on high pressure pumping operations.

Wash down facilities, with running water, should be available as well as SodiumBicarbonate for neutralising any spills. Full-face vapour type masks aremandatory.

It should be noted that the use of ordinary face shields and goggles is notacceptable.

The mixing area should be cleared of non essential personnel. In the case of highpressure displacement, no one should be close to or work across lines whilepumping. The rig floor should be kept clear of unnecessary personnel; those onwatch should be properly attired.

6.5.3 DRILLING DETERGENT

Drilling Detergent (DD) is primarily used in surface and intermediate holesections to minimise bit balling problems caused by reactive clays. Use shouldbe initiated prior to penetrating known problem zones. Treatment should becontinued as required until casing depth has been reached. DD should not beused as an integral part of a weighted mud system as it may cause settling ofBarytes. However, water base pills may be used to clear balled up bits of BHA.

6.5.4 LOST CIRCULATION

The procedure for detection, prevention and curing losses can be found in chapter8, "Hole Problems and Fishing". Information on the preparation of differenttypes of LCM is also available in chapter 8.


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6.5.5. STUCK PIPE AND SPOTTING FLUIDS

Information on mud treatment for stuck pipe and spotting fluids can be found inchapter 8 and in the "ABC of stuck pipe".

a) HOLE CLEANING

The drilling fluid recipe for cleaning of wellbore should be inclination dependent.Wells with angle of inclinations less than 50 degrees can be swept clean withonly high viscous pill pumped at regular interval. However, wells withinclination greater than 50 degrees suffer from cuttings bed formation which hasto be swept clean with a combination of low viscous light weighted and highviscous heavy weighted pills pumped in tandem every 3 or 4 stands drilled. Thelow vis pill's design and formulation should be done to ensure that the pill is inturbulent flow regime in the critical annulus downhole.

Optimum flow rate, pipe rotation coupled with controlled drilling rate andfrequent short trips will help in preventing the formation of cuttings bed in thehighly deviated/horizontal section. The recommended volume of tandem pills forhole cleaning is given in Table 6.6 below.

Table 6.6:

Tandem pills used in cleaning hole with inclination greater than 50 degrees.

Low viscosity Light weight High viscosity Heavy weight17 ½" hole 30 bbl 35 bbl12 ¼" hole 20 bbl 25 bbl8 1/2" hole 15 bbl 15 bbl6" hole 10 bbl 10 bbl

Note: If serious caving problem is encountered in highly deviated/horizontal wells,

the mud viscosity should be increased with YP at 40 - 50 lbs/100 ft2 . This

should be followed with periodic low viscous pill sweeps at optimum flowrate coupled with pipe rotation to create the required turbulence along thesection, where cuttings bed must have been formed.

b) MUD WEIGHT PREDICTIONUsing the right mud weight is crucial to the mechanical stability of wellboreirrespective of the mud types (WBM, LTOBM & POBM). Stabor was developed


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by SIEP to predict mud weight without taking into consideration the effect ofshale reactivity with the drilling fluid. Stabor input is either derived from log dataor obtained from shale cores. The accuracy of input determines the accuracy ofthe Stabor's output. Prediction from Stabor together with the data fromcorrelating wells should be used in determining the optimum mud weight for theparticular well. Stabor's Input Facilitator should be used to generate the requiredinput for stabor calculations.

6.5.6 USE OF MUD LUBRICANTS IN WATER-BASE MUD IN HIGH ANGLEWELLS

Additional friction between the drill string and borehole is expected when drillinga hole off-vertical. This implies that the torque and drag will increase withincrease in deviation. Moreover, the frictional resistance generated may requireconsiderable extra torque to rotate the string or force to raise or lower the pipe.Therefore, addition of certain lubricating agents to the mud can alleviate thisproblem.

Desirable characteristics of a lubricant for water-base mud are:

i) The material must perform well as a lubricantii) It must be non-toxic and biodegradableiii) It should have little tendency to form an oil slick on water.

Note: The use of diesel oil as lubricant has been discontinued because of (iii)above.

Commonly used lubricants in drilling operations are plastic beads, paraffin oils,fatty acid compounds and blends of triglycerides and alcohols.

Laboratory studies and field data show that the most effective lubricants are fattyacid compounds (e.g.: EP Mud lube, Bit lube) and blends of triglycerides andalcohols (e.g.: Torque trim, magcolube).Liquid lubricants are added to the mud system at a range of 2-6 Ib/bbl dependingon torque or drag severity, while lubrabead pill of about 8 Ib/bbl is usuallyspotted in the region of high torque and drag.


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6.6 MUD ENGINEERING PRACTICES

6.6.1 MUD TESTING PROCEDURES

Measurement should be made using standard procedures described in APIrecommended practice 13B - 1 "Standard Procedure for Field testing Waterbased Drilling Fluids". See appendix II - 1 of the drilling fluids manual (EP 88 -2637).

Note: A "Halliburton" pressurised true weight balance must be available ofthe rig and be used with aerated or gasified mud.

6.6.2 REPORTING GUIDELINES

The API daily Mud report should be completed in full each day. A minimum ofone representative check, taken active circulation, should be taken each tour.During extended periods of non-circulation, only one full mud check need to bereported each day.

Pit volumes and basic mud checks (e.g. mud density) should be taken regularly,as determined by the DSV in consultation with the Mud Engineer.

In addition, a complete mud test should be carried out on the followingoccasions.

1. When circulating prior to logging or running casing. The timing of the testsshould be such that a representative mud sample is tested and that there is stilltime to carry out any treatment found to be necessary from the results of the test.

2. When there are indications of formation fluid inflow.

6.6.3 QUALITY CONTROL

In order to ensure the quality of chemicals received on location, the followingprocedures should be adopted for new batches of chemicals, barytes andBentonite arriving on the rig. For the purposes of the list, the term "Chemicals"is taken to include all Barytes and bentonite as well as sacked chemicals.


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1. Ensure that individual bags are clearly marked with: Generic or brand name ofthe product, the unit weight and the supplier/manufacturer's name.

2. Each pallet must be clearly marked with the batch number.

3. Ensure that the chemicals are from an accredited supplier. A list of suppliersshould be available on the rig. If in doubt, contact SDE.

4. Send samples of all new batches to POX-CEM, where spot checks on the qualitywill be performed. All samples should be accompanied by an Analysis Requestform, which should be available on the rig.

5. Always pilot test new batches of chemicals. If they appear sub-standard, contactthe SDE and inform him of the chemical name, batch number and date a samplewas sent to POX-CEM. Do not use any of this batch until permission has beenobtained from the SDE.

6. All chemicals which are incorrectly bagged, labelled, damaged, contaminated,under-weight or from an unknown supplier, should be returned to the mudcontractor and marked with a large X. The SDE should be notified.

7. Mud samples, taken from the active system, should be sent on a weekly basis toPOX-CEM for testing. Additional samples may be sent if required. All mudsamples should be accompanied by the wellsite test data. Samples should beproperly packed and tagged.

A telex should be sent to the SDE and POX-CEM informing them that thesample has been sent.

6.6.4 PILOT TESTING

Pilot tests should be done:

1. On all new batches of chemicals, Barytes and bentonite.

2. When planning mud treatment with a new chemical, this test needs to be done inthe laboratory and also on the well site.


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For details on equipment and procedures, refer to chapter 2, section 1.3 of the'Drilling Fluids Manual' (EP 88-2637).

6.6.5 MUD LABORATORY TESTING EQUIPMENT AND CHEMICALS

Mud testing equipment and reagents should be provided by the mud engineeringcompany. The details of equipment and reagents required to conduct mudanalysis could be found in Appendix II - 1 of the Drilling Fluids Manual ReportEP88-2637 (1988). Below is a listing of all mud testing equipment required onthe well site.

a) Mud Testing Equipment:

1. Clock or Stop-watch 1 pc.2. Fann viscometer, model 34 1 pc.3. Transformer for Fann Viscometer 1 pc.4. Complete filter press kit 1 pc.5. Filter papers, Whatman No. 50, 9 cm diameter 1 pc.6. CO2 bottles 'Sparklets' for filter presses without hydraulic systems 4 boxes

(Alternative : Nitrogen bottle + pressure regulator)7. High speed mixer 1 pc.8. Stainless steel cup for the high speed mixer 1 pc.9. Mud cup 2 pcs.10. Marsh funnel and Marsh funnel mud receiver 2 pcs. each11. Mud balance + box 2 pcs.12. pH papers; range 8. 2 - 10 and 9.5 - 13 2 pkts. each13. Complete retort kit set (50 ml) 2 pcs.14. Sand content screen and funnel 1 pc. each15. Sand content tubes 2 pcs.16. Simple weighing balance for pilot tests 1 pc.

b) Glassware:

1. Test tube brushes (various sizes) 4 pcs.2. Porcelain dish for titrations 2 pcs.3. Long spatula 20 cm blade 2 pcs.4. Steel wool 4 pcs.


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5. Thermometer (50 - 150 deg. F.) 2 pcs.6. Mud sample cans (1/2 gallon) 5 pcs.7. Mini weight balance 1 pc.

c) Chemicals:

1. Methyl orange indicator 1 litre2. Silver nitrate solution 0.01N and 0.0282N (4.79 gms. in one litre)

and 0.0282N (47.9 gms. in one litre) 1 litre3. Distilled water in plastic cans 1 litre4. Potassium chromate 5% solution 1 btl.5. Phenolphthalein indicator (5 gms. in 100ml of water) 1 btl.6. Silver nitrate 0.1N solution 1 litre7. Sulphuric acid 0.2N solution 1 litre8. Standard versenate solution (EDTA) 1 litre9. Calcium buffer solution 1/2 litre10. Calcium indicator tablets 1 btl.

Note:

1. Silver nitrate solution is to be kept in a dark bottle. The date of preparationof all solutions should be marked on their bottles and solutions should bereplaced within three months.

2. The listed equipment and chemicals should be available in the well sitelaboratory of every drilling rig.

3. All laboratory equipment should be kept clean and in working order.Broken or faulty equipment must not be left in well site laboratorycupboards.

4. All electrical equipment, plugs and sockets should be of an explosion prooftype and should be switched off when not in use.

5. Before a rig move, make sure that all equipment are stored away safely inthe laboratory, to avoid breakages and losses during transport.

6. Wellsite laboratories should be of a positive pressure type.

6.7 SOLIDS CONTROL


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6.7.1 GENERAL

This topic is more comprehensively covered in SIEP Drilling Fluids Manual EP88-2637 to which reference should be made.

The incorporation of drilled solids (low specific gravity solids) in the mud systemis inevitable in all drilling operations. However, their presence leads to manyproblems which include:-

1. Unstable rheology

2. Expensive mud treatment

3. Differential sticking

4. Formation fracture/lost circulation

5. Reduced penetration rate.

6. Increased drilling problems (increased torque and drag)

7. Increased wear on drilling equipment

8. Impaired hole quality (thicker filter cakes and formation impairment).

9. Increased waste product volumes.

Consequently, it is essential that the build up of drilled solids be minimised at alltimes. This can be achieved by three methods, namely: -

a) Prevention

1. Using inhibitive drilling muds and correct mud weight to prevent wellboredestabilization and solids from being integrated into the mud systems (Polymermuds and Oil muds).

2. Encapsulating the cuttings to prevent them being integrated into the mud system(Polymer muds and oil muds).


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3. Good drilling practices bit selection, nozzle selection, hydraulics, BHA etc. toreduce hydraulic and mechanical damage to the formation.

b) Primary Solids Removal

1. Settling of coarse solids: Sand traps settling pits.

2. Mechanical separation using Solids Removal Equipment (SRE): Shale shakers,desanders, desilters, mud cleaners and centrifuges.

c) Secondary Solids Removal

Dilution of the mud system either with water or preferably, using freshly mixedmud (whole mud dilution). A description of the various mud types used in SPDCoperations together with their properties, advantages and recommended methodsof preparation and maintenance can be found in earlier sections of the manual.This section discusses exclusively the subject of primary solids removal.The ease with which drilled solids can be removed from a mud is dependent ontheir size. The larger the cutting, the easier it is to remove. It is thereforeessential that drilled solids are removed during their first stage through the solidsremoval equipment. Failure to do this may lead to a disintegration of the solidsinto smaller particle sizes.Presently, the following solids remova l equipment are used in SPDC operations :Shales shakers, hydrocyclones (desanders, desilters and mud cleaners) andcentrifuges.

6.7.2 SHALE SHAKERS

Shale shaker is a general term for vibrating devices used to screen solids from themud with the entire circulating volume passing over the screens. A variety oftypes of shakers exist; single deck single screen, single deck double screen,double deck double screen, single deck triple screen. They employ various typesof motion; circular, elliptical and linear motion.

a) Scalper Shakers

Scalper shakers are used on some SPDC rigs to increase the mud handlingcapacity of the main shale shakers, especially in top hole sections where the high


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flow rates and viscosity of the mud cake make it increasingly difficult for thelinear motion shakers to cope. Scalper shakers are shakers whose main objectiveis to remove the large cuttings and gumbo clays. In order that the whole mudvolume may pass over the scalper shakers, large screen sizes are used. Normally,10 x 10, 20 x 20 or 40 x 40. Using large screen sizes also has the benefit ofincreasing the screen life.

b) Linear Motion Shakers

Linear motion shakers are used on most SPDC drilling rigs. The continuouslinear motion provides rapid and efficient conveyance of drilled solids, allowingfine screening of the mud at high flow rates. The slope of the deck can beadjusted to allow for maximum retention of cuttings on the screen for improvedseparation of cuttings from the mud.

c) High G-shakers

High G-shakers are now being used to reduce considerably the amount of mud oncuttings lost into the environment. The efficient use of this shaker results inrelatively dry cuttings. High G-shakers are improved linear motion shakers withhigher "g force" to provide high vibratory force needed for drying the cuttings.The high G-shakers treat the moist cuttings discharged from the linear motionshakers. The screens on the high G-shakers should be finer than those on thelinear shale shakers. The underflows from the hydrocyclones should be directedto high G shakers for further mud recovery.

c) Shale Shaker Screens

The performance of a shale shaker is highly dependent on the type of shakerscreens used. The three principal criteria for screens are Screen surface area,mesh size and screen type.

Surface Area

The handling capacity for a shaker is partially governed by the surface area of thescreen. The larger the area, the better the performance. The screen surface islimited by the dimensions of the shale shaker.


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Mesh Size

The particle size of the screen is defined by the mesh size of the shaker screen,which is usually referred to be the number of openings per linear inch.

Table 6.4: Screen Sizes:

Mesh Screen Size Minimum Size (ins.) Separated Microns

8 x 8 0.097 2464

10 x 10 0.075 1905

12 x 12 0.060 1524

14 x 14 0.020 1295

16 x 16 0.0445 1130

18 x 18 0.0376 955

20 x 20 0.017 838

20 x 30 0.035 889

30 x 30 0.0213 541

30 x 40 0.0233 592

40 x 40 0.015 381

50 x 50 0.011 279

80 x 80 0.007 178

100 x 100 0.0055 140

120 x 120 0.0046 117

200 x 200 0.0029 74

Table 6.5: Drilling Solids Distribution

Solids Type Size (ins.) Microns

Cuttings/Cavings >0.0165 440 - 1500

API Sand 0.0165 - 0.0029 440 - 74

Silt <0.0029 74 - 2

Barytes " 60 - 2

Clay " 2 - 0.5

Note: Some manufacturers do not use API standard for their classificationof the screen size. Some equivalents for other manufacturers areincluded in the table 6.6.


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Table 6.6: Equivalent Screen Sizes

Brandt Derrick Equivalent mesh

S 30/ B 50/ B 60 D x 38 30 x 30

S 40 40 x 40

S 50/ B 80/ B 100 D x 50 50 x 50

S 60 60 x 60

S 80 / B 120 D x 70 80 x 80

S 100 D x 84 100 x 100

D x 110 120 x 120

Screen Type

The most common shaker screens are 'market grade' (MG). They have a squaremesh e.g. 80 x 80, with a supporting coarse mesh backing for support. MGscreens are also available in a rectangular (oblong) format e.g. 60 x 40.Rectangular screens are capable of handling higher flow rates and are lesssensitive to screen blinding. They are however less effective at solids separationin comparison to square mesh screen which have a mesh size equal to the smallerof the sides of the rectangular mesh. The corrugated type of screen (derrickscreen) is currently preferable as it demonstrates high efficiency in separatingcuttings from the mud.

Sandwich or layered screens consist of three layers. A fine top layer, a slightlycoarse middle layer and coarse bottom layer for support. The advantage of thissystem is that it is less sensitive to near-size particle plugging.

d) Shale Shaker Screen Selection

The correct selection of shaker screen size is essential if the potential of thesolids removal equipment is to be maximised. Selection depends on the shakerdesign, mud properties, flow rate, ROP, volume and type of cuttings. The finestscreens available should always be used such that the mud covers approximately75% of the screen length.

In general, the following screen sizes are recommended for use in SPDCoperations:


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For Surface Hole Sections (24", 17-1/2")

Screen Size Depth

30 x 30 Down to 4000 ft.

50 x 50 Down to 4000 ft.

For immediate and Lower Hole Sections (12-1/2", 8-1/2")

Screen Size Depth

80 x 80 Down to 7000 ft.

100 x 100 Down to 1000 ft.

120 x 120 Below 10,000 ft.

e) Operation and Maintenance

Shakers are rugged and require little maintenance. The most vulnerable parts areshakers screens, which are continuously exposed to erosive and abrasive fluidsand solids. Certain routine operating procedures will improve screen life andallow fine screens to be run at minimal cost. A few recommendations on theoperations and maintenance of shale shakers are listed below.

1. Use all available shale shakers. If the full mud flow can be handled withoutusing all the shakers, then change the screens to smaller sizes.

2. Select screens such that mud will cover 75% of the total length.

3. When using more than one shaker, ensure equal fluid and solid distribution. Aimfor uniform screen sizes on all shakers. The flow line should enter at the bottomcentre of the header tank.

4. Never by-pass the shale shakers during normal operations, including trips, toavoid large particles plugging downstream equipment.

5. Repair or replace torn screens immediately.


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6. A water hose should be provided to wash down the screens at the start of eachtrip or any long term interruption in circulation, thereby minimising blinding andabrasive wear at start up and the build up of corrosive chemicals.

7. Ensure that the proper screen support cushions are installed in accordance withthe manufacturer’s instructions and that screens are properly tensioned inaccordance with the manufacturer's instructions.

8. Ensure that the proper screen support cushions are installed in accordance withthe manufacturer's instructions and that screens are properly tensioned inaccordance with the manufacturer's recommendations. Check tension daily.

9. Clean and lubricate tensioning hardware on each screen change to ensure thattorque readings are reliable.

10. Ensure that solids are being conveyed uniformly off the discharge end of thescreens. Otherwise, a build up solids and subsequent tearing of the screen mayresult. Adjusting the screen tension may eliminate this.

11. Ensure that cuttings are being conveyed in the correct direction, if not, it is likelythat the vibrator motor is operating in the wrong direction.

12. Switch off the shaker when not circulating.

6.7.3 HYDROCYCLONES

Hydrocyclones (desanders, desilters and mud cleaners) are compact devices inwhich drilled solids are separated from the mud by means of forcedsedimentation.Solid laden mud is pumped into the hydrocyclone via a tangential inlet at a highvelocity, creating a swirl in the cylindrical and conical sections of the separationchamber, producing centrifugal forces. These forces direct the solids to thechamber wall where the downward velocity carries them towards the underflowsolids discharge.

The solids separate according to weights and sizes with the largest particlessettling first. As the cone section narrows, the liquid is reversed at the "balancepoint" due to the increasing centrifugal forces near the centre and leaves via theoverflow while, the solids are discharged at the underflow.


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When operating optimally, the discha rge should be of an "intermittent spray"type underflow. If it is of a "rope type" underflow, then the unit is eitheroverloaded or the feed pressure is low. A reduction of the solids load can beachieved by the installation of finer shaker screens, reducing the mud viscosityand installing more cyclone capacity. Increasing the underflow aperture willallow the equipment to handle greater solids content. If the discharge from theunderflow is a solid stream of mud with approximately the same density as thecirculating mud, then, it is likely that the hydrocyclone feed is plugged. The feedmanifold should be cleaned immediately. This problem is often caused by tornor by-passed shaker screens or by blockages to the feed pump.

Underflow plugging or severe cone wear may cause a total stoppage of discharge.Use a welding rod or brush to clean the obstruction. To ensure the optimaloperation of hydrocyclones, the following rules should be applied.

1. Start the hydrocyclones before the rig pumps are started and stop thehydrocyclones after the rig pumps are stopped.

2. Never by-pass the shaker and ensure that the correct screens are used and are nottorn.

3. Ensure that all cyclone suction pits are agitated and the correct feed pressure ismaintained.

4. If the discharge is not of an intermittent spray pattern, check for a blocked inlet,insufficient feed pressure or worn cone parts. A continuous wet, fine sprayindicates worn apex valves.

5. Check regularly, clean and service the equipment.

a) Desanders

Desanders are usually 10" or 12" hydrocyclones with a flow capacity ofapproximately 500 gal/min. They are designed to remove solids larger than 74microns, which have passed through the smaller shaker screens. Ensure that:


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1. It operates with a spray type underflow and that the correct feed pressure ismaintained (see operating manual for the exact figure), normally between 30 and40 psi depending on mud weight.

2. It is run continuously while drilling loose sand beds.

3. It is run as required to reduce overloading of the desilters while drilling withweighted or unweighted mud.

b) Desilters

Desilters are smaller hydrocyclones. Usually they have a cone diameter of 4inches or less, with a flow capacity of approximately, 50 gal/min. They aredesigned to remove solids larger than 4 microns (i.e.) 'Silt '. For optimalperformance:-

1. They should operate with a spray type underflow.

2. The correct feed pressure should be maintained (see operating manual for theexact figure). This is normally between 35 and 50 psi, depending on mud weight.

3. They should be run continuously while drilling loose sand beds and that at alltimes while using unweighted mud.

4. They should be run as required to reduce low gravity solids while using weightedmuds. large volumes of barytes will be discarded in this situation, it is thereforerecommended that the underflow is processed using a centrifuge.

c) Mud Cleaners :

Mud cleaners are hydrocyclones (desilters) mounted on a fine mesh (150 or 200mesh) shaker screen. The underflow from the desilters are discharged onto thescreen where most of the drilled solids are removed, allowing the barytes and theliquid phase of the mud, as well as the finer drill solids, to return to the activesystem. They should be used at all times when using weighted muds with theunderflow being processed by a centrifuge to prevent the build up of low gravitysolids.


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6.7.4 CENTRIFUGES

Centrifuges are used to remove fine solids, or liquids that contain colloidalparticles, from weighted and unweighted muds. The centrifuges used in SPDCoperations are of the 'decanting' type. A decanter is a centrifugal separatorhaving a horizontal axis of rotation. The liquid/slurry enters the rotating bowlwhere the solids are separated from the liquid phase. The solids are forced to theinner wall of the bowl while the liquid forms a ring inside the solids. A screwtype conveyor transports that solids towards the tapered end of a conical bowl.The liquid flows in a spiral path, established by the conveyor flights, to thedischarge ports at the large end of the bowl.

In normal operation, a centrifuge will separate most of the silt size and largersolids to the underflow. Most colloidal and clay size solids along with the liquid,will be discharged out of the overflow. Centrifuges can process flow rates of 200- 250 gals/min, however, the higher the specific gravity of the feed, the lower thethroughput.

a) Unweighted Muds

The build up of the solids in unweighted muds lead to increase mud weight andviscosity, both of which are undesirable and can lead to heavy usage of chemicalsdue to dilution. This can be counteracted by removing the solids from the activemud system and from the hydrocyclone underflow, using a centrifuge.

b) Weighted Muds

Drilled solids, which are of the same size range as Barytes cannot be removed byscreening or in hydrocyclones. However, due to the lower density of the drilledsolids, the barytes will be discharged through the underflow while, the overflowwill contain the small size and colloidal solids which are primarily responsiblefor increased viscosity. The overflow can be dumped, if the base fluid of themud is cheap or further treated using a high speed centrifuge with the solidunderflow dumped and the liquid routed to the active system.


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6.7.5 FLUID ROUTING

Proper routing of fluids through the solids removal equipment is essential ifmaximum solids removal is to be achieved.

1. The overflow of each piece of equipment discharges to the compartmentdownstream through the suction compartment of that piece of equipment.

2. The suction compartment for the desilter should be either the same to which thedesander overflow is discharged or one further downstream.

3. Two pieces of equipment must not have the same suction compartment or thesame overflow discharge compartment.

4. If the total processing capacity of the centrifuge(s) is less than the circulatingmud flow, the desilter underflow should be processed. In this way, the entirecirculating system is processed.

6.7.6 TROUBLE SHOOTING IN SOLIDS CONTROL EQUIPMENT

a) Shale ShakerTable 6.7 Trouble shooting in a shale shaker

Problem High Mud Losses Screen Overloading

Indications Mud losses over screenHeavy build up of cuttings on thescreen

CausesWrong screen size. High viscositymud. High circulating rate. screenblinding

High drilling rate Shaker angle too

steep.

Remedy Change to a larger screen size. Adjust

shaker angle. Ensure equitable flow over

shaker.

Reduce mud viscosity.

Reduce flow rate. Change to

Rectangular mesh screens.

Wash down shaker screens using

a high pressure water hose not a

wire-brush. Adjust shaker angle.

b) Hydrocyclones (Desanders, Desilters and Mud Cleaners)


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Table 6.8 Trouble shooting in Hydrocyclones

Problem High Mud Losses Screen Overloading

Indications Rope type flow from discharge.

High solids content (>50%) in

Discharge.

No discharge from underflow.

CausesIncorrect feed pressure. Overloadingof hydrocyclone. Improperlyinstalled or worn cone bladder.Plugged feed nozzle or feed inlet.

Underflow blocked.

Feed inlet blocked.

Remedy Adjust feed pressure: Check

Manufacturers pressure

recommendation. Install finer shaker

screens Remove cone and clean out

blockage. Replace improperly

installed or worn Bladder. Increase

underflow aperture.Reduce mud

viscosity.

Remove blockage from underflow

nozzle. Remove blockage from

inlet.

c) Centrifuges

Centrifuges should only be serviced by properly trained personnel. Ensuring thatcorrect start up and shut down procedures are followed which will preventplugging of the bowl/scroll and reduce service requirements.

6.7.7 SOLIDS CONTROL EQUIPMENT MAINTENANCE

After each well, during rig move, prior to spudding the well, or as requiredbetween hole sections carry out the following.

1. Flush the system with water to remove any caked solids that could lead to theplugging of feed nozzles.

2. Strip down the unit and check for worn bladders, cone wear (particularly the feednozzles) and liner body.

3. Check centrifugal pumps for impeller shaft or casing wear and motorperformance. Change or service as required.


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3. Run the installation with water. Ensure that the feed pressures are correct. Thesewill be slightly lower than as for mud.

5. Ensure sufficient spare parts and screens are on site.


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CHAPTER 6: Drilling Fluids Readers' Form

Use this form to communicate:

1. Any inaccuracies or confusion in the text.

2. Ease of finding information.

3. Adequacy of reference

4. Too many or too little detail

5. What is missing

6. What is not required

7. Any other suggestions or comments.

NAME: DEPT.: TELEPHONE:

Please despatch form to DWE-TEC or DWW-TEC

drilling fluids

Documents

mud preparation

mud consultant

mud testing procedures

bentonite mud systems

tame mud systems

silicte mud systems

description of mud systems

spdc mud programme