Drilling Fluids

STANDARD DRILLING PROCEDURES MANUAL DRILLING FLUIDS

CHAPTER 6 REVISION 2

PAGE 1 08-05-03

CHAPTER INDEX BULLETIN ITEM

PAGE

6.1

INTRODUCTION

3

6.2 6.2.1 6.2.2

PROGRAMMING AND REPORTING Programming Reporting

3 3 3

6.3 6.3.1 6.3.2

DESCRIPTION OF MUD SYSTEMS Functions of Drilling Fluids Minimum Bentonite and Barytes stocks

4 5 6

6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7 6.4.8

MUD PREPARATION AND TREATMENT General Bentonite Mud Systems KCl Polymer Mud Systems KCl Polymer/Gypsum Mud Systems Silicte Mud Systems TAME Mud Systems Pseudo Oil Based Mud (POBM) Low Toxicity Oil Based Mud (LTOBM)

6 6 8 10 12 13 15 17 19

6.5 6.5.1 6.5.2 6.5.3 6.5.4 6.5.5 6.5.6

SPECIAL PROCEDURES Mixing Caustic Soda and caustic Potash Acids Drilling Detergent Lost Circulation Stuck pipe and Spotting Fluids Use of Mud Lubricants in Water Base Mud in High Angle Wells

20 20 21 21 21 22

6.6 6.6.1 6.6.2 6.6.3

MUD ENGINEERING PRACTICES Mud Testing Procedures Reporting Guidelines Quality Control

24 24 25 25

6.6.4

Pilot Testing

25

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23



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6.6.5

Mud Laboratory Testing Equipment and Chemicals

26

6.7 6.7.1 6.7.2 6.7.3 6.7.4 6.7.5 6.7.6 6.7.7

SOLIDS CONTROL General Shale Shakers Hydrocyclones Centrifuges Fluid Routing Trouble Shooting in Solids Control Equipment Solids Control Equipment Maintenance

28 28 29 34 37 38 38 39

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6.1


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INTRODUCTION This chapter on drilling fluids (muds) has been prepared with the objective of providing a practical instruction in mud mixing and treatment. The different mud systems used in SPDC have been highlighted with a section on trouble shooting. The responsibility for programming, maintenance of the mud system and reporting are discussed. More detailed information on individual mud systems can be found in the Drilling Fluids Manual EP88 - 2637 and various mud engineering companies' manuals. Any variation from the recommended procedures should be approved by the Senior Drilling Engineer (SDE). Note: All chemicals used on the rig must have been approved by the PC Laboratory. 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 the Senior Drilling Engineer (SDE), in consultation with the mud consultant. A detailed programme, inclusive of mud type, properties, estimated consumption, costs and treatment should be requested from the contractor Mud Engineering company and will form the basis of the SPDC mud programme. Expert advice should be requested from POX-CEM and the DWX-TEC mud focal point as required and should always be requested in the case of non-routine mud systems or 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 maintenance of the mud system, within the guidelines of the drilling programme. He will report 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 with SPDC 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 measured every fifteen (15 ) minutes or as directed by the DSV. Any significant changes in mud properties must be reported immediately to the DSV and the Mud Engineer. The DSV reports to the SDE/office Drilling Engineer who will consult and relay information and advice from POX-CEM and DWX-TEC drilling fluid focal points.

6.3

DESCRIPTION OF MUD SYSTEMS Currently, water based mud (WBM) and pseudo oil based mud (POBM) are used in land/swamp drilling operations, while in offshore drilling activities WBM and Low Toxicity Oil Based Mud (LTOBM) are used. The mud type selected for a particular well section depends on the following considerations: • • • • • • • • • •

DWE:

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

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

WBM ü

POBM/LTOBM ü

ü* ü 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 presently restricted to the following:1. 2. 3. 4. 5. 6.

Spud water based muds. Non-dispersed polymer water based muds. KCl-Silicate-Polymer water based muds. Esters Pseudo Oil Based muds (POBM)/synthetic Based Muds. 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 be considered:

1. 2. 3. 4. 5. 6. 7. 8. DWE:

To transport drilled cuttings (or cavings) to the surface. To control sub-surface pressures. To cool and lubricate the bit and drill string. To minimise washouts and damage to the well bore. To suspend cuttings, weight material and other solids when circulation is stopped. To transmit hydraulic power to the bit. To form low permeability filter cake at the borehole face. To provide mechanical stability in uncased sections of the well bore. DATE:



9. 10.

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

6.3.2

MINIMUM BARYTES AND BENTONITE STOCKS

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The following minimum stock levels of barytes and bentonite (Table 6.2) are based on the requirement to increase the density gradient of the complete circulating system by 0.052 psi/ft (1.0 lbs/gal) in the event of a kick. The stocking level of Barytes and Bentonite must never fall below these levels. Should stocks fall below these levels, the rig must cease drilling operations, pull to 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 sufficient stock of all drilling chemicals, barytes and bentonite are maintained on the rig to allow uninterrupted drilling for 7 days.

6.4

MUD PREPARATION AND TREATMENT

6.4.1

GENERAL

1.

All mud preparation and treatment should be carried out under the supervision of the contract Mud Engineer. Good communication is imperative for a successful operation to be achieved. It is recommended that: The Mud Engineer is provided with a copy of the drilling programme.

2.

DWE:

No additions of chemicals should be made without the approval of the Mud Engineer.

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3.

The Mud Engineer should be present at tour changes to ensure that correct instructions 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 some form of interactive between the mud and formation. Remedial treatment or modification of the programmed properties will be required. The Mud Engineer should pilot-test suggested modifications of programmed mud properties 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 for approval. The SDE should notify POX-CEM and the DWX-TEC drilling fluids focal point of such changes, so that they can monitor proceedings and note them for future programming.

b)

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

1.

Dangerous chemicals (caustic, lime, acids, etc.) should be roped off and stored separately. A warning sign indicating that the chemicals are of a dangerous nature should be erected so that it is in clear view of all personnel working in the area.

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 the waste management team. Dumping of chemical wastes in the swamp is not an acceptable method of disposal.

DWE:

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c)


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Inventory Control Sufficient stocks of chemicals should be maintained at all times to enable a smooth drilling operation to progress. Due to logistical and supply problems, it is recommended that sufficient chemicals are kept on site to allow the present section to be drilled. Where this is not possible, a minimum of two weeks stocks should be maintained. Adequate lead times should be allowed for the re-supply of chemicals. If chemical stocks fall to a level such that the correct treatment of the mud system is not possible, then drilling must stop until such time as sufficient chemicals are available. Chemical stocks should be physically checked on a regular basis. A weekly physical stock check is mandatory.

6.4.2

BENTONITE MUD SYSTEMS In a sea water (salt water) environment, bentonite yield is greatly reduced (low viscosity) because of the presence of ions like Ca++, Mg++, Cl- , etc. For such areas, the limitation is overcome by prehydrating the bentonite in fresh water. The prehydrated bentonite is blended with sea water. Higher rheology can be achieved by using HV polymers. Below the surface casing shoe, LV filteration control materials can be added to adjust the water loss properties. As most of the source water is saline, prehydration is always desired to obtain maximum yield. Water for prehydrated bentonite must be fresh (salinity < 2000 mg/l). The water should 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. Soda Ash addition is to reduce the Ca ++ and Mg++ ions present in the water. Allowing some hydration time initially, will deflocculate the mud and allow more bentonite addition. 4.

Dilute prehydrated Bentonite slurry to 20 - 25 lbs/bbl by adding equal volume of sea 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 to mix fresh mud in the mixing pit and slowly bleed it into the active system than to dilute with water and add chemicals directly into the active system. Table 6.3 gives a summary of the most common problems encountered while drilling with a Bentonite mud system together with suggested causes and treatment. More details can be found in the Drilling Fluids Manual. Table 6.3:

Contaminant Clays & shales Sand & Silt Cement

MBT

pH

+++

Fluid Loss +

++

-

Alkalinity -

+(PV)

+

+

+++

++

++

++

Anhydrite

+++

++

Lignite

--

-

---

++

Surface water

+ DWE:

Den sity +

Visc.

+

The Effects of Contaminants on Bentonite Mud

-

: :

Cl -

Ca ++

++

+

-

-

-

Treatment Required Dilution. Treat with Caustic Soda Solids removal. Dilution / addition of fresh mud. Pre-treat with Sodium Bicarbonate. Add filteration materials to restore fluid loss. Dump contaminated mud. Add Soda Ash. Restore fluid loss. Consider conversion to GYP. System. Increase viscosity and treat for pH.

-

++ +

++

Parameter increases with contaminant Parameter decreases with contaminant. DATE:

Increase density and rheology. Reduce fluid loss.



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

a)

Composition and Preparation Dilute 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/bbl KOH/Caustis Soda 0.23 – 0.30 Ibs/bbl KCl 25 – 30 Ibs/bbl Starch 4 Ibs/bbl Pac-R 1 – 1.5 Ibs/bbl Pac-L 0.5 – 1.0 Ibs/bbl *XCD Polymer 0.25 – 0.5 Ibs/bbl Lubricant 0.5 – 2.0 % v/v Barite (as required) Contingency LCM Shale stabilizers Pipe free agent * optional (for suspension)

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

Treatment Whole new mud dilution is essential for effective treatment of KCl/Polymer muds. Fresh mud should be prepared in the reserve pits and slowly added to the active system. Fresh whole mud should be prepared as above. However, no further Bentonite should be added. KCl/Polymer muds are highly sensitive to drilled solids, with an associated detrimental effect on both viscosity and gels. It is therefore very important that all solids removal equipment is functioning in an optimal manner. Table 6.4 gives a summary of the most common problems encountered while drilling with a KCl/Polymer mud system together with suggested causes and treatment. More details can be found in the Drilling Fluids manual.

Table 6.4: Contaminant Clays & shales

Den sity +

Visc.

Sand & Silt

+

MBT

PH

++

Fluid Loss +

++

-

Alkalinity -

+(PV)

+

+

++

++

Cement

Anhydri te Surface water

The Effects of Contaminants on KCl/Polymer Muds

+

--

DWE:

+++

++

---

+

Cl -

a+

K + and Polymer --

++

--

++ -

-

-

--

--

DATE:

Treatment Required Add KCl and Polymer. Dump part of old mud and replace by fresh mud. Maintain MBT < 25. Improve solids removal. Solids removal. Dilution/addition of fresh mud. Pre-treat with Sodium Bicarbonate. Restore fluid loss and Polymer content. Dump contaminated mud. No treatment required. Increase density and rheology. Add Caustic or Potash 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 contaminant with +++ 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 hrdratable shales. The system combines the inhibitive properties of K+ and Ca++ to stabilise the formation. The rheology of the system is easy to control.

a)

Composition and Preparation Note: Pre-hydrate Bentonite in fresh water. Volume increases can be made with sea water. Bentonite KOH KCl Gypsum Starch Pac-R/L Contingency XCD Lube LCM Shale stabilizer Barite for weight

b)

-

0 – 5 Ibs/bbl 0.2 – 0.35 Ibs/bbl 25 – 30 Ibs/bbl (as reqd.) 4 – 6 Ibs/bbl 4 Ibs/bbl 1.5 – 2 Ibs/bbl

-

0.5 – 0.75 Ibs/bbl 2 – 4 % v/v

Treatment

Whole new mud dilution is essential for effective treatment of muds. Dump sand trap through waste management system inorder to create space for new mud volume. Fresh mud should be prepared in the reserve pits and slowly added to the active system in batches. Fresh whole mud should be prepared as above. However, no further Bentonite 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 to drilled solids, with an associated detrimental effect on both viscosity and gels. It is therefore very important that all solids removal equipment is functioning in an optimal manner. 6.4.5

SILICATE MUD SYSTEM

Silicate mud system is a highly inhibitive and environmentally friendly water based mud systems. It is particularly suitable for drilling reactive formations such as clays and Shales, highly dispersible formation such as chalk and unconsolidated sands. This mud system utilises sodium or potassium silicate to provide primary inhibition while monovalent salts such as sodium and potassium chloride provide the secondary inhibtion. Commercial silicate liquors have molecular ratios typically between 1.5 and 3.3. Silicate liquor with molar ratio of 2.0 is preferred for mixing silicate mud systems. Silicate mud provides shale inhibition by two mechanisms. Monosilicates polymerise easily to form negatively charged oligomers which can easily penetrate the shale micropore 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 pore fluids ( pH +/- 7). The pore fluids dilute the filtrate (lowers the pH ) resulting in the formation 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 the shale pores to form insoluble precipitates. The gelled and precipitated silicates thus stabilise the shales by providing physical barriers, which prevents further mud filtrate invasion and pore pressure penetration. The wellbore and shale are thus ‘pressure isolated’. 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 lead to the need for large volume dump and dilution to restore rheology and filtration properties. 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 81/2” holes may be adhered to. The build up of low gravity solids should be avoided early enough 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 is attributed to silicates preferentially wetting steel tubulars, mud pits and contaminants in the circulating system. Depletion occurs also due to contact of silicates with formation. Depletion levels are higher in anhydrite, dolomite and highly reactive shales, lower in low reactive shales and lowest in clean sands. Acid gases can also deplete silicates rapidly as the silicates leave solution due to declining pH. To re-establish the recommended silicate ion level for a given application, addition of fresh sodium silicate liquor need to be done at intervals. Therefore in addition to the API mud check, SiO 2 and Na2O concentration has to be included in routine mud check. Refer to the contractors drilling fluids manuals on 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 will depend on the application. Moreover, most conventional lubricants are not effective in Silicate mud. Therefore, polystyrene beads may be added in high angle wells. However, the following formulation and order of addition could serve as a guide. Water KCL Sodium silicate PAC R (or equivalent) PAC L (or equivalent) Starch XC-Polymer (or equivalent) Barite Lubricant

DWE:

: : : : : : : :

0.9 bbl 30 lbs/bbl 640 lb/bbl : 0.5 lb/bbl 1.8 lbs/bbl 5 lbs/bbl 0.6 lb/bbl as required 5% v/v

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Table 6.4.5: The Effects of Contaminants on Silicate Mud.

Contaminant

Symptoms

Treatment required

CO2

Total SiO 2 depletion

Raise pH with NaOH

Serious gelling of system

Replace silicates

Decreased pH

Dilute with new mud

Increase in fluid loss

Raise density Replace polymers

Anhydrite

Slight-high SiO2 depletion

Replace silicates

Increased PV/YP

Dilute and add polymers as required

Increase in fluid loss Reduced pH H2 S

High SiO2 depletion

Raise pH with NaOH

Gelling of mud

Replace silicates

Slight decrease in Ph

Dilute with new mud


Raise density Add scavenger (e.g. zinc oxide)

Clay solids

Cement

Slight SiO2 depletion

Replace silicates

Slight increase in PV

Dilute mud

Increase in MBT

Run solids control equipment

High SiO2 depletion

Replace silicates


Add polymers

Slight/no drop in PV/YP

6.4.6

TAME MUD SYSTEM Thermally activated mud emulsion (TAME) drilling fluid is an improved KCl/polymer mud for the minimisation of mud related borehole instability problems in shales. TAME drilling fluid is an inhibitive mud system, which is capable of minimising fluid invasion into shale pores, thereby reducing the effect of the associated pore pressure penetration. The inhibitive property of the mud is derived 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 as polyalkylene 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 the shale pores is impeded by TAME due to its possession of viscosified filtrate and its ability to cloud-out and block shale pores at the elevated temperature normally experienced downhole. The Cloud Point Temperature (CPT) of a polyglycol is the temperature at which it changes from water soluble to water insoluble. When CPT is reached, the polyglycol starts to come out of solution and an otherwise clear liquid starts to become opaque and cloudy. Control of CPT is critical for the inhibitive characteristics of the fluid and is a function of salinity and type and concentration of polyglycol. TAME should be designed and formulated by ensuring that its down hole cloud point temperature (true CPT) is equal or above the bottom hole circulating temperature (BHCT) but below the bottom hole static temperature (BHST). The BHCT data to be used in TAME design, are those taken with the measurement while drilling (MWD) tool. Surface CPT is the cloud point temperature taken at surface under atmospheric pressure. 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. If total hardness is above 400ppm, treat with Soda ash (Na2 CO3 ).

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.

DWE:

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b)


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Treatment Whole new mud dilution is essential for effective treatment of TAME drilling fluid. Fresh mud should be prepared in the reserve pits and slowly added to the active 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 associated detrimental effect on both viscosity and gels. It is therefore very important that all 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 while Synteq and Ultidrill has isomerised olefins and linear alkyl olefins (LAO) respectively as the continous phase. These muds systems are claimed to be nontoxic and biodegradable, therefore environmentally tolerable. Disposal of the cuttings generated while drilling with these POBMs should be in accordance with DPR guidelines and standards. The olefins or ester, which is in contact with the formation, prevents shale from its 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 CaCl2 brine in the POBM osmotically dehydrates the shale penetrated and the drilled cuttings to ensure minimal mud maintenance and effective solids removal at surface. These positive attributes of OBM provide the opportunity to drill shales at a 0.024 - 0.12 SG lower mud weight than would be required with a water based mud. POBM gives excellent lubricity resulting in low torque and overpull.

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

The table below provides a guide for the oil:water ratio to use for a given mud density. Note that these values could vary depending on the kinematic viscosity of 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 DWE:

DATE:



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

mud gradient range (psi/ft) < 0.47 0.47 – 0.57 0.57 – 0.72 0.72 – 0.83 0.83 – 0.98 > 0.98

oil:water ratio 60:40 65:35 70:30 75:25 80:20 90:10

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

Source

Recommended action(s)

Starting POBM

Confirm mud volume prepared/received using calibrated dipstick

volume 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

Displace with large volume of spacer, e.g.

when cementing Dumped (mud

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

tank dead

cylindrical or bevelled mud tanks where available.

volume) 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 control program will ultimately improve overall efficiency. DWE:

DATE:


6.4.8


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LOW TOXICITY OIL BASED MUD (LTOBM) The low toxicity oil based mud currently being used in SPDC is Enviromul. The LTOBM has mineral oil as the continuous liquid phase. These muds are nonbiodegradable, therefore environmentally unfriendly.

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

Although oil muds have a greater tolerance to drill solids, a correct solids control program 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

Adjust density. add lime, salt emulsifiers

Note: 1) 2) DWE:

Effects are minimal if cement is not green. Add oil wetting agent.

Zn or Fe Add fresh water scavengers. Add to dissolve lime. Increase insoluble salt. density if required.

> - means Increase < - means Decrease DATE:

Dilute with oil. Add emulsifier. Improve solids control.



6.5

SPECIAL PROCEDURES

6.5.1

MIXING CAUSTIC SODA AND CAUSTIC POTASH:

PAGE 20 08-05-03

Caustic Soda and other such products are highly corrosive and require special handling and mixing procedures. The proper safety equipment and clothing must be provided and used at all times. a)

Safety Equipment and Clothing

1. 2. 3.

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

4. 5. 6. 7.

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

b)

Mixing Equipment Chemical barrel: This should be light-weight and easily movable with a top mounted, hand cranked paddle mixer and 1" bottom discharge valve to which a hose 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 must be supplied in drums, not sacks).

3.

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

6.5.2

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

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transferred using transfer pump only. Handling by untrained rig labour should be minimal and only under strict supervision complying with safety regulations. A safety meeting should be held immediately prior to the handling of acids, with all parties made aware of the operations to be performed, potential hazards and remedial actions to be taken in case of spillage or leaks. Extra care and vigilance is required on high pressure pumping operations. Wash down facilities, with running water, should be available as well as Sodium Bicarbonate for neutralising any spills. Full-face vapour type masks are mandatory. It should be noted that the use of ordinary face shields and goggles is not acceptable. The mixing area should be cleared of non essential personnel. In the case of high pressure displacement, no one should be close to or work across lines while pumping. The rig floor should be kept clear of unnecessary personnel; those on watch should be properly attired.

6.5.3

DRILLING DETERGENT Drilling Detergent (DD) is primarily used in surface and intermediate hole sections to minimise bit balling problems caused by reactive clays. Use should be initiated prior to penetrating known problem zones. Treatment should be continued as required until casing depth has been reached. DD should not be used as an integral part of a weighted mud system as it may cause settling of Barytes. 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 chapter 8, "Hole Problems and Fishing". Information on the preparation of different types 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 in chapter 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 with only high viscous pill pumped at regular interval. However, wells with inclination greater than 50 degrees suffer from cuttings bed formation which has to be swept clean with a combination of low viscous light weighted and high viscous heavy weighted pills pumped in tandem every 3 or 4 stands drilled. The low vis pill's design and formulation should be done to ensure that the pill is in turbulent flow regime in the critical annulus downhole. Optimum flow rate, pipe rotation coupled with controlled drilling rate and frequent short trips will help in preventing the formation of cuttings bed in the highly deviated/horizontal section. The recommended volume of tandem pills for hole cleaning is given in Table 6.6 below. Table 6.6: Tandem pills used in cleaning hole with inclination greater than 50 degrees.

17 ½" hole 12 ¼" hole 8 1/2" hole 6" hole

Low viscosity Light weight 30 bbl 20 bbl 15 bbl 10 bbl

High viscosity Heavy weight 35 bbl 25 bbl 15 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 flow rate coupled with pipe rotation to create the required turbulence along the section, where cuttings bed must have been formed.

b)

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MUD WEIGHT PREDICTION Using the right mud weight is crucial to the mechanical stability of wellbore irrespective of the mud types (WBM, LTOBM & POBM). Stabor was developed DATE:



PAGE 23 08-05-03

by SIEP to predict mud weight without taking into consideration the effect of shale reactivity with the drilling fluid. Stabor input is either derived from log data or obtained from shale cores. The accuracy of input determines the accuracy of the Stabor's output. Prediction from Stabor together with the data from correlating wells should be used in determining the optimum mud weight for the particular well. Stabor's Input Facilitator should be used to generate the required input for stabor calculations. 6.5.6

USE OF MUD LUBRICANTS IN WATER-BASE MUD IN HIGH ANGLE WELLS Additional friction between the drill string and borehole is expected when drilling a hole off-vertical. This implies that the torque and drag will increase with increase in deviation. Moreover, the frictional resistance generated may require considerable 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 this problem. Desirable characteristics of a lubricant for water-base mud are: i) ii) iii)

The material must perform well as a lubricant It must be non-toxic and biodegradable 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 fatty acid compounds (e.g.: EP Mud lube, Bit lube) and blends of triglycerides and alcohols (e.g.: Torque trim, magcolube). Liquid lubricants are added to the mud system at a range of 2-6 Ib/bbl depending on torque or drag severity, while lubrabead pill of about 8 Ib/bbl is usually spotted in the region of high torque and drag.

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6.6

MUD ENGINEERING PRACTICES

6.6.1

MUD TESTING PROCEDURES


PAGE 24 08-05-03

Measurement should be made using standard procedures described in API recommended practice 13B - 1 "Standard Procedure for Field testing Water based Drilling Fluids". See appendix II - 1 of the drilling fluids manual (EP 88 2637). Note: A "Halliburton" pressurised true weight balance must be available of the 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 of one representative check, taken active circulation, should be taken each tour. During extended periods of non-circulation, only one full mud check need to be reported 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 following occasions.

1.

When circulating prior to logging or running casing. The timing of the tests should be such that a representative mud sample is tested and that there is still time 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 following procedures should be adopted for new batches of chemicals, barytes and Bentonite 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 of the 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 suppliers should 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 quality will be performed. All samples should be accompanied by an Analysis Request form, which should be available on the rig.

5.

Always pilot test new batches of chemicals. If they appear sub-standard, contact the SDE and inform him of the chemical name, batch number and date a sample was sent to POX-CEM. Do not use any of this batch until permission has been obtained 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 mud contractor 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 to POX-CEM for testing. Additional samples may be sent if required. All mud samples should be accompanied by the wellsite test data. Samples should be properly packed and tagged. A telex should be sent to the SDE and POX-CEM informing them that the sample 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 in the 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 engineering company. The details of equipment and reagents required to conduct mud analysis could be found in Appendix II - 1 of the Drilling Fluids Manual Report EP88-2637 (1988). Below is a listing of all mud testing equipment required on the well site.

a)

Mud Testing Equipment:

1. 2. 3. 4. 5. 6.

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

1 pc. 1 pc. 1 pc. 1 pc. 1 pc. 4 boxes

7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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

1 pc. 1 pc. 2 pcs. 2 pcs. each 2 pcs. 2 pkts. each 2 pcs. 1 pc. each 2 pcs. 1 pc.

b)

Glassware:

1. 2. 3. 4.

Test tube brushes (various sizes) Porcelain dish for titrations Long spatula 20 cm blade Steel wool

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4 pcs. 2 pcs. 2 pcs. 4 pcs. DATE:



5. 6. 7.

Thermometer (50 - 150 deg. F.) Mud sample cans (1/2 gallon) Mini weight balance

c)

Chemicals:

1. 2.

Methyl orange indicator Silver nitrate solution 0.01N and 0.0282N (4.79 gms. in one litre) and 0.0282N (47.9 gms. in one litre) Distilled water in plastic cans Potassium chromate 5% solution Phenolphthalein indicator (5 gms. in 100ml of water) Silver nitrate 0.1N solution Sulphuric acid 0.2N solution Standard versenate solution (EDTA) Calcium buffer solution Calcium indicator tablets

3. 4. 5. 6. 7. 8. 9. 10.

PAGE 27 08-05-03

2 pcs. 5 pcs. 1 pc.

1 litre 1 litre 1 litre 1 btl. 1 btl. 1 litre 1 litre 1 litre 1/2 litre 1 btl.

Note: 1.

6.

Silver nitrate solution is to be kept in a dark bottle. The date of preparation of all solutions should be marked on their bottles and solutions should be replaced within three months. The listed equipment and chemicals should be available in the well site laboratory of every drilling rig. All laboratory equipment should be kept clean and in working order. Broken or faulty equipment must not be left in well site laboratory cupboards. All electrical equipment, plugs and sockets should be of an explosion proof type and should be switched off when not in use. Before a rig move, make sure that all equipment are stored away safely in the laboratory, to avoid breakages and losses during transport. Wellsite laboratories should be of a positive pressure type.

6.7

SOLIDS CONTROL

2. 3.

4. 5.

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6.7.1


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GENERAL This topic is more comprehensively covered in SIEP Drilling Fluids Manual EP 88-2637 to which reference should be made. The incorporation of drilled solids (low specific gravity solids) in the mud system is inevitable in all drilling operations. However, their presence leads to many problems 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 all times. This can be achieved by three methods, namely: -

a)

Prevention

1.

Using inhibitive drilling muds and correct mud weight to prevent wellbore destabilization and solids from being integrated into the mud systems (Polymer muds 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. to reduce 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 mixed mud (whole mud dilution). A description of the various mud types used in SPDC operations together with their properties, advantages and recommended methods of 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 on their size. The larger the cutting, the easier it is to remove. It is therefore essential that drilled solids are removed during their first stage through the solids removal equipment. Failure to do this may lead to a disintegration of the solids into smaller particle sizes. Presently, the following solids remova l equipment are used in SPDC operations : Shales shakers, hydrocyclones (desanders, desilters and mud cleaners) and centrifuges.

6.7.2

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

a)

Scalper Shakers Scalper shakers are used on some SPDC rigs to increase the mud handling capacity 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 the linear motion shakers to cope. Scalper shakers are shakers whose main objective is to remove the large cuttings and gumbo clays. In order that the whole mud volume 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 of increasing the screen life. b)

Linear Motion Shakers Linear motion shakers are used on most SPDC drilling rigs. The continuous linear motion provides rapid and efficient conveyance of drilled solids, allowing fine screening of the mud at high flow rates. The slope of the deck can be adjusted to allow for maximum retention of cuttings on the screen for improved separation of cuttings from the mud.

c)

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

c)

Shale Shaker Screens The performance of a shale shaker is highly dependent on the type of shaker screens 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 the screen. The larger the area, the better the performance. The screen surface is limited 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

8x8

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 classification of the screen size. Some equivalents for other manufacturers are included in the table 6.6.

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Table 6.6:


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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

S 60

50 x 50 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 square mesh e.g. 80 x 80, with a supporting coarse mesh backing for support. MG screens are also available in a rectangular (oblong) format e.g. 60 x 40. Rectangular screens are capable of handling higher flow rates and are less sensitive to screen blinding. They are however less effective at solids separation in comparison to square mesh screen which have a mesh size equal to the smaller of the sides of the rectangular mesh. The corrugated type of screen (derrick screen) is currently preferable as it demonstrates high efficiency in separating cuttings from the mud. Sandwich or layered screens consist of three layers. A fine top layer, a slightly coarse middle layer and coarse bottom layer for support. The advantage of this system 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 the solids removal equipment is to be maximised. Selection depends on the shaker design, mud properties, flow rate, ROP, volume and type of cuttings. The finest screens available should always be used such that the mud covers approximately 75% of the screen length. In general, the following screen sizes are recommended for use in SPDC operations:

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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 are shakers screens, which are continuously exposed to erosive and abrasive fluids and solids. Certain routine operating procedures will improve screen life and allow fine screens to be run at minimal cost. A few recommendations on the operations and maintenance of shale shakers are listed below.

1.

Use all available shale shakers. If the full mud flow can be handled without using 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. Aim for uniform screen sizes on all shakers. The flow line should enter at the bottom centre of the header tank.

4.

Never by-pass the shale shakers during normal operations, including trips, to avoid 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 each trip or any long term interruption in circulation, thereby minimising blinding and abrasive wear at start up and the build up of corrosive chemicals.

7.

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

8.

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

9.

Clean and lubricate tensioning hardware on each screen change to ensure that torque readings are reliable.

10.

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

11.

Ensure that cuttings are being conveyed in the correct direction, if not, it is likely that 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 in which drilled solids are separated from the mud by means of forced sedimentation. Solid laden mud is pumped into the hydrocyclone via a tangential inlet at a high velocity, creating a swirl in the cylindrical and conical sections of the separation chamber, producing centrifugal forces. These forces direct the solids to the chamber wall where the downward velocity carries them towards the underflow solids discharge. The solids separate according to weights and sizes with the largest particles settling first. As the cone section narrows, the liquid is reversed at the "balance point" due to the increasing centrifugal forces near the centre and leaves via the overflow 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 either overloaded or the feed pressure is low. A reduction of the solids load can be achieved by the installation of finer shaker screens, reducing the mud viscosity and installing more cyclone capacity. Increasing the underflow aperture will allow the equipment to handle greater solids content. If the discharge from the underflow is a solid stream of mud with approximately the same density as the circulating mud, then, it is likely that the hydrocyclone feed is plugged. The feed manifold should be cleaned immediately. This problem is often caused by torn or 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 optimal operation of hydrocyclones, the following rules should be applied. 1.

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

2.

Never by-pass the shaker and ensure that the correct screens are used and are not torn.

3.

Ensure that all cyclone suction pits are agitated and the correct feed pressure is maintained.

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 spray indicates worn apex valves.

5.

Check regularly, clean and service the equipment.

a)

Desanders Desanders are usually 10" or 12" hydrocyclones with a flow capacity of approximately 500 gal/min. They are designed to remove solids larger than 74 microns, 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 is maintained (see operating manual for the exact figure), normally between 30 and 40 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 with weighted or unweighted mud.

b)

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

1.

They should operate with a spray type underflow.

2.

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

3. 4.

They should be run continuously while drilling loose sand beds and that at all times while using unweighted mud. They should be run as required to reduce low gravity solids while using weighted muds. large volumes of barytes will be discarded in this situation, it is therefore recommended that the underflow is processed using a centrifuge.

c)

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

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6.7.4


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CENTRIFUGES Centrifuges are used to remove fine solids, or liquids that contain colloidal particles, from weighted and unweighted muds. The centrifuges used in SPDC operations are of the 'decanting' type. A decanter is a centrifugal separator having a horizontal axis of rotation. The liquid/slurry enters the rotating bowl where the solids are separated from the liquid phase. The solids are forced to the inner wall of the bowl while the liquid forms a ring inside the solids. A screw type 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 the discharge ports at the large end of the bowl. In normal operation, a centrifuge will separate most of the silt size and larger solids 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 the throughput.

a)

Unweighted Muds The build up of the solids in unweighted muds lead to increase mud weight and viscosity, both of which are undesirable and can lead to heavy usage of chemicals due to dilution. This can be counteracted by removing the solids from the active mud 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 by screening or in hydrocyclones. However, due to the lower density of the drilled solids, the barytes will be discharged through the underflow while, the overflow will contain the small size and colloidal solids which are primarily responsible for increased viscosity. The overflow can be dumped, if the base fluid of the mud is cheap or further treated using a high speed centrifuge with the solid underflow dumped and the liquid routed to the active system.

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6.7.5


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FLUID ROUTING Proper routing of fluids through the solids removal equipment is essential if maximum solids removal is to be achieved.

1.

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

2.

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

3.

Two pieces of equipment must not ha ve the same suction compartment or the same overflow discharge compartment.

4.

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

6.7.6

TROUBLE SHOOTING IN SOLIDS CONTROL EQUIPMENT

a)

Shale Shaker Table 6.7

Problem

High Mud Losses

Indications

Mud losses over screen

Trouble shooting in a shale shaker

Wrong screen size. High viscosity mud. High circulating rate. screen blinding

Causes

Remedy

Screen Overloading Heavy build up of cuttings on the screen High drilling rate Shaker angle too steep.

Change to a larger screen size. Adjust

Wash down shaker screens using

shaker angle. Ensure equitable flow over

a high pressure water hose not a

shaker.

wire-brush. Adjust shaker angle.

Reduce mud viscosity. Reduce flow rate. Change to Rectangular mesh screens.

b)

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Hydrocyclones (Desanders, Desilters and Mud Cleaners)

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Table 6.8


PAGE 39 08-05-03

Trouble shooting in Hydrocyclones

Problem

High Mud Losses

Indications

Rope type flow from discharge.

Screen Overloading No discharge from underflow.

High solids content (>50%) in Discharge. Incorrect feed pressure. Overloading of hydrocyclone. Improperly installed or worn cone bladder. Plugged feed nozzle or feed inlet.

Causes

Remedy

Underflow blocked. Feed inlet blocked.

Adjust feed pressure: Check

Remove blockage from underflow

Manufacturers pressure

nozzle. Remove blockage from

recommendation. Install finer shaker

inlet.

screens Remove cone and clean out blockage. Replace improperly installed or worn Bladder. Increase underflow aperture.Reduce mud viscosity.

c)

Centrifuges Centrifuges should only be serviced by properly trained personnel. Ensuring that correct start up and shut down procedures are followed which will prevent plugging 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 required between hole sections carry out the following.

1.

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

2.

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

3.

Check centrifugal pumps for impeller shaft or casing wear and motor performance. Change or service as required.

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PAGE 40 08-05-03

3.

Run the installation with water. Ensure that the feed pressures are correct. These will 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

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PAGE 41 08-05-03

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Drilling Fluids

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