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Section 1100 SOLIDS CONTROL HANDBOOK Schlumberger Dowell

Tank Design and Equipment Arrangements

January 1998 Page 1 of 18

Tank Design and Equipment Arrangements 1 Tank Design.........................................................................................................................1 1.1 Compartment Equalization.............................................................................................2 1.2 Sand Trap......................................................................................................................3 1.3 Slug Tank ......................................................................................................................3 1.4 Equipment Arrangement................................................................................................4 1.5 General Guidelines for Surface System Arrangements..................................................5

2 Equipment Arrangements...................................................................................................6 2.1 Unweighted Mud - Centrifuge Processing Active System...............................................6 2.2 Unweighted Mud - Centrifuge Processing Hydrocyclone Underflow ...............................8 2.3 Unweighted Mud - Centrifuge Processing Mud Cleaner Underflow .............................. 10 2.4 Weighted Water-Based Mud - Single-Stage Centrifuging (Barite Recovery) ................ 12 2.5 Weighted Mud - Two-Stage Centrifuging ..................................................................... 14 2.6 Complete System Layout For Both Weighted and Unweighted Mud ............................ 16

3 Summary............................................................................................................................ 18 FIGURES Fig. 1. Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6.

Unweighted mud - centrifuge processing active system. ..............................................7 Unweighted mud - centrifuge processing hydrocyclone underflow................................9 Unweighted mud - centrifuge processing mud cleaner underflow. .............................. 11 Weighted water-based mud - single-stage centrifuging (Barite recovery) ................... 13 Weighted mud - two stage centrifuging. ..................................................................... 15 Generic - complete system. ....................................................................................... 17

1 Tank Design The surface pits that comprise the active circulating system should be designed to contain enough usable mud to maintain mud properties and to fill the hole during a wet trip at the rig’s maximum rated depth. Usable mud is defined as the mud volume which can be pumped before suction is lost. For example, a typical 10,000 ft well will normally require a minimum active system tank volume of 500 bbls.

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Section 1100 January 1998 Page 2 of 18

SOLIDS CONTROL HANDBOOK


Schlumberger Dowell

The active surface system can be divided into two sections: Solids Removal and Addition-Suction. All solids removal equipment and degassing occurs in the Solids Removal section. The Addition-Suction section is used to add fresh mud to the circulating system and provide sufficient residence time for proper mixing to occur before being pumped downhole. A slug tank is usually available to pump small “pills” such as LCM or barite slugs for tripping. Each section must be further divided into enough compartments to efficiently carry out its designed function. The number of compartments needed will depend upon the amount and type of solids removal equipment, system size and circulation rate. Each compartment must have enough surface area to allow entrained air to break out of the mud. A rule of thumb for the minimum surface area is calculated by: Area (sq ft) = Maximum Circulating Rate (GPM)/40 To maximize solids suspension and usable volume, the best tank shape is round with a conical bottom. Next best is a square or rectangular shape with a V-bottom. The least-preferred shape is the square or rectangular box with a flat bottom. The ideal tank depth is equal to the width or diameter of the tank. This design provides sufficient pump suction head and is best for complete stirring.

1.1 Compartment Equalization Equalization height between compartments will depend upon the duty of the compartment. As a rule, an adjustable equalizer is needed only between the Solids Removal section and the Addition-Suction section. An adjustable swing-arm equalizer is recommended. Normally, the equalizer will take mud from the bottom of the last solids removal compartment and discharge mud near the top of the first compartment in the Addition-Suction section (high equalization). This keeps the fluid level high in the Solids Removal section to maintain sufficient suction head for the centrifugal pumps, prevents vortexing by the stirrers and provides sufficient mixing volume in barite-recovery operations. In emergencies, the swing-arm can be lowered to provide access to the full surface volume. High equalization between the Solids Removal and Addition-Suction sections also increases the ability to detect volume changes due to influx or losses to formation. Because the volume of the Solids Removal Section remains constant, any volume change is apparent as a liquid level change in the Addition-Suction section only. This increases the sensitivity to volume fluctuations since the change in fluid level will be more pronounced per unit volume. The minimum equivalent diameter of the equalizer for adequate flow between compartments can be estimated by the following calculation: 1/2

Diameter, in. = (Qmax, gpm/15)

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Recommended equalization between specific compartments is summarized below: Location

Equalization

Sand Trap Exit

High

Degasser

High

Desander

Low

Desilter

Low

Centrifuge

Low

Solids Removal - Addition

High (Adjustable)

Addition-Blend

Low

Blend-Suction

Low

1.2 Sand Trap A sand trap is the settling compartment located downstream of the shale shakers. It should be the ONLY settling compartment and should not be used in closed-loop systems. Its main function is to remove large solids that might plug the downstream hydrocyclones. With the fine-screen capabilities of today’s shale shakers, the sand trap mainly serves as a backup should the shakers be bypassed or operated with torn screens. The sand trap should be the first compartment the mud enters after passing through the shaker screens. Since it is a settling tank, it should not be stirred and the mud should exit the sand trap over a high weir. The sand trap floor should have a 45° slope to its outlet. A 20 to 30 bbl volume is sufficient. A quick opening solids dump valve that can be closed against the mud flow is recommended to reduce mud losses. The sand trap should be dumped only when nearly filled with solids, since whole mud is lost when the sand trap is dumped.

1.3 Slug Tank A slug tank is a small compartment (10-50 bbls) isolated from the rest of the active system in the Addition/Suction section. Slug tanks provide the ability to mix small volumes of specialized fluids and materials. The mud pump suction is manifolded to provide access to the slug tank. The slug tank is routinely used to mix small slugs of material to be pumped directly downhole, such as high density pills for placement in the drill pipe prior to tripping. It is also commonly used for preparation of LCM pills, spotting fluid for differential sticking, and viscous sweeps. The mixing hopper must be manifolded to permit isolation of the slug tank for mixing these pills.

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Section 1100 January 1998



Page 4 of 18

Schlumberger Dowell

1.4 Equipment Arrangement The solids removal equipment should be arranged to sequentially remove finer solids as the mud moves from the flowline to the suction pit. The purpose of this arrangement is to reduce the solids loading on the next piece of equipment. Each device must take mud from an upstream compartment and discharge into the next compartment downstream. This applies to both unweighted and weighted mud equipment arrangements. The amount and type of equipment required will depend upon the drilling conditions and economics specific to each well. Unless a detailed economic analysis is made, it is usually better to overestimate solids removal equipment requirements. Underequipping the rigs will usually result in more dire consequences, such as low penetration rates, differential sticking, high material consumption and excessive dilution and disposal volumes. Proper routing of fluids through the solids removal system is essential to achieve maximum solids removal efficiency. Mistakes in fluid routing can drastically reduce separation performance by causing a large percentage of the circulation rate to be bypassed. These errors are most commonly associated with mud cleaners and hydrocyclones. In addition to suction and discharge routing, overflow discharges to mud ditches and mud gun use are other common sources of routing errors. Ideally, each piece of solids control equipment should be fed by a singlepurpose pump with no routing option. Mud cleaners, desilters and desanders should not, under any condition, require multiple suction locations. In practice, complex routing with multiple suction options is the rule rather than the exception. When this is the case, the internal configuration of the mud tanks during rig up must be inspected to trace all lines. Do not rely on “as built” schematics; they are usually incorrect. Color-coding of the correct routing schematic to correspond with color-coded valves on the manifolds can greatly assist rig crew members in making correct routing decisions. “Hard-plumbed” routing errors should be corrected as soon as possible.

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1.5 General Guidelines for Surface System Arrangements The following guidelines are common to all equipment arrangements. 1.

All removal compartments except the sand trap should be well-agitated to ensure even solids loading.

2.

Mechanical stirrers are recommended. Check that they are properlysized and installed correctly.

3.

Mud guns are not recommended for the Solids Removal section.

4.

When installed, the degasser should be located immediately downstream of the shale shaker and upstream of any equipment fed from a centrifugal pump.

5.

Use a high equalizer between degasser suction and discharge.

6.

All solids removal equipment should downstream of their suction compartments.

7.

All equipment except the centrifuge should process at least 100% of the circulation rate. Backflow should be observed in these compartments.

8.

Low equalization between suction and discharge for all solids removal equipment.

9.

Different solids control devices must not share suction compartments or share discharge compartments unless they are making the same cut. For example, two desilters may share the same fluid routing, but a desander and desilter should not.

discharge

immediately

10. Adjustable equalizer between Solids Removal section and AdditionSuction section. This equalizer should normally be high except when access to the additional volume in the solids removal section is desired. 11. No solids removal equipment should discharge into the suction pit.

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Section 1100 SOLIDS CONTROL HANDBOOK

January 1998


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Schlumberger Dowell

2 Equipment Arrangements 2.1 Unweighted Mud - Centrifuge Processing Active System This arrangement may be used with unweighted muds having low formulation costs, where liquid disposal costs are negligible. Some whole mud is discarded with the hydrocyclone underflows. Typically used with environmentally benign water-based muds. A mud cleaner should be used only if there are insufficient desilter cones to process the entire circulation rate; it should be run in parallel with the desilter. Blank off the screen and discharge underflow. Use of a centrifuge will depend on the economics of the specific application. Equipment

Median Separation

Comments

Shale Shakers

< 147 mm

Capable of running 100 mesh (d50=147 microns) at maximum circulation rate.

Degasser

na

Desander

70 mm

If required. Processing Rate = 110% of maximum circulating rate. Discard Underflow.

Mud Cleaner

25 mm

Use as a desilter if required to achieve 110% of circulation rate. Run in parallel with other desilter manifolds.

Desilter

25 mm

Total Processing Rate (including mud cleaner cones) = 110% of maximum circulating rate. Discard Underflow.

Centrifuge

4 mm

Process at least 25% of maximum circulating rate. High G, high capacity machine. Discard Cake (Solids).

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Fig. 1. Unweighted mud - centrifuge processing active system.

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2.2 Unweighted Mud - Centrifuge Processing Hydrocyclone Underflow Used to reduce liquid discharged with cuttings while maintaining high separation efficiency. This arrangement is preferred when the liquid phase is expensive or when free liquid discharge must be limited. Hydrocyclones concentrate solids to centrifuge. Use caution when processing abrasive desander underflow; it may cause premature centrifuge wear. Centrifuge should process in excess of hydrocyclone underflow rate, with makeup mud from the active system. Refer to the centrifuge chapter for details of feed compartment design and routing. Equipment

Median Separation

Comments

Shale Shakers

< 147 mm


Degasser

na

Desander

70 mm

If required. Processing Rate = 110% of maximum circulating rate. Underflow to Centrifuge.

Desilter

25 mm

Processing Rate = 110% of maximum circulating rate. Underflow to Centrifuge.

Centrifuge

4 mm

Processing Rate > Hydrocyclone underflow rate. High G, high capacity machine. Feed from hydrocyclone underflows, plus active system. Cake (wet solids) are discarded.

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Fig. 2. Unweighted mud - centrifuge processing hydrocyclone underflow.

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2.3 Unweighted Mud - Centrifuge Processing Mud Cleaner Underflow Recommended when large sections of sand are expected and free liquid must be recovered from desander underflow. The mudcleaner screen receives desander underflow. Sand is removed by the mud cleaner screen. Screen unders are processed by the centrifuge. Best alternative is to provide enough shale shakers to screen down to desander separation efficiency (74 microns) or use full size shaker to process cone unders. Equipment

Median Separation

Comments

Shale Shakers

147 mm


Degasser

na

If required.

Desander

70 mm

Processing Rate circulating rate.

=

110%

of

maximum

Underflow to mud cleaner screen. Mud Cleaner

25 mm

Total processing rate should exceed maximum circulating rate. Both desander and mud cleaner cone underflows screened before processing by centrifuge.

Centrifuge

4 mm

Processing Rate > Hydrocyclone underflow rate. High G, high capacity machine. Feed from hydrocyclone underflows, plus active system. Discard Cake (wet solids) are discarded.

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Fig. 3. Unweighted mud - centrifuge processing mud cleaner underflow.

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2.4 Weighted Water-Based Mud - Single-Stage Centrifuging (Barite Recovery) This is the standard equipment arrangement for weighted water-based muds when fluid costs are low and liquid discharge is permitted. The mud cleaner may be used when the shakers cannot screen down to 200 mesh, but monitor barite losses. The centrifuge removes liquid and colloidal solids while recovering barite. Low centrifuge feed rates at high g-force and continuous processing are recommended to maximize barite recovery. Equipment

Median Separation

Comments

Shale Shakers

> 74 mm

Capable of screening to 200 mesh at maximum circulation rate. Monitor solids discharge for barite content.

Degasser Mud Cleaner

na 74-100 mm

If required. Run only if insufficient shaker capacity. 150 mesh screens recommended. Monitor screen discharge for barite content.

Centrifuge

4 mm

Process 10-15% of circulation rate. Return solids to well-agitated compartment, upstream of addition section. Dilute feed. Discard centrate.

Note: Refer to the Dewatering chapter for addition of chemically-enhanced dewatering unit to this system.

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Fig. 4. Weighted water-based mud - single-stage centrifuging (Barite recovery)

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2.5 Weighted Mud - Two-Stage Centrifuging This arrangement is used when liquid discharge must be minimized. The first centrifuge operates as a barite recovery unit. The second centrifuge, operating at maximum g-force, processes the centrate (overflow) from the barite recovery centrifuge. The solids are discharged and the centrate is returned to the active system. Colloidal solids are not removed. Equipment

Median Separation

Comments

Shale Shakers

> 74 mm

Screen with finest mesh possible, down to 200 mesh (d50=74 microns) at maximum circulation rate. Monitor solids discharge for barite content.

Degasser Mud Cleaner

na 74-100 mm

If required. Run only if insufficient shaker capacity. 150 mesh screens recommended. Monitor solids discharge for barite content.

Centrifuge #1

Barite recovery mode, high capacity machine. Return barite to well-agitated compartment upstream of addition section. Dilute feed. Run at highest G-force conditions will allow. Centrate to centrifuge #2.

Centrifuge #2

4 mm

Run at maximum rpm, high-G machine. Discard solids. Return centrate to active system.

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Fig. 5. Weighted mud - two stage centrifuging. CONFIDENTIAL





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2.6 Complete System Layout For Both Weighted and Unweighted Mud In many cases, multiple suction and discharge locations cannot be avoided. For example, centrifuges that will process both unweighted and weighted systems must be located to permit routing both the cake and centrate streams to either the active system or to discharge. The following schematics show the fluid routing requirements for a solids removal system which must process either unweighted or weighted mud. Note: The centrifuge to be used for barite recovery must be positioned so the solids may be routed either to discharge (unweighted) or returned to the active system (weighted mud). Use a high capacity machine for treating out coarse desilter underflows or recovering barite. The second unit should be a high-G machine capable of removing fine solids. If only one machine is used, it should be a high-G unit.

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Fig. 6. Generic - complete system. CONFIDENTIAL





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3 Summary ·

The mud pits must contain enough usable mud to maintain mud properties and to fill the hole during a wet trip at maximum depth.

·

The active circulating system is divided into two sections: Solids Removal and Addition-Suction. The purpose of each is self-explanatory. Each section is further divided into enough compartments to carry out its designed function. Additional tankage includes the slug tank for mixing and pumping small pills, the trip tank for accurately metering pipe displacement during trips, and the premix tank discussed in Chapter 10, Addition/Mixing Systems.

·

The best compartment shape is round with a conical bottom, followed by square with a V-bottom. Each must have enough surface area to allow entrained air to break out.

·

Equalization height between compartments will depend upon the duty of the compartment. Refer to the discussion in this chapter for specific recommendations.

·

The sand trap, located under the shale shakers, is the only settling compartment and should not be used in closed loop systems.

·

Equipment arrangements for a variety of unweighted and weighted muds are illustrated in this chapter. Also included is a complete system arrangement when both unweighted and weighted muds must be processed during the course of the well.

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