Solids-control equipment guidelines – Surface Systems

This section provides additional thoughts and considerations concerning solids-control equipment. The practical operational guidelines for equipment discussed here may not apply to all drilling applications. These guidelines (in italics) were developed as part of API RP 13C. The discussion beneath each captures some of the comments by committee members as they debated the guideline before approval.

Surface Systems

  1. The surface system should be divided into three sections each having a distinct function: removal section, additions section, and check/suction section. Undesirable drilled solids should be removed in the removal section. All mud material and liquid additions should be made in the additions section. The check/suction section provides volume for blending of new mud materials and verification of desired mud properties.

This is a simple concept that requires each surface system to have three easily identifiable sections. If not, changes will quickly pay for themselves.

2.  Minimum recommended ‘‘usable’’ surface mud volume is 100 barrels (less for slim holes) plus enough to fill the hole when the largest drill string the rig can handle is pulled wet and all the mud inside the string is lost. In order to maintain fluid properties in large diameter, soft, fast drilling holes, the minimum surface volume should be at least five or six times the volume of the hole drilled per day.

For safety reasons a rig must have enough drilling fluid to fill the hole at all times. This is the situation described above. A second consideration is not a safety feature but a recognition of practicality. When rapidly drilling a large-diameter hole and removing drilled solids from the system, new fluid must be built quickly. The volume of new fluid is the sum of the solids removed and the drilling fluid clinging to them. Mixing equipment on rigs is not usually geared to rapid additions of drilling-fluid products. Drilling operations experience fewer problems if the drilling fluid properties are controlled, which is difficult with large, rapid additions of drilling fluid products.

3. All removal compartments, except the sand trap, should be well stirred or agitated to ensure even loading of solids-removal equipment.

Solids-control equipment works best when the solids loading remains constant. Slugs of large quantity of solids tend to plug the lower discharge opening in desilters. When this occurs, drilled solids will not be removed until the plugged cones are cleaned. On a rig, even with diligent crews, some cones will usually remain plugged, which leads to increased drilled solids in the drilling fluid.

4. The ideal tank depth would be approximately equal to the width, or the diameter, the tanks. If deeper, special considerations may be necessary for stirring; if shallower, adequate stirring without vortexing will be difficult or impossible.

Baffles will help prevent vortexing. When using vertical blade stirrers in circular tanks, baffles are a necessity.

5. Use top equalization for the sand trap.

To take advantage of the maximum settling time, fluid should enter the upstream end of the sand trap. After the fluid moves through the compartment, it exits through an overflow weir into the next compartment. An underflow arrangement would carry settled solids into the next compartment. The ability to use API 200 screens on shale shakers means most of the sand-sized particles are removed and few drilled solids are available to settle. (API defines anything larger than 74 microns, or API 200, as sand.) With the introduction of linear motion shale shakers (using API 200 screens) and the emphasis on minimizing rig discharges, fewer rigs are including sand traps in their removal systems. Many rigs place their desander pump suctions in the former sand traps. This requires that these compartments be well agitated.

6. Use top equalization between the degasser suction and discharge compartments.

The degasser should process more fluid than is entering its suction compartment. This will create a backflow from the discharge compartment into the suction compartment. Only processed drilling fluid can flow over the top weir. Any drilling fluid still containing gas will overflow over the top weir back to the degasser suction compartment.

7. Use bottom equalization between the suction and discharge compartments of desanders, desilters, mud cleaners, and centrifuges.

Openings in the partitions between the suction and discharges are needed primarily to allow a small backflow. Since the level can vary in these compartments, an underflow is needed so that no adjustment is necessary while drilling. Also, an overflow tends to entrain air as the fluid cascades into the upstream tank.

8. Use an adjustable between the removal and additions sections when cyclones and/or centrifuges are being used. Run with the high position on the downstream side.

An adjustable equalizer is usually a curved pipe that can swivel in the equalizing line. The upper end is in the downstream compartment. Fluid exits the bottom of the removal section and flows through the adjustable equalizer. The downstream end of the equalizer is elevated so that the liquid level in the removal section remains constant.

9. Use bottom equalization in the additions section and in the check/ suction section.

This will allow the maximum use of the fluid contained in these sections.

10. For removal devices processing flow rates greater than the rig circulating rate, equalizing flows should always be in the reverse (or upstream) direction.

A centrifuge usually processes only a small portion of the total rig flow. Backflow should not be expected between the centrifuge suction and discharge compartments. All other removal equipment (degasser, desander, desilter, and mud cleaner) should process all of the fluid entering the suction compartment. This may exceed the rig flow if drilling fluid enters upstream from another process or from mud guns.

11. Based on experience, a rule of thumb for the minimum square feet of horizontal area for a compartment is as follows: horizontal surface area (sq ft) = max circulating rate (gpm)/40 It has been found from experience that this rule of thumb provides fluid velocities low enough to allow entrained air bubbles to rise to the surface and break out. Note: This rule-of-thumb was developed by George Ormsby and was included in the IADC Mud Equipment Manual, Handbook 2: ‘‘Mud System Arrangements,’’ pp. 2–17.

This is strictly an empirical guideline. Following this guideline usually results in most of the air leaving the drilling fluid, although many types of drilling fluids (polymer, synthetics, relaxed fluid-loss oil muds, mineral oil muds, etc.) were not in existence when this rule was developed. Since the air breakout is a function of the interfacial tension and the low-shear-rate viscosity of the fluid, this formula should be used with caution. This is also the reason that all tanks should be adequately stirred. The entrained air must be brought to the surface so that it can leave the system.

12. Mechanical stirrers are preferred for stirring removal compartments.

All fluid entering a suction compartment should be processed. Mud guns suctioning from a downstream compartment will increase the quantity of fluid that must be processed. Mechanical stirrers eliminate this problem. Mud guns can be used in removal compartments if each centrifugal pump stirs its own suction.

13. Mechanical stirrers should be properly sized and installed according to manufacturer’s recommendations.

Mechanical stirrers are available in two stirring blade shapes. One type is canted to actually pump fluid vertically downward. The second type has vertical blades, similar to the impeller blades in a centrifugal pump, which propel the fluid outward. Both types are designed to move all of the fluid within a certain volume. The turnover rate (TOR) depends on how much fluid is moving. The TOR must be large enough to adequately blend the fluid within the compartment. TOR is calculated as 60 times the tank volume divided by displacement. Displacement or flow rate associated with each type of blade, based on projected area of blade, is available from manufacturers.

14. Baffles may be installed around each mechanical stirrer to prevent air vortices and settling in corners. A typical baffle can be 1 inch thick by 12 inches wide and extend from the tank bottom to 6 inches above the top agitator blade. Four baffles are installed around each agitator. They are installed 6 inches past the tips of the agitator blades, along lines connecting the center of the agitator blade with the four actual corners of a square pit or compartment. For a long rectangular pit, with two or more agitators, the tank is divided into imaginary square compartments and a baffle is pointed at each corner (either actual or imaginary).

The purpose of installing baffles is to prevent the drilling fluid from swirling in a manner that creates a vortex, which pulls air into the drilling fluid. The baffles can be created in a variety of shapes and positions and still function properly.

15. Mud guns should not be used in the removal section except where the feed mud to the mud gun(s) comes from the compartment being stirred by the mud gun(s).

see item 12.

16. Mud guns can be used in the additions and check/suction sections of the surface system and provide the benefits of shear and dividing and reblending newly added mud materials.

Mud guns do an excellent job of shearing new material, enabling it to disperse, and are effective at blending new material into the surface system. Agitators may aid and assist mud guns in these activities. The flow rate through a mud gun can be calculated from the following equation:

flow rate, gpm = 19.4(D)²√H

where D = nozzle diameter, in.; H=head, ft. For example, if a 1-inch sway heavily is attached to the end of a mud gun line and 85 feet of head is applied, the flow rate would be= 19.4 × 1 sq.in√85ft = 179 gpm

17. The sand trap is the only settling compartment in the surface mud system. It should not be stirred, nor should any pump take its suction from the sand trap.

Sand traps are becoming obsolete except for fast drilling surface holes, where seawater is used as a drilling fluid and coarse mesh screens are used on the shale shakers.

18. If a sand trap is used, the bottom should slope to its outlet at 45°or steeper. The outlet valve should be large, non-plugging, and quick opening and closing.

The bottom slope is needed so that the settled solids can be easily removed from the compartment. The quick opening and closing valve is needed to reduce drilling fluid loss. The valve is normally shaped like a plate or rectangular piece of metal.

19. The degasser (if needed) should be installed immediately downstream of the shaker and upstream of any piece of equipment requiring feed from a centrifugal pump.

Degassers are used to remove entrained hydrocarbon gasses from drilling fluid. Another benefit of degassers is to prevent air or gas from entering centrifugal pumps, where even small quantities significantly reduce pump effectiveness. As liquid (or drilling fluid) is thrust to the outside of the impeller chamber, air or gas collects at the center. Eventually, enough air or gas will collect to completely block the suction and no liquid will be able to enter the pump.

20. The solids-removal equipment should be arranged sequentially so that each piece of equipment removes successively finer solids. Although every piece of equipment may not be used or needed, general arrangements are as follows:

Unweighted Mud Weighted Mud
Gumbo remover Gumbo remover
Shale shaker Shale shaker
Degasser Degasser
Desilter Centrifuge
Centrifuge
Dewatering units

Including the degasser in the unweighted mud list was not agreed upon by the entire committee. Influx of gas into the drilling fluid normally requires the addition of a weighting agent to the drilling fluid. Very few unweighted drilling fluids are degassed.

21. The overflow for each piece of solids-control equipment should discharge to the compartment downstream from the suction compartment for that piece of equipment. This is termed proper piping, plumbing, or fluid routing.

The compartments do not need to be large. Simple partitions in a larger tank can frequently be added to improper systems to improve their performance. Since a backflow is desired between compartments, the partitions do not require a complete seal. A tank can be divided into several compartments by building a simple wall of boards—as long as each compartment is agitated.

22. Improper fluid routing always leads to solids-laden fluid bypassing the removal device.

Unfortunately, improper routing is all too common on drilling rigs. In the 1980s, approximately 90% of the rigs had flaws in their removal system, and the situation did not improve much in the 1990s. Poor tank arrangements, especially on jackup rigs, cost more money than almost any other problem associated with the drilling fluid system.

23. Two different pieces of solids equipment should not simultaneously operate out of the same suction compartment. Note: Different means, for example, degasser and desander or desander and desilter.

If a desander and a desilter take suction from the same compartment and then discharge into the next compartment, some fluid will be desanded and some will be desilted. This is referred to as connecting the equipment in parallel. Desanders are usually used to decrease solids loading in the desilters. If these two pieces of equipment operate in parallel and a significant amount of solids are being processed, the desilters will probably plug.
In some poorly designed systems, where an insufficient number of compartments are available, the degasser and the desander may be connected in parallel. This assumes that only the degasser, and not the desander, will be used on weighted drilling fluid. Obviously, if they are connected in this manner (parallel), both cannot operate simultaneously.

24. If two of the same piece of solids-control equipment are used simultaneously, the same suction and discharge compartment should be used for both. Example: If two desilter units are used, both should be properly rigged up and have the same suction and discharge compartments.

This situation is frequently encountered with desilter banks. In the upper part of a borehole, many hydrocyclones are needed to handle the volume (based on 50 gpm per 4-inch hydrocyclone, 1200 gpm will require 24 cones). While some of these cones can be mounted on mud cleaners (the screen is blanked to discard all underflow), all cones should use the same suction compartment and discharge downstream to the next compartment. Another problem can arise when a separate rig pump is used to increase annular velocity in risers. The additional flow rate onto the shale shakers may require adding additional units in parallel. The degasser capacity requirements may demand additional degassers be added to the mud tank system. These degassers should also be connected in parallel.

25. The degassers, desanders, desilters, and mud cleaners should process 100% of the mud entering their individual suction compartments. In a properly designed system, the processing rate should be at least 10–25% more than the rig circulating rate.

The intent of this rule is to provide some guideline so that a backflow will exist. As long as there is more fluid processed than is entering the compartment, except for the backflow, all of the fluid entering the compartment will be processed. If a desilter overflow, or cleaned drilling fluid, is returned to an upstream compartment or back into its own suction compartment, less than 50% of the drilling fluid coming from the well will be processed.

26. If rules 21, 24, and 25 are applicable and are followed, equalizing flow between compartments will be in the reverse (or upstream) direction. Backflow confirms that all mud entering the compartment is being processed。

In normal drilling operations rules 21, 24, and 25 are applicable.

27. Mud should never be pumped from one removal compartment to another except through solids-removal equipment.

The intent of this rule is to ensure that all drilling fluid is processed in an orderly fashion. Drilling fluid should not bypass any solids-removal equipment or be pumped upstream from a suction compartment.

28. Mud should never enter any removal compartment from outside the removal section to feed mud guns, mixers, or the eductor jet of a vacuum degasser.

When a system is designed, the equipment is set up to treat a certain quantity of drilling fluid. Increasing the flow rate demand by returning drilling fluid from downstream usually results in an inefficient removal system.

29. The power mud of the eductor jet for a vacuum degasser should come
from the degasser discharge compartment.

The drilling fluid to pull the fluid from the vacuum degasser usually comes form a centrifugal pump. This fluid must be degassed. The fluid in the degasser discharge compartment is pumped through the eductor and returns to the same compartment. This will not interfere with solids-removal efficiency.

30. Single-purpose pumps are necessary in the removal section to ensure proper fluid routing. One suction and one discharge should be used. Suction and discharges should not be manifolded.

Manifolding ruins more good drilling fluid systems than just about any other single design. For this reason, dedicated pumps should be used. Since multiple leaky valves will confuse proper fluid routing, a standby pump should be purchased instead of valves. When the desilter pump is on, the system is being desilted and there should be no question concerning routing. All systems, whether dispersed, non-dispersed, polymer, water-based, oil-based, synthetic, or saltwater, will need the same sequential treatment by the removal equipment.

31. In a properly designed system, solids-control devices should not overflow into mud ditches.

Many drilling fluid systems have a square channel (approximately 2×2 ft) along the top of one side of the tanks. Metal plate openings (ditches) are provided so that drilling fluid can bypass compartments. As drilling fluid drops from a ditch opening into the top of the fluid in the tank, air is entrained. Theses ditches can be useful for completion fluids and other specialty conditions; however, their use in drilling operations may result in drilling fluid bypassing solids removal equipment.

32. Exception to rule 31: Based on field experience, mud foaming problems can be reduced by routing the overflow of a desander or desilter into a mud ditch for a horizontal distance of about 10 feet before the fluid enters its discharge compartment to allow entrained air to break out. If this is done, ensure that the fluid routing is correct.

Flowing drilling fluid down a ditch allows the fluid/air interface to expose more of the entrained air to the surface. Sometimes, however, air does not breakout in this distance and more air is entrained as the fluid drops into the active system. Obviously, experts have many differing opinions concerning ditches.

33. All mud material additions should be made after the removal section. All removal, including all centrifuging, must be finished before the mud material addition begins.

This rule is a result of problems incurred when adding a weighting agent upstream from mud cleaners (or pumping fresh weighting agent upstream through mud guns). API barite allows 3% weight larger than API 200, or 75 microns, most of which will be removed by a mud cleaner. When mud cleaners were first introduced, they had to be turned off during weight-up to prevent discarding too much of the newly added barite. This happens because the fresh drilling fluid was pumped back upstream to the removal section before it went downhole, or the mud cleaner was located downstream from the additions tank. All undesirables must be removed
before new products are added. One exception to this rule is the addition of flocculants, or other materials, to aid removal of drilled solids.

34. Mud foaming problems can also be reduced by using a non–airen-training mud mixing hopper. Jet and venturi hoppers suck air into the mud during mixing.

Hoppers should be turned off when they are not being used. At the discharge end of the additions line, an inexpensive air removal cylinder can be added without creating much backpressure. A welder can fabricate it from a piece of 13-3/8-inch to 20-inch casing approximately 1-1/2 feet tall, welded vertically to the end of the hopper discharge line. A plate with an 8- to 10-inch diameter hole is welded on the top of the casing. Fluid enters tangentially and is swirled as it encounters the piece of casing. This swirling action causes drilling fluid to move to the outside wall, and the air moves to the inside. This acts as a centrifugal separator. Air exits through the hole at the top and the drilling fluid drops freely into the pits.

35. Jet hoppers should include venturi for better mixing.

A venturi is needed if the flow line rises to an elevated position. The device converts a velocity head to a pressure head. Without it, fluid does not have enough pressure to rise over the tank wall.

36. The check/suction section of the surface system should contain a 20–50 barrel slugging tank, which includes a mud gun system for stirring and mixing.

An agitator may be used in addition to the mud gun. The mud gun system can be connected to the pump that is used to fill the slug tank. Usually the slug tank is used to prepare a drilling fluid with a higher density. This ‘‘slug’’ is pumped into the drill string. When tripping drill pipe, the fluid level inside the drill pipe will remain below the surface. This prevents spilling drilling fluid when a stand is removed from the drill string. Failure to slug the pipe, or get a good ‘‘slug,’’ results in drilling fluid splashing the rig crew as the pipe is pulled and racked.

37. Mud premix systems should be used on any mud system whose additives require time and shear for proper mixing. Premix systems should especially be used on systems requiring the addition of bentonite, or hard-to-mix polymers, such as CMC, PHPA, XC, etc. Do not add dry bentonite to a drilling fluid.

To be effective, bentonite must be pre-hydrated and dispersed into platelets as small as possible. It should be added to a well-agitated tank of freshwater. No other additives are required. The addition of lignosulfonate will inhibit dispersion as it thins the slurry. Bentonite should be allowed to hydrate for 24 hours (8 hours minimum). Polymers, such as HP007, require many hours of prehydration and shear before use.

38. Special shear and mixing devices are recommended for premix systems for mixing polymers (especially PHPA, spotting fluids, specialized coring fluids, and for hydrating bentonite).

Centrifugal pumps are available that have modified impellers with holes or nozzles through which the fluid shears. These systems are very effective for shearing polymers.

39. High-shear devices should not be used on the active system because they will rapidly reduce mud solids to colloidal size. Drilled solids are not processed in the same manner as bentonite.

The purpose of dispersing bentonite is to take advantage of the very thin clay platelets and their electric charges. Drilled solids usually will not grind as thin as bentonite can disperse. Although they become colloidal, they are still 1000 times larger than bentonite platelets. Increasing the colloidal content will increase the plastic viscosity, which needs to be as low as possible. Bentonite is the ideal clay because only a small amount is necessary to build yield point or control filtration. The capability of hydrated bentonite to disperse is much greater than drilled solids.

40. The surface system should include a trip tank.

Trip tanks are needed to ensure that the well bore receives the correct amount of fluid as the drill string is pulled. For example, after a 10-bbl volume of steel is removed during a trip, the liquid level in the well bore should drop. This might result in an influx of formation fluid at the bottom of the hole. The trip tank continuously supplies fluid to the bell nipple to keep the well full of drilling fluid. If only 3 bbl of drilling fluid are needed to fill the hole after 10 bbl of steel are removed, the additional fluid must be entering the wellbore from the formations. A blowout is imminent. The pipe is run back to the bottom and the situation corrected.

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