Drilling fluid maintenance cost, clean up & disposal cost as well as the over all cost of boring, can be reduced dramatically when proper solids control techniques are utilized. These facts were recognized in the oil industry in the late 1800’s. When open earthen pits we used to separate the cuttings from the borehole. This was accomplished by a series of weirs and settling pits that allowed the solids to naturally settle out by using gravity. The clean mud then flowed into a suction pit to be re-pumped down hole. This was the first solids control technique ever used.

The next innovation in solids control came when the shale shakers were introduced in the early 1930’s in the oil industry. The shale shakers were derived from technology used in the mining industry. The shale shaker remains today the primary piece of solids control equipment utilized in the industry.

Another machine borrowed from the mining industry in the 1930’s was the cone classifier or hydrocyclone. The basic principle of this device involves the centrifugal forces brought about by the high velocity of the drilling fluid spinning in the cone forcing the larger and heavier solids to settle outward toward the cyclone wall and downward toward the underflow solids discharge. Together with the shale shaker hydrocyclones have become an integral part of today’s solid control system.

State of The Art

The state of the art solids control systems includes improved versions of this original equipment, first introduce into the oil industry many years ago. Although much more efficient and robust the core technology has change little over the past few decades.

Future

The future path of solids control systems will continue to increase the overall removal efficiency of undesirable solids from the drilling mud. This will include continued improvements in shale shakers and screen life. Research investigating alternate technology such as using vacuum techniques and different motions may prove more effective in the future.

The continuing trend of more stringent environmental regulations around the world will require more and more solids control systems to be implemented to minimize haul off of drilling waste not to mention the cost saving on equipment such as mud pumps and mud motors.

Solids Removal Theory

Drilling fluid and equipment maintenance cost can decrease greatly when proper solids control practices are utilized. From a fluid control stand point it would be desirable in most cases to remove all drilled solids. Although this is possible, it would be cost prohibitive. The goal of a solids control system is to achieve the balance between mechanical solids separation and dilution that will result in drill solids being maintained at an acceptable level with a minimum of cost.

Particle Size & Effects

Drilling fluid is classified as water base utilizing water as the liquid phase. The solid phase of any drilling fluid is either commercial solids or drilled solids. Most commercial solids such as bentonite have a relative particle size of less than one micron. (.000039 inches).

Drilled solids are those particles that enter the mud system in the form of cuttings from the bit or back reamer or from borehole debris. These solids vary in size from less than one micron & larger depending on the carrying capabilities of the drilling fluid. Note that spindle speed and the amount of push or pull force require to drill play an important factor on the particle size of the cuttings.

One of the most important objectives in solids control is to remove as many of the large particles as is practical the first time that these solids are pumped out of the borehole. This requires properly designed and installed solids removal mechanical treating equipment sized to process a minimum of 100% up to 125% of the mud circulation rate. Solids that are not removed during the first circulation through the surface equipment are subjected to mechanical degradation by the drill bit, reamers and mud pumps during each circulation cycle until they are too fine for removal by mechanical means.

In order to evaluate the removal capabilities of the various pieces of mechanical treating equipment, it is necessary to consider the source of the solids and classify them according to the following sizes:

440 microns or larger	large drilled solids (*cuttings)
74 to 440 microns	sand
2 to 74 microns	silt
0.5 to 2 microns	clay
0.5 microns & smaller	colloids

Note: ( .001 inches = 25.4 microns so 1” = 25,400 microns )

Benefits of Low Solids In Drilling Mud

Increased drilling penetration
Increase bit or back reamer life
Reduce mud cost
Reduce triplex mud pump, mud motor & surface equipment maintenance cost.
Reduced clean up & haul off or disposal cost

These benefits are the result of planning prior to boring and are accomplished through the use of properly designed, sized and operated solids removal equipment. It is the responsibility of the boring crew to become knowledgeable in the proper use of the equipment; otherwise the potential benefits may be reduced or nullified.

Methods of Controlling Solids

Mechanical treatment
Chemical treatment
Dilution of mud with water
Discard mud & mix new mud.

All of the above with the exception of Mechanical treatment in most cases are cost prohibitive.

Mechanical Treatment

This is the method of mechanically removing solids using shale shakers or hydrocyclone devices such as; desanders and desilters. Each piece of equipment is general limited to the following range of particle removal:

Standard shale shaker - 440 microns or larger (also referred to as the scalping shaker)
Fine screen shaker - 74 microns or larger
Desanders - 100 microns & larger
Desilters - 15 microns & larger

Correctly implemented each piece of mechanical equipment is effective within a certain particle size range. Utilizing all or a combination of the above equipment throughout your boring program will produce maximum benefits and result in a cost effective means of controlling your solids within an affordable budget.

Mechanical Separation Basics

Mechanical separation equipment employs mass differences, size differences or a combination of both to selectively reject undesirable solids and retain desirable drilling fluid. The desanders and desilters utilize centrifuge force and mass difference between the solids density and liquid density for solids removal. The shale shakers employ a vibrating screen of various micron-sized differences.

A standard rig shaker or fine screen shaker is vital to the solids control and should process 100% of the mud returning from the starting pit before allowing this mud to be processed by any of the down stream equipment you may utilize in your solids control system.

Located directly down stream from the shale shaker will be one of the hydrocyclone devices:

Desander
Desilter

These should be sized to process at least 125% of the rig circulation rate while discarding undesirable cuttings & solids down to the 50 micron size range. The desander removes the majority of the solids down to the 100micron size range and prevents the desilter from being over loaded. The desilter removes the majority of the solids down to the 15micron range.

Summary of Effective Solids Control

Obtain solids removal equipment for your operation
Remove as many drilled cuttings as possible before being pumped back down the bore hole
Do not by pass the shale shaker or other solid control equipment while drilling
Use the smallest mesh screen possible on the shale shaker. This will change from formation to formation
Maintain an adequate inventory of recommended spare parts.
Train & assign rig personnel to be responsible for equipment operation & maintenance

Solids Control Equipment

Shale Shakers

SEPARATION by VIBRATORY SCREENING

One method of removing solids from drilling mud is to pass the mud over the surface of a vibrating screen. Particles smaller than the openings in the screen pass through the holes of the screen along with the liquid phase of the mud. Particles too large to pass through the screen are thereby separated from the mud for disposal. Basically, a screen acts as a “go-no-go” gauge: Either a particle is small enough to pass through the screen or it is not.

Screening Surface. Screening surfaces used in solids control equipment are generally made of woven wire screen cloth, in many different sizes and shapes. The following characteristics of screen cloth are important in solids applications.

Screens may be constructed with one or more LAYERS. NON-LAYERED screens have a single screen cloth mounted in a screen panel. These screens will have openings that are regular in size and shape. LAYERED screens have two or more screen cloths, usually of different mesh, mounted in a screen panel. These screens will have openings that vary greatly in size & shape.

Mesh is defined as the number of openings per linear inch. Mesh, can be measured by starting at the center of one wire and counting the number of openings to a point one inch away.

SIZE OF OPENING is the distance between wires in the screen cloth & is usually measured in fractions of an inch or microns.

Screens of the same mesh may have different sized openings depending on the diameter of the wire used to weave the screen cloth.

The smaller the diameter of the wire results in larger screen openings allowing larger particles to pass through the screen. The larger the diameter of the wire, the smaller the particles that will pass through the screen Remember: It is the size of the opening in a screen not the mesh count that determines the size of the particles separated by the screen.

PERCENT OF OPEN AREA: is the amount of the screen surface, which is not blocked by wire. The greater the wire diameter of a given mesh screen, the less open space between the wires. For example; a 4 mesh screen made of thin wire has a greater percent of opening than a 4 mesh screen made of thick wire.

The open area of a non-layered, square or oblong mesh can be calculated and should be available from your screen supplier.

Vibrating Mechanism

The purpose of vibrating the screen in solids control is to tumble the solids particles to separate fluids from solids and increase through put capacity. This vibrating action causes rapid separation of whole mud from the oversize solids, reducing the amount of mud lost with the solids.

For maximum efficiency, the solids on the screen surface must travel in a predetermined pattern. For example; some devices use elliptical motion and others use Linear motion to increase efficiency. The combined effect of the screen and vibration, result in the separation and removal of oversized particles from drilling mud.

Shale Shakers

The first line of defense for a properly designed solids control system has been and will continue to be, the shale shaker. Without proper screening of the drilling fluid during this initial removal step, reduced efficiency and effectiveness of all downstream solids control equipment in the system is assured. The downstream hydrocyclones will simply be overloaded beyond their designed capacity.

The shale shaker, in various forums has played a prominent role in solids control systems for several decades. The two main shale shakers utilized in the utility industry are:

1. Elliptical, “unbalanced” design

2. Linear “straight line” design

Shale Shaker Maintenance

Because of their greater efficiency & complexity the use of finer mesh screens is essential. This requires more attention than the standard scalping shale shaker screen.

Besides periodic lubrication, fine screen use requires the following care:

Wash down screens with power washer regularly
Check screens for tear or rips
Make sure screens are properly mounted
Make sure shaker is at proper angle
Check wiring or a regular basis high speed shakers tend to wear through wiring insulation

In addition, frequent checks must be made for plugging or blinding screens. The shaker angle can be increased or decreased to help eliminate this problem. For blinding screens a more coarse or finer screen may be installed to help solve the problem.

Remember your shale shaker is your first line of defense in your solids control system, Choose a shale shaker that is correct for your application and have rig personnel learn to use it correctly.

Hydrocyclones

Hydrocyclones are simple mechanical devices, with out moving parts, designed to speed up the settling process. Feed pressure is transformed into centrifugal force inside the cyclone or cone to accelerate particle settling in accordance with Stoke’s Law #1. In essence, a cyclone is a miniature settling pit which alloys very rapid settling of solids under controlled conditions.

Hydrocyclones have become important in solid control systems because of their ability to efficiently remove particles smaller than the finest mesh screens. They are also uncomplicated devices, which make them easy to use and maintain.

A hydrocyclone consist of a conical shell with a small opening at the bottom for the underflow discharge, a larger opening at the top for liquid discharge through an internal “vortex finder” and a feed nozzle on the side of the body near the wide (top) end of the cone.

Drilling mud enters the cyclone under pressure from a centrifugal feed pump. The velocity of the mud causes the particles to rotate rapidly within the main chamber of the cyclone. Light, fine solids and the liquid phase of the mud tend to spiral inward and upward for discharge through the liquid outlet. Heavy, coarse solids and the liquid film around them tend to spiral outward and downward for discharge through the solid outlet or under flow.

Design features of cyclones units vary widely according to the size. The cyclones are made of composite materials and hold up to wear quite well. The size of cyclones in use varies from 12” down to 2”. The measurement refers to the inside diameter of the largest, cylinders section of the cyclone. In general, but not always, the larger the cone, the larger the cut point and the greater the thru put. Refer to the figures below:

Cone Size:	4”	5”	10”
Capacity GPM	50-75	70-80	400-500
Feed Pressure PSI	32-40	32-40	20-30
Cut Point microns	15-20	20-25	30-40

Manifolding multiple cyclones in parallel can provide sufficient capacity to handle the required circulating volume plus some reserve as necessary. Manifolding may orient the cyclones in a vertical position or nearly horizontal the choice is one of convenience & system design parameters. The position does not affect cyclone performance.

The internal geometry of a cyclone also has a great deal to do with its operating efficiency. The length & angle of the conical section, the size and adjustment means of the underflow opening all play important roles in a cyclones effective separation of solids particles.

Operating efficiencies of cyclones may be measured in several ways, but since the purpose of a cyclone is to discard maximum abrasive solids with minimum fluid loss, both aspects must be considered.

Hydrocyclones are another important line of defense in the battle against the removal of solids from your drilling fluid.

Desanders

Desanders are hydrocyclones larger in diameter than 5”. Desanders are installed down stream from the shale shaker. The desander removes sand size particles and larger drilled solids, which have passed through the shale shaker screens. These solids are discarded along with some liquid into the waste pit. The clean mud is then discharged into the next pit ready to be run through the desilters.

When installing a desander, follow these general recommendations:

Size the desander to process 100-125% of the total mud circulation rate.
Keep all lines as short & straight as possible with a minimum of pipe fittings. This will reduce loss of pressure head on the feed line & minimize back-pressure on the overflow discharge line.
Do not reduce the diameter of the overflow line from that of the overflow discharge manifold.
Direct the overflow line downward into the next downstream compartment at an angle of approximately 45%. The overflow discharge line should never be installed in a vertical position, doing so may cause excessive vacuum on the discharge header and pull solids through the cyclone overflow, reducing the cyclones efficiency.
Keep the end of the discharge line above the surface of the mud to avoid creating a vacuum in the line.
Install a low equalizer line to permit backflow in to the desander section.

Operating desanders at peak efficiency is a simple matter, since most desanders are relatively uncomplicated devices. Here are a few fundamental principles to keep in mind.

Operate the desander unit at the recommended feed manifold pressure, usually around 30 PSI. A feed pressure too low decreases efficiency and a pressure too high puts un-due wear & tear on the cyclone.
Check cones regularly to ensure the discharge orifice is not plugged.
Run the desander continuously let fluid overflow back and process over & over.
Operate the desander with a light spray rather than a rope discharge to maintain peak efficiency.
Maintenance of desanders normally entails no more than checking all cone parts for excessive wear and flushing out the feed manifold between bores. Large trash may collect in feed manifold, which could cause cone plugging during operation. Preventive maintenance minimizes downtime & repairs are simpler between bores, rather than during drilling operations.

Desilters

A desilter uses smaller hydrocyclones usually 5” or smaller. The smaller cones enable desilters to make the finest particle size separation of any full flow solids control equipment. Removing particles of 15 microns and larger. Multiple cones are normally used in this application to obtain the required GPM you need for your solids control system.

When installing a desilter, follow these general recommendations:

Size the desilter to process 100-125% of the total mud circulation rate.
Take the desilter suction from the compartment receiving fluid processed by the desander.
Keep all lines as short & straight as possible.
Do not use the same centrifugal pump to feed both the desander & desilter. If both pieces of equipment are to be operated at the same time, they should be installed in series and each should have its own pump.
Direct the overflow line downward into the next downstream compartment at an angle of approximately 45%.
Keep the end of the discharge line above the surface of the mud to avoid creating a vacuum in the line.
Install a low equalizer line to permit backflow in to the desander section.
Install a guard screen over the suction with ½” slots to prevent large trash from entering the unit and plugging the cones.

Operating a desander ahead of the desilter takes a big load off the desilter and improves its efficiency.

Operating desilters at peak efficiency is much the same as operating a desander. Here are a few fundamental principles to keep in mind.

Operate the desilter at the recommend pressure 32 to 40 PSI.
As solids increase, the cones bottom can be opened slightly to help increase solids removal.
Check cones regularly for bottom plugging or flooding, since a plugged cone allows solids to return to the active mud system.
If a cone bottom becomes plugged, unplug it with a welding rod or nail. If a cone is flooding it may need to be adjusted or the feed may be partially blocked off. It should also be inspected to make sure the cone is not worn out.
Run the desander continuously let fluid overflow back and process over & over.

Maintenance. A desilters smaller cyclones are more likely than a desander cones to become plugged with oversized solids, so it is important to inspect them often for wear & plugging. This may generally be done between bores, unless a failure occurs. The feed manifold needs to be flushed & checked for debris between bores.

Mud Tank Separation

There are many ways to configure your mud tanks below your solids control equipment. The most common practice requires three sections. Under your shale shaker you have the first section known as the sand trap where the mud to be pumped from this section goes through your Desander. This cleaned mud is then dumped into the second section known as a silt tank. The mud is picked up then pumped through your Desilters. The Desilters dumps the cleaned mud into the third section, know as the clean mud or mixing tank. This will be picked up by the pump to be pumped back into your bore hole.

Combination Desanders/Desilters are common when the volume being pumped is under 500 GPM. The tank separation in this application can be achieved with two sections in the mud tank. A clean section where mud to be mixed and pumped down hole is located. The second section is under the shale shaker where the contaminated mud is picked up and processed by the Desander/Desilter combination.

Centrifugal Pumps

Another important aspect of a well engineered solids control system is the use of centrifugal pumps which not only mix the mud to be pumped down the bore hole, but also provide the feed pressure and volume required to operate the hydrocyclones. Maintenance of these pumps is essential to the operation of your solids control equipment. The following is a list of proper maintenance procedures.

Check alignment between motor & pump. This alignment must be correct.
Check the bearing oil or grease on the mechanical end of the pump.
Check that gland packing is adjusted correctly not too tight.
Grease gland packing every two hours.

This article was contributed by:

Wiley Brice

Drillers Mud Systems, LLC

www.DrillersMudSystemsLLC.com

WileyTBrice@aol.com