Drilling Solids Removal Theory -Basic Knowledge

Drilling fluid maintenance costs can decrease greatly when proper solids control techniques are utilized. From a fluid control standpoint, it would be desirable in most cases to remove all drilled solids. Although this is possible with the use of chemical enhancement prior to separation, it is not always the most economical approach. 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 the minimum cost.

Particle Sizes And Effects

Drilling fluids are classified as water-based or oil-based with most systems utilizing water as the liquid phase. Oil-based muds employ oil as the continuous liquid phase and are generally more expensive than water-based muds.

The solids phase of any drilling mud are two basic types:  a) Commercial solids; and b) Drilled solids.

Most commercial solids, with the exception of barite and lost circulation material, have a relative particle size of less than one micron (.000039 inches). Barites generally have a size range of 1 to 74 microns with approximately 45% being less than 14 microns in size. API standards require at least 97% by weight of barites pass through a 74-micron opening (200 mesh) and not more than 95 % pass through a 44-micron opening (325 mesh screen). Lost circulation materials are available in a wide range of sizes but are considerably larger than barite. Barite lost circulation materials, and other commercial solids such as bentonite, lignite, and starch are available from the mud companies and are added to the mud system to obtain certain desired properties and perform certain functions.

Drilled solids are those particles that enter a mud system in the form of cuttings from the bit; or as formation, particles that slough or fall into the annulus. These solids vary in size from less than 1 micron to over 2000 microns and reflect the composition of the formation being drilled and shape of the bit teeth. A bit having large teeth will produce relatively large cuttings whereas button-head bits produce extremely fine particles. NOTE: Rotary speed and weight on the bit are also factors that contribute to particle size.

Solids in drilling fluids can be separated essentially into two density groups, high and low specific gravities. High specific gravity will refer to those solids with a specific gravity of 4.2 and above. The most commonly used for density increases are barite, which is 4.2, and hematite, which is 5.0. Low specific gravity solids may range from a low of 1.1 for lignite to 2.9 for dense lime. 2.6 is normally used in solids analyses as the average specific gravity of
drilled solids.

One of the most important objectives in solids control is to remove as many of the larger particles as is practical the first time that these solids are circulated to the surface. This requires properly designed and installed mechanical treating equipment sized to process at least 100-125 % 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 and mud pump during each circulation cycle until they are too fine for removal by mechanical means. Breaking down a large particle into hundreds of fine particles increases the surface area of the particles and requires adding liquid to the mud system to accommodate the increase.

In order to evaluate the removal capabilities of the various pieces of mechanical treating equipment, it is necessary to consider the source of said solids and classify them according to size as follows: (.001 inches = 25.4 microns)

• 440 microns and larger – large drilled solids (cuttings)
• 74 to 440 microns – sand
• 2 to 74 microns – silt
• 0.5 to 2 microns – clay
• 0.5 micron and smaller – colloids

All solids in the colloidal range are not detrimental to a mud system: Some finer particles in the colloid range are necessary for building a thin, slick wall cake in the borehole and reduce the possibility of differential pressure sticking of the drill string. However, it is highly important that drilled solids are removed the first time they are circulated to the surface or they would eventually degrade to a colloid size by continuous circulation through the mud pumps, drill pipe, bit jets, bit teeth, etc.

As an example, one particle having a diameter of 100 microns will become 125,000 particles with a diameter of 2 microns and require 50 times as much liquid to coat the surface of this same mass of drilled solids without any
reduction in solids concentration. This thickening process, occurring without an absolute increase in solids concentration, is referred to as viscosity or the resistance to flow.

Adding water or oil to the system reduces the concentration of those solids, thus reducing the viscosity. Removal of drilled solids during the early circulation stages with solids removal equipment at the surface is much more
simple and less expensive. Water-soluble chemicals, such as lignites, lignosulfonates, phosphates, quebracho, may be added to the water phase to control the extremely fine clays in the mud. Also, some flocculants are effective in agglomerating many fine solids into one large floc that can be removed by settling in the tanks or by removal equipment.

Benefits Of Low Solids Mud

1. Increased drilling penetration rate.
2. Increased bit of life.
3. Reduced mud costs.
4. Reduced main mud pump maintenance cost.
5. Reduced differential pressure sticking.
6. Bore-hole is closer to gauge.
7. Reduced water dilution.
8. Increased cementing efficiency.
9. Increased accuracy of geological information retrieved from the wellbore.
10. Reduced drill pipe torque.
11. Increased control of mud properties.

Obviously, these benefits are the result of planning prior to drilling a well and are accomplished through the use of properly designed, sized, and operated solids removal equipment. It is the obligation of the drilling crew to become knowledgeable in the proper use of the equipment; otherwise, its potential benefits may be reduced or nullified.

Methods Of Controlling Solids

• Mechanical treatment.
• Dilution of whole mud.
• Chemical treatment.
• Jetting or discarding whole mud.

Each of the above methods is effective at the proper time and place; however the last two categories are quite often employed due to lack of planning when mechanical treatment would be more effective and economical; especially during the early phases of the drilling program.

MECHANICAL TREATMENT

This is the method of mechanically removing solids using shale shakers, desanders, desilters, mud cleaners and centrifuges with each piece of equipment generally limited to the following range of particle removal:

1. Standard Shale Shaker – 440 microns and larger.
2. Fine Screen Shaker – 74 microns and larger (weighted muds). – 44 microns and larger – (unweighted muds).
3. Mud cleaner – 74 microns and larger (weighted muds). – 44 microns and larger(unweighted muds)
4. Desanders – 100 microns and larger.
5. Desilters – 15 microns and larger.
6. Centrifuge – 4 to 8 microns and smaller (weighted muds); 4 to 8 microns and larger (unweighted muds).

Each piece of mechanical equipment is effective within a certain particle size range. Utilizing all of the above items throughout a drilling program will produce maximum benefits without overloading any one piece of equipment.
None of the above items will take the place of another piece of equipment; however no piece of equipment operating at optimum efficiency should cause downstream equipment to become overloaded.

Removing solids from spud of a drilling program is the first priority in solids control as it is much easier to remove one particle 100 microns in diameter with a fine screen shaker than to attempt to remove 125,000 particles of 2-micron size with a centrifuge.

In unweighted water-base muds, the fine screen shaker, desander, and desirer are generally used until the point of adding barites. Centrifuges are added to increase drilled solids removal. With weighted water-base muds and all oil
base muds, fine screen shaker, mud cleaner, and centrifuge are utilized.

CHEMICAL TREATMENT

Chemical treatment of a water-base mud for solids removal involves adding a “flocculant” to the mud system. This causes extremely fine solids to agglomerate together in order to be removed mechanically or allowed to settle by gravity in the mud tanks. Normally, the flocculant is used in conjunction with mechanical treatment. For example, flocculants can be added at the shaker screen to increase apparent particle size. Polymer flocculant may also be injected into the centrifuge feed to improve centrifuge performance.

Deflocculants such as lignosulfonates may be added to a water base mud to increase the solid’s tolerance of the fluid. These “thinners” allow more solids to be incorporated into the mud before viscosity becomes too much of a problem.

DILUTION OF WHOLE MUD

Dilution of whole mud requires the addition of water or oil as a means to thin the mud. This increases the volume of whole mud while retaining all of the drilled solids and, in effect, reduces the solids concentration and viscosity to some degree.

JETTING OR DISCARDING WHOLE MUD

This is a simple means of discarding a certain amount of whole mud because of excessive volume in the system, generally caused by severe solids contamination or dilution. Jetting whole mud allows some solids to be discarded with the liquid phase, but only at the same concentration as the liquid/solids ratio of the whole mud. Therefore, jetting is a very expensive method of reducing solids concentration as the removal of 80 barrels of whole mud reduces the volume of the solid of a 2000 barrel system by only 4 percent.

Mechanical Separation – Basics

Mechanical separation equipment employs mass differences, size differences, or a combination of both to selectively reject undesirable solids and retain desirable solids in a drilling fluid. The desander, desilter, and centrifuge
utilize centrifugal force and mass difference between the solids density and liquid density for solids removal. The shale shakers and mud cleaners employ a vibrating screen of various micron-sized openings to selectively classify
particles by size difference. The centrifuge is limited to treating only a portion of the total mud circulating rate while the other equipment is sized to process at least 100-125 percent of the rig circulating rate.

A standard rig shaker or a fine screen shaker is vital to solids control and should process all of the mud returning through the flow line since any particles will degrade to such a degree that concentration and viscosity will be difficult and expensive to control. A standard rig shaker performs adequately for small rigs operating at shallow depths with low solids native mud; however, fine screen shale shakers are generally more efficient and represent the latest technique of screen support, screen motion, vibratory design, and other desirable features.

Located directly downstream from the shale shaker are the desander and desirer solids removing equipment (unweighted water-based muds). These should be sized to process at least 125% of the rig circulation rate while
discarding undesirable cuttings and solids down to the 50-micron size range when used in series. The desander removes the majority of the solids down to the 100-micron size range and prevents the desilter from being overloaded. The desilter removes the majority of solids down to the 15-micron range.

Liquid loss from desanding and desilting an unweighted water-based mud is relatively insignificant compared to the amount of whole mud jetted to remove the same amount of solids unless you are drilling in an environmentally sensitive area.

When drilling with a weighted water-based mud, or an unweighted oil-based system, the desander, and desilter cannot be economically utilized because they would tend to discard the valuable barite from a weighted mud and
lose the expensive liquid phase in an oil-based system. Therefore, a fine screen shaker or mud cleaner is employed to remove solids down to 74 microns.

It is important that all of the mud be processed by a fine screen shaker even though most of the solids smaller than the screen mesh will be returned to the active mud system. This process of screening and allowing some fine particles to return to the active system is desirable when the alternatives are considered:

•  Removing no solids in this range.
• A significant loss of barite and liquid.

Shale shakers and fine screen shakers, operated under optimum conditions, will produce a relatively clean mud; however, a centrifuge may be required in several instances where:

• A hard formation is being drilled.
• Diamond bits are being used.
• Used mud, containing drilled solids, is added during the drilling process.
• Improperly operated and maintained solids removal equipment being used.
• The shale shaker has been bypassed.
• The mud has been used for several months and a build-up of fine degraded particles has occurred.
• Unweighted systems with expensive liquid phases are required.
• Environmental constraints are a factor.

Summary – Effective Solids Control

1. Obtain solids removal equipment from a reputable manufacturer and size it to process mud at the manufacturer’s recommended capacity. The process rate should be 100-125% of the maximum rig circulating rate except for the centrifuge.
2. Remove as many drilled cuttings as possible the first time they are circulated to the surface.
3. Do not bypass the shale shaker or other equipment if at all possible.
4. Use of the smallest screen meshes possibly on the shale shakers.
5. Maintain an adequate inventory of recommended spare parts.
6. Assign rig personnel on each tour to be responsible for equipment operation and maintenance.