Solids Control Equipment

The purpose of the solids control equipment is to reduce to a minimum the amount of inert solids and gases in the drilling fluid. They are:

1. Shale shakers, 2. Degassers,

3. Desanders (hydrocyclones), 4. Desilters (hydrocyclones), 5. Centrifuges,

6. Mud cleaners.

Figure 2.18 shows a sketch of a typical solids control system (for unweighted fluid). Fine particles of inactive solids are continuously added to the fluid dur-ing drilldur-ing. These solids increase the density of the fluid and also the friction pressure drop, but do not contribute to the carrying capacity of the fluid. The amount of inert solids must be kept as low as possible.

Figure 2.18: Solids control system.

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Figure 2.19: Shale shaker configurations.

Figure 2.20: A two–screen shale shaker.

2.3.2.1 Shale Shakers

The shale shaker removes the coarse solids (cuttings) generated during drilling.

It is located at the end of the flow line. It constitutes of one or more vibrating screens in the range of 10 to 150 mesh over which the mud passes before it is fed to the mud pits. (See Figure 2.19.)

The screens are vibrated by eccentric heavy cylinders connected to electric motors. The vibration promotes an efficient separation without loss of fluid.

Figure 2.20 shows a typical two–screen shale shaker.

2.3.2.2 Degassers

Gases that might enter the fluid must also be removed. Even when the fluid is overbalanced, the gas contained in the rock cut by the bit will enter the fluid and must be removed. The degasser removes gas from the gas cut fluid by creating a vacuum in a vacuum chamber. The fluid flows down an inclined flat surface as a thin layer. The vacuum enlarges and coalesce the bubbles. Degassed

Figure 2.21: A vacuum chamber degasser.

fluid is draw from chamber by a fluid jet located at the discharge line. A typical degasser diagram is shown in Figure 2.21.

2.3.2.3 Hydrocyclones (Desanders and Desilters)

Hydrocyclones are simple devices with no internal moving parts. The drilling fluid enters the device through a tangential opening in the cylindrical section, impelled by a centrifugal pump. The centrifugal force generated by the whirling motion pushes the solid particles towards the internal wall of the inverted cone.

As the whirling flux moves downwards the rotating speed increased and the diameters decreases. The fluid free of solid particles is “squeezed” out of the flow and swirls upwards in a vortex motion, leaving the hydrocyclone from the upper exit. The solids leave the hydrocyclone from the apex of the cone (under-flow). For maximum efficiency, the discharge from the apex exit of hydrocyclone should be in a spray in the shape of a hollow cone rather than a rope shape.

Figure 2.22 shows the fluid/solids paths in a hydrocyclone.

Hydrocyclones are classified according to the size of the particles removed as desanders (cut point in the 40–45µm size range) or desilters (cut point in the 10–20µm size range). At the cut point of a hydrocyclone 50% of the parti-cles of that size is discarded. The desander is a set of two or three 8in or 10in hydrocyclones, and are positioned after the shale shaker and the degasser (if used). The desilter is a set of eight to twelve 4in or 5in hydrocyclones. It re-moves particles that can not be removed by the desander. Figures 2.23 shows a desander (a), and a desilter (b). Note the size and number of hydrocyclones in each case.

A typical drilling solid particle distribution and particle size range classifica-tion are shown in the diagram in Figure 2.24.

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Figure 2.22: Flow path in a hydrocyclone.

(a) Desander. (b) Desilter.

Figure 2.23: Solid control equipment.

Figure 2.24: Particle size classification.

2.3.2.4 Centrifuges

The centrifuge is a solids control equipment which separates particles even smaller, which can not be removed by the hydrocyclones. It consists of a rotat-ing cone–shape drum, with a screw conveyor. (See Figure 2.25.) Drillrotat-ing fluid is fed through the hollow conveyor. The drum rotates at a high speed and creates a centrifugal force that causes the heavier solids to decant. The screw rotates in the same direction of the drum but at a slight slower speed, pushing the solids toward the discharge line. The colloidal suspension exits the drum through the overflow ports. The drums are enclosed in an external, non–rotating casing not shown in the figure.

2.3.2.5 Mud Cleaners

Inert solids in weighted fluid (drilling fluid with weight material like barite, iron oxide, etc) can not be treated with hydrocyclones alone because the particle sizes of the weighting material are within the operational range of desanders and desilters.⁴ This is shown in the diagram in Figure 2.24, which includes the particle size distribution of typical industrial barite used in drilling fluids.

A mud cleaner is a desilter unit in which the underflow is further processed by a fine vibrating screen, mounted directly under the cones. The mud cleaner separates the low density inert solids (undesirable) from the high density

weight-4Weighting material are relatively expensive additives, which must be saved.

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Figure 2.25: Internal view of a centrifuge.

ing particles. (See Figure 2.26-a.)

Hydrocyclones discriminate light particles from heavy particles. Bentonite are lighter than formation solids because they are of colloidal size (although of the same density). Barite particles are smaller than formation solids because they are denser.

The desilter removes the barite and the formation solids particles in the un-derflow, leaving only a clean mud with bentonite particles in a colloidal suspen-sion in the overflow. The thick slurry in the underflow goes to the fine screen, which separate the large (low density) particles (formation solids) from the small (high density) barite particles, thus conserving weighting agent and the liquid phase but at the same time returning many fine solids to the active system.

The thick barite rich slurry is treated with dilution and mixed with the clean mud (colloidal bentonite). The resulting mud is treated to the right density and vis-cosity and re–circulates in the hole. A diagram of a mud cleaner is shown in Figure 2.26-b.

Mud cleaners are used mainly with oil– and synthetic–base fluids where the liquid discharge from the cone cannot be discharged, either for environmental or economic reasons. It may also be used with weighted water–base fluids to conserve barite and the liquid phase.