GN Solids Control Brochure
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Jan
30
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The types and quantities of solids (insoluble components) present in drilling mud systems play major roles in the fluid’s density, viscosity, filter-cake quality/filtration control, and other chemical and mechanical properties. The type of solid and its concentration influences mud and well costs, including factors such as drilling rate, hydraulics, dilution rate, torque and drag, surge and swab pressures, differential sticking, lost circulation, hole stability, and balling of the bit and the bottom-hole assembly. These, in turn, influence the service life of bits, pumps, and other mechanical equipment. Insoluble polymers, clays, and weighting materials are added to drilling mud to achieve various desirable properties.
Drilled solids, consisting of rock and low-yielding clays, are incorporated into the mud continuously while drilling. To a limited extent, they can be tolerated and may even be beneficial. Dispersion of clay-bearing drilled solids creates highly charged colloidal particles (<2 mm) that generate significant viscosity, particularly at low shear rates, which aids in suspension of all solids. If the clays are sodium montmorillonite, the solids will also form thin filter cakes and control filtration (loss of liquid phase) into the drilled formation(GN Solids Control). Above a concentration of a few weight percent, dispersed drilled solids can generate excessive low-shear-rate and high-shear-rate viscosities, greatly reduced drilling rates, and excessively thick filter cakes.
Fig. 1 Effect of Mud Weight and MBT on Viscosity of Acceptable WBM
As shown in Figure 1, increasing the mud density from 10 lb/gal to 18 lb/gal requires that the MBT be reduced by half. Different mud densities require different strategies to maintain the concentration of drilled solids within an acceptable range. Whereas low mud densities may require only mud dilution in combination with a simple mechanical separator, high mud densities may require a more complex strategy: (a) chemical treatment to limit dispersion of the drilled solids (e.g., use of a shale inhibitor or deflocculant like lignosulfonate), (b) more frequent dilution of the drilling fluid with base fluid, and (c) more complex solidsremoval equipment, such as mud cleaners and centrifuges.
Fig. 2 Drilling Fluid Circulating System
In either case, solids removal is one of the most important aspects of mud system control, since it has a direct bearing on drilling efficiency and represents an opportunity to reduce overall drilling costs. A diagram of a typical mud circulating system, including various solids-control devices, is shown in Figure 2. While some dilution with fresh treated mud is necessary and even desirable, sole reliance on dilution to control buildup of drilled solids in the mud is very costly.
Solids removal on the rig is accomplished by one or more of the following techniques:
. Screening: Shale shakers, gumbo removal devices
. Hydrocycloning: Desanders, desilters
. Centrifugation: Scalping and decanting centrifuges
. Gravitational settling: Sumps, dewatering units
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