Before hydrocyclones and centrifuges became available for drilling applications, shale shakers and dilution were the only means of controlling the solids content of drilling fluids. Consequently, solids too fine to be separated by the shaker screens in use could be controlled only by dilution. During drilling with weighted muds, once the solids content reached the maximum acceptable level, the continuing and unavoidable incorporation of drilled solids made it necessary to add a continuous stream of water to control viscosity, while adding barite to control the mud weight. This was, obviously, a costly procedure that generated large quantities of excess drilling fluid.
Consideration must be given to the effects of the presence of drilled solids and barite on different types of drilling fluids: water-based and nonaqueous fluids (NAFs), unweighted and weighted.
Stokes’ Law and Drilling Fluids
Drilling fluids normally contain two categories of solids: (1) commercial clays and drilled solids, both low gravity, with specific gravities (SGs) of about 2.6, and (2) weighting agents, usually barite, with an assumed SG of 4.2. If all of the solids particles were of the same size, decanter centrifuges could be used to separate the weighting agent from the low-gravity solids, because the barite particles, due to their higher SG, would be heavier. Drilling fluids, of course, are not slurries of particles of equal size. Weighted drilling fluids always contain solids of both categories, ranging from colloidal particles too fine to settle, even in pure water, to particles 70 microns () in size and larger. Consequently, the centrifuge cannot separate barite from low-gravity solids. What it does, when operated properly, is separate larger barite particles from smaller ones and larger low-gravity-solids particles from smaller ones. Failure to recognize this very important fact frequently leads to the misuse of centrifuges.
Decanting centrifuges are mechanical devices used for the separation of solids from slurries in many industrial processes. In oilwell drilling, centrifuges are used to condition drilling fluids by dividing the fluid into high-density and low-density streams, permitting one to be separated from the other. The division is achieved by accelerated sedimentation. As the drilling fluid is passed through a rapidly rotating bowl, centrifugal force moves the heavier particles to the bowl wall, where they are scraped toward the underflow (heavy slurry) discharge ports by a concentric auger, also called a scroll or conveyor, which rotates at a slightly slower rate than the bowl. The separation of the heavier particles divides the processed fluid into two streams: the heavy phase, also called the underflow or cake; and the lighter phase, which is called the overflow, light slurry, effluent, or centrate. (see picture 1.)