Drilling Mud Pit Construction

The necessity of using high-viscosity drilling fluids for coring unconsolidated materials presents problems in settling out cuttings, which, if not settled out, would cause excessive weight of the drilling fluid resulting in greater hydrostatic head of the drilling mud column and consequently deeper drilling-fluid invasion. The manner and size of the drilling-mud pit construction is important to the cuttings settling process because the drilling mud must keep moving, if the cuttings are to settle out of suspension. High-gel-strength drilling fluid is a semisolid when not moving and will not allow cuttings to settle out. Construct two pits, each to a depth of about 6 ft, a length of 10 ft, and a width not to exceed 2 ft (the narrow width keeps the mud moving). In addition to these settling pits, two or three shallow (about 1-ft deep) primary settling pits should be excavated in the narrow mud ditch leading from the core hole to the first settling pit. (The mud ditch should be at least 15 ft long.) During the coring operation, these primary settling pits and the ditch must be constantly cleaned out, because the higher velocity of mud flow in the ditch maintains highest fluid level of the gel, resulting in the cuttings settling out. If proper clean-out of the primary settling pits is maintained and the first settling pit is periodically stirred or agitated, half of the suspended cuttings will be removed. However, if the periodic stirring and agitation of the drilling fluid in the first settling pit does not adequately remove enough cuttings to lower the drilling-mud weight sufficiently (less than 9 lb/gal), the fluid in the pit can be continuously circulated. The intake end of the centrifugal pump hose used for circulating the cuttings-laden drilling fluid should be submerged to about one-third the depth of the settling pit so as not to disturb the cuttings already settled in the pit bottom. Also, the discharge hose should be horizontally submerged in the settling pit so as not to stir the cuttings up from the bottom of the pit. If the viscosity of the drilling mud is increased by the addition of natural clays during the drilling process, drilling-mud thinning additives or water need to be used to decrease the drilling-mud viscosity. This technique will result in better settling out of cuttings; however, the drilling-mud viscosity should not be reduced to a point where mud invasion of the core can occur. The final suggested method for maintaining proper mud control, if the previous methods are not successful, is to drain and clean the settling pits out and refill them with a new drilling-mud mixture.

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