selectively separate most of the drilling fluids from the cuttings 

 (Gettleson and Laird 1980). 



Drilling fluids are circulated into the borehole during drilling 

 activities to cool and lubricate the drilling bit and drill pipe, remove 

 formation cuttings from the hole, insure controlled and efficient 

 drilling through maintenance of well pressures and integrity of the 

 borehole, permit logging and geological evaluations, and minimize 

 corrosion. Drilling fluids are usually water-based, colloidal 

 suspensions with additions of barite (barium sulfate) to increase 

 density, bentonite to increase viscosity, and other components to 

 control fluid loss, corrosion, and other mud properties. Barite and 

 bentonite clays are the major components used in drilling fluids. The 

 exact composition of the drilling fluids used varies with the depth of 

 the hole and from site to site, depending on the formations and drilling 

 conditions encountered. Normally, the initial 300 m of an offshore well 

 are drilled with seawater and the natural formation clays. Seawater 

 gel, consisting of added bentonite clay and minor amounts of barite, 

 sodium hydroxide, mica flakes and cellulose fibers in seawater, is used 

 below 300 m . From 1 ,200 to 1 ,520 m, the seawater gel drilling fluid is 

 converted to a lignosulfonate mud by adding barite, lignosulfonate and 

 other products. Freshwater is usually used with the seawater from 1,520 

 to 3,000 m and in place of the seawater below 3.000 m to maintain the 

 desired properties of the drilling fluid. The percentage of barite and 

 other components is normally increased with the depth of the well. 



Although a typical well usually requires from 1 to 3 months to 

 drill, depending on mechanical problems and hole depth, only 30$ to 50? 

 of the time is spent actually drilling. Drilling discharges may be 

 divided into the categories of (1) cuttings with minor amounts of 

 adhering drilling fluids and (2) drilling fluids. The cuttings, which 

 are small pieces of the drilled formation produced by the drill bit, 

 range in size from microns to a few centimeters. These cuttings with 

 their adhering drilling fluids and additional minor quantities of fluids 

 are discharged into the environment from the various mechanical devices 

 used to separate the cuttings from the drilling mud. During drilling 

 periods the discharge from the shale shaker is continuous, while that 

 from the other devices is periodic. Discharges of drilling fluids 

 without cuttings occur eight to ten times during the drilling of an 

 offshore well (Offshore Operators Committee 1978) and usually originate 

 from the mud tanks. Discharges normally result from mud changes at the 

 time of cementing, at the end of drilling the well, and when excessive 

 concentrations of colloidal solids build up in the drilling fluids. 



Observations of the discharges as they enter the water column 

 indicate that the large cuttings fall almost straight to the bottom with 

 the adhering drilling fluids washing free during the descent. 

 Flocculated clay spheroids appear to drift downward, while the drilling 

 fluids and very fine cuttings form a plume of suspended clays that is 

 diluted as it moves away from the discharge pipe (Ray and Shinn 1975; 

 Zingula 1975; ECOMAR Inc. 1978). The effective differential settling 



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