fluids, even with controlled low filter loss, do not adequately retard the sloughing 

 and caving of shale and clay formations nor the excessive dispersement of bit 

 cuttings into the mud. These special fluids contain combinations of salt, gypsum, 

 calcium acetate, calcium chloride, lime, and potassium chloride designed to 

 eliminate the foregoing problems. Surfactant drilling fluids are usually used as 

 water-base emulsions. The cuttings and wall cavings introduced into these 

 drilling fluids are rendered preferentially oil-wet by the surfactant and thereby 

 are coated with oil from the emulsion. This oil coating adheres tightly to the 

 cuttings because of the surfactant, and the coating usually is very effective in 

 retarding the swelling and dispersement of the cuttings until they are removed 

 by the screens. Both inhibited and surfactant drilling fluids, either with or 

 without filter loss control additives, usually recover more of the cuttings than do 

 ordinary water-base clay drilling fluids. 



Salt-Water Muds 



These muds usually result from drilling salt sections or from encountering 

 salt-water flows. The mud usually is undersaturated with salt; however, satu- 

 rated salt muds sometimes are used for avoiding hole enlargements when 

 salt beds or salt-dome overhangs are being drilled. Sea water sometimes is 

 used in drilling fluids, usually in lime-treated mud systems, for drilling shale. 

 Organic-type additives such as pregelatinized starch, sodium carboxymethyl 

 cellulose, synthetic polymers, or natural gums are used to control fluid loss. 



Salt-water muds containing from 20 to 30 percent salt have very low 

 resistivity, which makes interpretation of electric logs difficult. Often a separate 

 batch of fresh-water-clay mud must be spotted in the hole for obtaining suitable 

 electric logs. 



Emulsion Muds 



These fluids fall into two classes: water-base emulsions and oil-base 

 emulsions. Water-base emulsions are those in which the oil is dispersed 

 as small droplets through the water, whereas oil-base emulsions are those in 

 which the water is dispersed throughout the oil. 



Water-base emulsion muds are compounded from virtually all of the water- 

 base muds by addition of approximately 3 to 30 percent of various oils. Emul- 

 sifiers also are used frequently to stabilize the emulsions. Emulsion muds have 

 become quite popular and are used to increase drilling rates, reduce torque on 

 drill pipe and give greater bit life and holes more nearly to gage. Sometimes 

 misleading shows of oil may be found in cores cut with emulsion mud, but 

 this problem can be avoided largely by having the emulsion mud properly 

 emulsified and stabilized prior to starting coring operations, by not subsequently 

 adding additional oil, and by using refined oils such as diesel fuel in preparing 

 the mud. Crude oil should be avoided in preparing emulsion mud where the 



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