Fresh-Water-Clay Muds 



Natural muds are formed from the drilled formations. With continued 

 drilling, continual dilution with water often is required to maintain the mud 

 so that it is easy to pump. 



Phosphate muds are natural muds treated with one of more of the complex 

 phosphates to thin the mud and aid in reduction of filter loss. Complex phos- 

 phates commonly used are sodium hexametaphosphate, sodium acid pyrophos- 

 phate, sodium tripolyphosphate, sodium tetraphosphate, and tetrasodium pyro- 

 phosphate. Phosphate muds are suitable where there is little contamination with 

 anhydrite or where the salt content is less than 1 percent. 



High-pH red mud is usually natural mud treated with relatively large 

 concentrations of caustic soda and one or more of the organic-type thinners 

 such as quebracho, lignite, hemlock extract, mangrove bark, and tara bean 

 extract. This mud is chosen ordinarily because of its somewhat greater tolerance 

 to contamination with salt. Starch is frequently used to reduce filter loss. 



Lime-treated mud is similar to high-pH red mud in composition except for 

 addition of slaked lime to convert the mud to calcium clay base. Lime mud is 

 used primarily in deep drilling where mud-making shales are exposed or where 

 considerable salt water contamination occurs. Calcium lignosulfonate sometimes 

 is substituted for most of the quebracho or other organic thinners. Lime mud 

 can resist salt contamination of several percent as well as contamination with 

 gypsum, anhydrite, and cement to an extent exceeding the tolerance of muds 

 not treated by lime. Lime muds generally have low viscosities and exceptionally 

 low gels, and at temperatures lower than 250F, lime muds have the ability to 

 remain quiescent for long periods without excessive gelling. These properties 

 permit ready degassing, high circulating rates, and minimum shale sloughing. 

 Lime-treated mud tends to solidify at temperatures above 250F. Centrifugal 

 separators or hydraulic cyclones may be used to reduce the clay solids content of 

 the mud and thereby overcome the solidification problem. Another approach 

 to this problem has been the preparation of the lime-treated mud with selected 

 optimum proportions of caustic soda and a minimum amount of lime and an 

 organic thinner. More recently, a barium-base mud has been developed to 

 combat the solidification problem. In some instances high-pH lime-treated 

 muds have interfered with electric logging because of their relatively low re- 

 sistivities. 



Gypsum muds consist of native fresh-water muds to which are added con- 

 trolled amounts of Plaster of Paris to convert the clay solids to calcium-base 

 clays and starch to control filter loss. The muds are designed specifically for 

 drilling thick sections of anhydrite that may be followed by sections of salt. 

 These muds usually have desirable electric-logging properties. 



Inhibited and surfactant drilling fluids are relatively new water-base 

 drilling fluids used in drilling formations in which other water-base drilling 



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