Development of a new mud designed to give the desirable drilling characteristics of the conventional lime mud but without its inherent limitations is discussed. The work includes both laboratory and field testing of the mud.
The mud has been used successfully in nine wells with considerable cost reduction. It is resistant to contaminants such as anhydrite, gyp, cement, and salt water and does not gel at high temperature. Field results have shown that the mud can compete economically with low- or high-pH caustic-quebracho muds with the added advantage that it yields better gauge holes and its filtrate is not as damaging to the pay formation.
The use of conventional lime-base drilling muds has made possible the drilling of wells in areas where contamination from gyp, anhydrite, and salt water flows are encountered. Their desirable drilling characteristics and ease of maintenance have allowed the drilling of long sections of open hole and have often eliminated the necessity of a protective intermediate casing string. Conventional lime muds, however, suffer from several disadvantages which often make their usage impractical.
First, and of greatest importance in deeper holes, is the occurrence of high temperature gelation which prevents the successful application of various logging tools and perforating guns due to the solidification of the mud in the bottom of the hole. Further evidence of this gelation of conventional lime muds will be more apparent in the near future as workover jobs require washover operations in order to pull the packers set in lime mud.
Of almost equal importance is the high cost of the maintenance of a lime mud. This eliminates or sharply curbs its usage in areas where contamination from gyp, anhydrite, and salt is light enough that it can be successfully handled by use of low-pH or high-pH caustic-quebracho muds. Unfortunately, its elimination from a cost standpoint also limits the use of muds with filtrates containing calcium ion which is effective in reducing the swelling and sloughing of shales, thus yielding better gauge holes.
