A potassium-based polymer mud has been used successfully for controlling troublesome shales in many wells around the world. Shale stability results by combining potassium chloride with a high-molecular-weight, partially hydrolyzed polyacrylamide. Field application of the polyacrylamide/ potassium-chloride mud has been successful in stabilizing hard, sloughing potassium-chloride mud has been successful in stabilizing hard, sloughing shales in Canada and in reducing shale-related hole problems in several offshore areas. Introduction Drilling water-sensitive shales with conventional waterbase muds frequently results in increased costs arising from shale-related hole problems. These problems can range from minor annoyances, in which case they are generally ignored, to situations where further drilling is impossible and the well is sidetracked or abandoned. Darley and Chenevert were instrumental in determining basic mechanisms that cause shales to become unstable when exposed to water-base fluids. Various solutions have been suggested for dealing with troublesome shales encountered when drilling. Guidelines for choosing muds for drilling various types of shales have been presented by Kelly and by Allred and McCaleb . Oil-base fluids have been used successfully to drill water-sensitive formations; however, the use of oil-base muds is expensive and presents environmental problems, particularly for presents environmental problems, particularly for offshore use. A recent advance in drilling-fluid technology is the introduction of water-base muds containing potassium salts. The ability of potassium ions to reduce the swelling tendencies of shales has been studied by O'Brien and Chenevert, who also found that the use of certain polymers in combination with potassium ions showed promise as the basis of potassium ions showed promise as the basis of shale-protective drilling fluids. Mondshine presented data on the development of a potassium-chloride/ potassium-lignite system for stabilizing potassium-lignite system for stabilizing water-sensitive shales. This paper discusses the development of a potassium-based polymer mud that has been successful potassium-based polymer mud that has been successful in controlling troublesome shales in many wells around the world. Shale stability is achieved by combining potassium chloride with a high-molecular-weight, potassium chloride with a high-molecular-weight, partially hydrolyzed polyacrylamide.* Results of a partially hydrolyzed polyacrylamide.* Results of a laboratory evaluation of the shale-stabilizing ability of a number of polymers in combination with potassium chloride are presented. Information concerning formulation, handling, and treatment of the system is given, along with results of field application of the polyacrylamide/potassium-chloride mud in a variety of polyacrylamide/potassium-chloride mud in a variety of shales and geographical locations. Laboratory Evaluation of Shale-Stabilizing Fluids Test Equipment and Procedures The laboratory equipment used for evaluating the shale-protective ability of water-base fluids is the mud triaxial tester described by Darley. A schematic drawing of the tester is shown in Fig. 1. The mud triaxial tester allows mud to flow through a simulated borehole (1/4 in. in diameter) in the shale test specimen (1 in. thick × 2 in. in diameter) while the shale is under stress. Axial and radial stresses can be varied independently, although standard procedure is to set them equal. JPT P. 719