Fundamentals and Use of Potassium/Polymer Drilling Fluids To Minimize Drilling and Completion Problems Associated With Hydratable Clays

Abstract

Summary Water-sensitive shales cause expensive problems and may defeat the purpose of drilling a well. Clay hydration can produce drilling problems such as wellbore instability, stuck pipe, bottomhole fill, torque, drag, and solids buildup in the drilling fluid. It also can produce completion problems such as formation damage in shaly sands, logging and coring failures, hole washout, and poor cement jobs. Proper application of an inhibitive drilling fluid will reduce drilling costs, rig time formation damage, and completion costs. The potassium ion, when used at the proper concentration, is a powerful shale inhibitor. It interacts with clays, such as illite or montmorillonite, lowers the hydration energy, and reduces swelling. Relatively simple potassium/polymer drilling fluid systems, which provide excellent rheological and filtration properties, have been formulated at moderate costs. The systems, when properly used, are quite stable and easily maintained. These potassium/polymer drilling fluids have been used successfully on many wells around the world. Many different types of shales have been stabilized when the fluid system has been tailored to the specific shale. In areas notorious for poor cement ‘jobs and high completion costs, the problems were eliminated with potassium/polymer systems. Applications in drilling shaly sands showed substantial reduction in formation damage and improved productivity. Logging and coring operations were improved dramatically in exploration wells drilled in troublesome formations with potassium/polymer fluids. Introduction Four key steps are required to solve problems caused by hydratable clays. They are:determine the hydration characteristics of the shale,match the optimal inhibitor concentration to the shale,formulate a moderately priced fluid system with the proper inhibitor concentration, andprovide fluid processing equipment necessary to maintain the fluid properties at reasonable costs. This paper focuses on field results and on four areas that allow one to follow the steps given to achieve good field results. The topics covered are:a general description of clay hydration and problem shales,effects of potassium ion on shales and the results of shale tests with potassium and other materials,drilling fluid formulations,field preparation and maintenance, andresults of field applications. Shales Clays Shales are fine-grained sedimentary rocks that contain significant amounts of clay minerals. Some clays have very high hydration energies and thus will adsorb water and swell. 1 The most common water-sensitive clays, in order of greatest to least sensitivity, are montmorillonites, mixed-layered or interlayered montmorillonite/illites, and illites. Chlorite and kaolinite clays commonly are found in shales, but they exhibit no appreciable swelling tendencies. Swelling clays are disordered microcrystals of layered aluminosilicates with defect strctures. Each clay layer unit consists of an octahedral aluminate layer sandwiched between two tetrahedral silicate layers. The silicate surfaces of the microcrystals have net negative charges because of isomorphous substitution in the octahedral and tetrahedral layers. For montmorillonite. the substitution is predominantly iron or magnesium for aluminum in the octahedral or middle layer, which produces a negative charge at the surface. The charges are balanced by positive ions, usually sodium and calcium adsorbed to the surface. JPT P. 1661^