Borehole-Stability Model To Couple the Mechanics and Chemistry of Drilling-Fluid/Shale Interactions

Abstract

Summary A borehole-stability model that uniquely couples the mechanical and chemical aspects of drilling-fluid/shale interactions was developed. The model allows the user to determine the optimum drilling parameters (e.g., mud weight and salt concentration) to alleviate borehole-stability-related problems with oil- or water-based drilling-fluid systems. Chemically induced stress alteration based on the thermodynamics of differences in water molar free energies of the drilling fluid and shale is combined with mechanically induced stress. These two potentials are coupled by use of the framework of poroelasticity theory to formulate the physiochemical basis of this borehole-stability model. Introduction Shales make up more than 75% of drilled formations and cause more than 90% of wellbore-instability problems. Drilling shale can result in a variety of problems, ranging from washout to complete hole collapse. More typical drilling problems in shales are bit balling, sloughing, or creep. The problems are severe and have been estimated conservatively to be a $500 million/yr problem for the industry. These engineering problems are closely connected with the "bulk properties" of the shale, such as strength and deformation as a function of depositional environment, porosity, water content, clay content, composition, and compaction rates. The drilling-fluid bulk properties (such as the chemical makeup and concentration of the continuous phase of the mud; the composition and type of an internal phase, if present; the type of additives associated with the continuous phase; and the maintenance of the system) also are of utmost engineering importance. Other factors - in-situ stresses, pore pressure, temperature, time in open hole, depth and length of openhole interval, surrounding geological environment (salt stringers, dome, etc.) - also directly affect drilling and completion operations and must be integrated into any model and new mud system development. These variables are interconnected and influence the overall behavior of shales during drilling.