An experimental investigation was begun to understand fundamental borehole stability mechanisms. This laboratory study was directed toward understanding why and how drilling-fluid chemistry affects borehole stability. The ability of drilling-fluid chemistry to alter shale mechanical behavior and the borehole stress state as a result of shale water content alteration was investigated. Experimental results demonstrate the influence of various drilling-fluid parameters (e.g., salt concentration, salt type, and diesel/water ratio) in oil-based drilling fluids on the water content, compressive strength, and mechanical properties of five shales. The influences of temperature and drilling-fluid exposure time on shale strength alteration also are addressed. The results are explained on the basis of chemical potential differences between oil-based drilling fluid and shale. The change in the shale water content caused by these differences is identified as the predominant factor leading to alteration of shale mechanical behavior and hence borehole stability.
As oil reserves are depleted and costs increase, the need to drill extended-reach holes with long openhole intervals also will increase. This requirement will be linked with the need to protect the environment against pollution. In the past, oil-based muds have been the workhorse of the industry for difficult drilling. Their application typically has been justified on the basis of borehole stability, fluid loss, filter-cake quality, lubricity, and temperature stability. As environmental concerns restrict the use of oil-based muds, the drilling industry must provide innovative means to obtain oilbased-mud performance without the adverse environmental effect. Therefore, mud-system designs must provide the shale-stabilizing, lubricity, and filter-cake-quality characteristics of an oil-based mud while minimizing risk of acute toxicity and maximizing biodegradability in such harsh environments as the North Sea (a low temperature, anaerobic environment). Current design efforts have centered on the development of new additives and mud systems that will provide the benefits of oil-based muds without the environmental limitations. These efforts have had mixed reviews: with the implementation of better drilling practices, solids control processes, and new mud systems, however, progress is being made. Various investigators have tried to develop a fundamental understanding of how different chemical and physical characteristics of oil-based mud provide better borehole stability, lower friction coefficients, and better filter cakes. In terms of borehole stability, oilbased muds generally are considered superior to water-based muds. Work undertaken to understand the fundamental factors that provide the characteristics necessary for the oil-based mud to improve borehole stability has continued. From this basis, we hypothesize that implementation of these fundamental oil-based-mud concepts can be used to design an effective water-based mud.