Impact of rock strength degradation by fluid intrusion on borehole stability in shale

The interaction between shale bedding planes and fluids significantly weakens their structural integrity, profoundly affecting borehole stability in shale reservoirs. However, traditional analyses often overlook fluid intrusion from the borehole into the bedding planes, leading to an inaccurate understanding of the mechanisms behind shale deterioration and inadequate guidance for drilling engineering design. This study models the process of drilling fluid permeating bedding shale through fluid intrusion experiments. It evaluates how forces acting on the bedding plane and the drilling cycle affect strength evolution, deriving rules governing changes in the mechanical parameters of both the shale matrix and the bedding planes. We developed a borehole stability calculation model that incorporates bedding plane considerations by integrating the established rules for mechanical parameter changes. The model analyzes the effects of the bedding plane, well inclination angle, wellbore azimuth angle, bedding plane inclination angle, and drilling cycle on the collapse pressure and collapse area with different types of drilling fluids. The results indicate that the presence of bedding planes significantly influences borehole stability. Therefore, both matrix and bedding plane damage should be considered to accurately calculate the collapse pressure and area. The well inclination angle, wellbore azimuth angle, and bedding plane inclination angle also impact borehole stability. It is recommended that the horizontal section of the wellbore be drilled in the direction of the minimum horizontal in situ stress. As the drilling cycle extends, the collapse pressure gradually increases, with the largest increase occurring in the direction of the minimum stress. Additionally, the increase in collapse pressure is greater when using water-based drilling fluid than when using oil-based drilling fluid. These findings provide theoretical insights for drilling engineering design in bedding shale environments, aiming to enhance borehole drilling safety.