This paper investigates the stability of stratified shale boreholes in the Longmaxi Formation of the Silurian system in the Sichuan Basin, with shale gas reservoirs ranging in thickness from 65m to 516m. Considering that borehole stability is a crucial factor in drilling operations, this study combines theoretical analysis and laboratory experiments to explore the anisotropic strength characteristics of stratified shale and its borehole instability mechanisms. The research indicates that geo-stress, temperature, chemical interactions, and seepage effects significantly impact borehole stability. When considering only stress effects, the critical lower limit of drilling fluid density to prevent borehole collapse ranges from 1.3 to 1.7 g/mL. Particularly in horizontal wells with azimuth angles of 0°-40° and 180°-360°, the critical lower limit of drilling fluid density is the highest, reaching over 1.65 g/mL. However, under multi-field coupling effects, the distribution range of critical drilling fluid density increases to 1.75-2 g/mL, and the stability of the borehole shows significant changes in the required drilling fluid density. In horizontal wells with azimuth angles of 0°-40° and 180°-360°, the critical lower limit of drilling fluid density is the highest, exceeding 2 g/mL. In contrast, in boreholes with an azimuth angle of 120° and an inclination angle of 60°, the critical lower limit of drilling fluid density is the lowest, indicating better borehole stability in this region. The study of drilling fluid density distribution under different working conditions reveals that borehole trajectory significantly affects borehole stability. Adjustments in drilling fluid density are necessary to ensure borehole stability under specific combinations of azimuth and inclination angles. The results suggest that in actual drilling operations, it is essential to comprehensively consider factors such as stress, temperature, chemical interactions, and seepage to optimize drilling fluid density, thereby enhancing borehole stability and drilling efficiency. In conclusion, this paper establishes a multi-field coupled borehole stability model, elucidates the stability patterns of anisotropic shale under complex borehole trajectories and multi-field coupling effects, and proposes corresponding optimization recommendations for drilling fluid density. This provides theoretical foundations and technical support for the safe and efficient development of shale gas reservoirs.
Layered shale; Wellbore stability; Multiple coupling; Anisoropic; Collapse pressure
Zhang Z. Study on the Stability of Stratified Shale Boreholes under Multi-Field Coupling. Int. J. Eng. Sci. Technol., 2024, 2(2), doi: 10.58531/ijest/2/2/2
