Study on Reservoir Internal Stress Variation and Mud Shale Horizontal Section Slippage Induced by Interregional Pore Pressure Based on Stick-Slip Theory

Casing damage caused by shear stress has been detected in several areas of the Daqing oilfield. Such damage can be caused by marker bed slip in mud shale sections. However, the mechanism of slippage stress in mud shale horizontal sections is currently not well understood. Based on the effect of differences in interregional pore pressure, we have established a mechanical model of a cracked horizontal plane and obtained analytical solutions for the formation stress and displacement. In order to quantitatively characterize the relative slip distance in a horizontal section, the fault stick-slip and sliding weakening theories have been used. In this paper, based on a stick-slip theory, we propose a model to calculate the slip of a mud shale horizontal section under differing interregional pore pressures. The results show, that under the influence of differing formation pore pressure between the regions, the upper layer of the horizontal section intermittently slips towards the high-pressure center. When the pore water enters the formation, the friction between the sections decreases, causing a decrease in the weakened slip phenomenon. Consequently, the slippage of the formation section will increase under the conditions. The results of the study can provide useful background for preventing casing failures in mud shale marker beds.