Fault Friction and Casing Deformation Analysis of Shale in Southern Sichuan Based on Mineral Composition

With the exploration and development of shale gas resources in southern Sichuan, casing deformation related to fracture and fault shear slip occurs frequently, severely restricting the construction and production of deep shale gas. Therefore, it is significant to determine the friction coefficient of shale gouge under the high-temperature and high-pressure conditions of deep geologic environments from the perspective of fracture/fault friction. In-situ shale samples from three wells (A, B, and C) at different depths of the Longmaxi Formation were used to prepare fault gouges for friction shear tests. The clay mineral content of the samples ranged from 10% to 45%. The results of high-temperature, high-pressure triaxial shear friction tests show that the friction coefficient of the deep shale is between 0.52 and 0.70, and it decreases linearly as the clay mineral content increases. Based on the mineral composition of the Longmaxi shale, the distribution of the friction coefficient was predicted, revealing that the friction coefficient increases with relative depth within a single well. Finally, the slip amount of fractures/faults in the Longmaxi shale was calculated from the stress drop observed in the shear tests. This calculated slip corresponds well with actual casing deformation measurements. These results further illustrate the relationship between fracture/fault slip and casing deformation and provide a corresponding calculation method.