Effect of Temperature and Pressure on Mechanical Behavior of Reservoir Shales at Different Depths: Implications for Deep Shale Gas Extraction

Understanding the mechanical and brittle-ductile behavior of shales at different depths, controlled by in-situ stresses and reservoir temperatures, is critical for borehole stability evaluation and extraction design, which determines whether deep resources can be exploited safely and efficiently. In this work, triaxial compression tests were performed on reservoir shales at varying depths of 3300-4300 m under confining pressures (pc) ranging from 20 to 110 MPa, and with temperatures (T) ranging from 25 to 160 degrees C. The brittle-ductile transition mechanism of the shale was investigated by micro-CT scanning. Finally, a new brittleness grading system was established to reflect the fracture characteristics and mechanical behavior of shales in different brittleness intervals. Results show that the mechanical properties of shales are strongly dependent on pc, whereas T (<= 130 degrees C) has a minor effect on the shale's performance except for clay-rich shales. At T = 160 degrees C, the quartz-rich shale is mechanically strengthened and the clay-rich shale is weakened by heating at high confining pressure. The reservoir shales exhibit mainly semi-brittle failure at applied pc-T conditions, indicating that the deep shale reservoirs shallower than 4500 m are suitable for fracturing stimulation. However, the clay-rich (51%) shale exhibits ductile behavior at 70 MPa-160 degrees C and 110 MPa-25 degrees C, which is not conducive to fracturing. Two-dimensional X-ray imaging shows that the cracks within the brittle damaged samples are mainly shear cracks, whereas the ductile damaged samples are dominated by a cataclastic texture covered with tiny micro-cracks, indicating that the T- and pc-induced brittle-ductile transition is associated with primarily cataclastic deformation. The above findings would be of great value for the efficient exploitation of deep shale gas resources.