Research on Evolutionary Laws of Mechanical Properties and Pore Structure during CO 2 Pre-Injection Fracturing in Shale Reservoirs

CO 2 pre-injection fracturing is a promising technology for shale reservoirs development, with multiple advantages for improving the complexity of fractures, the production of crude oil, and the sequestration of CO 2 . Previous research mostly focused on the CO 2 effect on macroscopic mechanical properties of shale. However, there are many phenomena closely related to shale micro mechanical behavior. Therefore, this study presents a systematic investigation into the effects of CO 2 on both macro and micro mechanical properties, as well as pore-fracture structures during CO 2 pre-injection fracturing in shale reservoirs. The results show that CO 2 can significantly decrease the tensile strength, uniaxial compressive strength, and elastic modulus of shale. With the increasing CO 2 treatment time, the macro mechanical properties of shale decrease gradually. The microscopic experiments show that this significant decrease may be due to two mechanisms. The first is the significant decrease in the micro-mechanical properties of shale. The results of indentation analysis show that the microscopic elastic modulus and hardness of shale decrease by 51.3% and 63.3% after CO 2 treatment. The second is the changes of the original shale framework. Pore-fractures structure analysis showed that after CO 2 treatment, a large number of dissolution pores are generated in the shale matrix. Meanwhile, there are propagation of original fractures and opening of structural weak planes, which lead to the form of new fractures. Under the action of these two mechanisms, the macro mechanical strength of shale is reduced significantly. Therefore, in the field application, proper soaking following CO 2 injection could lead to a significant overall reduction in mechanical strength, potentially lowering formation breakdown pressure, easing the requirements for treatment equipment, and enhancing fracturing effects.