Fracture extension behavior and micro-mechanical damage mechanism under different CO2 fracturing methods in continental shale reservoirs

Continental shale reservoirs are resilient, and the fracture networks formed by conventional fracturing technologies are limited in size. The CO2 fracturing technology can be adapted for the development of continental shale reservoirs. However, shale reservoirs have strong heterogeneity in the microscopic pore-throat system. The fracture extension and micro-mechanical mechanisms of CO2 fracturing in shale reservoirs are not defined. To study the CO2 fracturing mechanism in shale reservoirs, typical core samples were selected from the Chang 7 shale reservoir of the Yanchang Formation in the Ordos Basin. Moreover, true triaxial simulated fracturing, CO2- fluid-rock microscopic damage, and rock mechanics experiments were conducted before and after CO2 fracturing. The experimental results were used to analyze the fracture initiation, steering, extension, and stopping criteria under different CO2 fracturing methods, quantitatively characterize the damage degree of the microscopic pore-throat system in shale reservoirs after CO2-fluid-rock interaction, evaluate the deterioration mechanism of CO2 fracturing on the mechanical properties of the shale reservoir, and clarify the complex fracture network formation mechanism as well as microscopic and mechanical control effects under different CO2 fracturing methods in shale reservoirs. The results show that CO2-fracturing shale reservoirs have lower fracture initiation pressure. The main fracture communicates with the natural bedding planes of the shale, and various fracture modes, such as tensile damage, shear damage, and combined tensile-shear damage, are apparent. The degree of damage to the microscopic pore-throat system by the CO2-fluid-rock interaction ranges from 7.78% to 9.28%. The longitudinal and transverse wave velocities of the rock are reduced, Young's modulus and compressive strength are decreased, and the mechanical properties of the rock deteriorate. The L-CO2 fracturing of shale forms large-scale fractures with high fracture complexity. The microscopic damage was dominated by physical damage, the damage of liquid CO2 (L-CO2) is more significant for medium and large pores. The deterioration of the rock mechanical properties was evident, and the rock brittleness index was reduced. Small-scale fractures form owing to the SC-CO2 fracturing shale reservoirs. Microscopic damage is dominated by chemical damage, strong dissolution-acidification. The structural damage of supercritical CO2 (SC-CO2) is evident for small pores. Rock brittleness is higher after CO2 fracturing. The results of this study reveals the fracture expansion mechanism of CO2 fracturing shale reservoirs and the control law of microscopic and mechanical damage effects on fracture network formation. This study enriches the basic theoretical system and has high economic, environmental and scientific value for enhanced oil recovery in shale reservoirs.