The fracture initiation pressure of conventional hydraulic fracturing is the highest, the fractures produced are limited in scale and single in fracture shape. The fracture initiation pressure of supercritical CO2 fracturing in shale is about 11.58% lower than that of hydraulic fracturing. The fracture initiation pressure of liquid CO2 fracturing in shale is the lowest. In the process of fracturing shale with liquid CO2 , the fractures are cracked and expanded many times, and there are many secondary fractures, forming a complex fracture network. After the shale reacts with slick water, supercritical CO2 and liquid CO2 , the elastic modulus of the rock increases, while the Poisson's ratio and compressive strength decrease. Due to its low temperature properties, liquid CO2 has the most significant degradation effect on the mechanical properties of shale.
Shale reservoirs, as unconventional oil and gas resources have become the main force of the world's oil and gas growth (Stringfellow et al., 2017; Shrestha et al., 2017; Zou et al., 2019; Zou et al., 2020). The Chang 7 shale reservoir in Ordos Basin has the low development degree of natural micro-fractures, but high brittle mineral content and TOC content. It is beneficial to generate new micro-fractures or expand the original micro-fractures through later fracturing to improve shale oil recovery (Cui et al, 2019; Lin et al, 2019; You et.al, 2021). When CO2 fracturing in shale reservoirs, one of its main advantages is that it has lower fracturing pressure, which is more conducive to rock fracturing (Richard et al, 2015; Middleton et al, 2015; He et al, 2016). In addition, compared with other unconventional reservoirs, the natural fractures and natural bedding of shale have a significant effect on fracture propagation. Compressed fractures tend to communicate with natural fractures to form a complex fracture network, and can also induce fishbone fractures, so that branch fractures extend along the bedding surface (Guo et al, 2014; Tan et al, 2017; Ranjith et al, 2019). Compared with conventional hydraulic fracturing, the fractures generated by CO2 fracturing are the products of comprehensive tensile and shear failure. When fracturing shale, a complex fracture network is more likely to be formed, and the fracture surface is rough (Zhang et al, 2017; Zhang et al, 2019; Chen et al, 2021).