The mode I fracture toughness alternation and crack propagation behavior evolution due to long-term Sc-CO2 saturation

The mode I fracture toughness is a critical parameter which defines the rock's resistance to crack propagation, especially in hydraulic fracturing. Recently, Supercritical carbon dioxide (Sc-CO2) has been proposed as a fracturing fluid candidate in hydraulic fracturing stimulations of shale reservoirs. However, its effects on fracture toughness transition and crack propagation behaviors have not been understood well. In this study, we performed a series of semicircular bend speci-mens (SCB) before the Longmaxi shale specimens' saturation. Three-point bending tests along divider orientation showed that after Sc-CO2 saturation, mode I fracture toughness (KIC), elastic modulus (E) and absorbed energy (U-e) of shale were decreased by 22.1%, 24.5% and 44.3%, respectively. High speed camera images indicated that after Sc-CO2 saturation, mode I crack directly propagated straight along artificial pre-crack direction, decreasing the degree of crack deviation as in KIC. The results of Cronos high-precision 3D scanning system and scanning elec-tron microscopy (SEM) revealed complicated fracture mechanisms (transgranular, intergranular and mutual coupling crack mechanisms) of shale after Sc-CO2 saturation, which reduced the roughness and area of fracture crack surface. The generation of pore and cracks was the main reason for the decrease of shale resistance to fracture. Furthermore, Sc-CO2 saturated shale specimens only needed to absorb less energy to more rapidly cause the initiation and propagation of mode I cracks with the main fracture mode being transgranular cracks.

Automated summary

This study investigates the effects of supercritical carbon dioxide (Sc-CO2) on the fracture behavior of shale. The results show that after Sc-CO2 saturation, the fracture toughness, elastic modulus, and absorbed energy of shale decrease. High-speed camera images and scanning electron microscopy reveal changes in fracture mechanisms and surface roughness of shale after saturation. The generation of pore and cracks is identified as the main reason for the decrease in shale resistance to fracture. Additionally, saturated shale specimens require less energy to initiate and propagate cracks, with a main fracture mode of transgranular cracks.