Fracture initiation and propagation under different perforation orientation angles in supercritical CO2 fracturing

Supercritical carbon dioxide (SC-CO2) fracturing has attracted considerable attention on exploiting shale gas and tight gas. This study investigated the influence of the perforation orientation on fracture initiation and propagation induced by SC-CO2 fracturing through a series of laboratory fracturing experiments under tri-axial stress states and water was also used as a fracturing fluid to compare with SC-CO2. It was found that breakdown pressure increases with the increase of perforation angle according to the pressure curves in SC-CO2 fracturing and hydraulic fracturing. AE energy characteristic has shown that AE energy release rate of SC-CO2 fracturing is larger than that of hydraulic fracturing, indicating that fractures induced by SC-CO2 fracturing are more complex. Fracture geometry and energy surges verified that there were more fractures induced by SC-CO2 fracturing with increase of the perforation orientation angle. It was observed that hydraulic fracturing mainly induced a biwing fracture while SC-CO2 fracturing induced bi-wing and tri-wing fracture. At low perforation angle (alpha <= 45 degrees), perforation orientation has little influence on fracture propagation direction and fracture extends along the direction of the maximum principle direction in SC-CO2 fracturing. At high perforation angle (alpha <= 60 degrees), fracture propagation shows an independence of loading stress, fractures may extend in three directions. The findings are beneficial for further understanding the mechanism of SC-CO2 fracturing and optimizing perforation parameters in SC-CO2 field fracturing.