Experimental Study of CO₂ Fracturing Behaviour in Ultralow Permeability Formations

CO₂ fracturing with many unique advantages has been successfully used as an alternative to water-based fracturing in unconventional gas production. However, the great difference of fracturing behaviours between water and CO₂ fracturing for different gas reservoirs is still scarcely studied. The intention of this study is to investigate the fracturing mechanism and to compare the fracture geometry characters between water and CO₂ fracturing, and a series of fracturing tests on sedimentary rocks was therefore conducted. Experimental results show that breakdown pressure of CO₂ fracturing is much lower than that of water fracturing for siltstone samples, and the difference of breakdown pressure between water and CO₂ fracturing for shale samples with much lower permeability is much greater. The much higher breakdown pressure for shale samples than siltstone samples indicates that high pore pressure within rock matrix due to the strong penetration ability can greatly decrease the breakdown pressure. CO₂ fracturing creates more loose particles and discrete blocks acting like proppants, and fracture aperture of CO₂ fracturing is 2-5 times that of water fracturing. Multi-parallel fractures and bigger discrete blocks contribute to the low fracture compressibility, and the secondary shear fractures along the weak bedding planes in shale samples greatly increase the complexity of the fracture network.