Comparison of fracturing unconventional gas reservoirs using CO₂ and water: an experimental study

CO₂-based fracturing (CBF) has been regarded as the most promising alternative to water-based fracturing (WBF) in water-sensitive unconventional gas reservoirs, and the fracture initiation and propagation at different in-site stresses created by CBF is crucial to the flow conductivity of fracture network. The main aim of this study is to compare the fracturing efficiency of WBF and CBF by conducting a series of fracturing tests on siltstone samples. According to the experimental results, the breakdown pressure linearly increases with the confining pressure, and the slope is 1.3928 and 1.6422 for CBF and WBF, respectively, and the much lower breakdown pressure for CBF is because of the fast penetration of CO₂ through the rock matrix, which significantly reduces the effective stress around the borehole. Importantly, the observed fracture aperture for CBF is much greater than that for WBF, the maximum fracture aperture for CBF is around 2 to 5 times greater than that for WBF under the same test conditions. The difference in fracture aperture depends on the presence of discrete blocks and particles, and CBF can create greater and more loose particles in both mechanically-created method and hydraulically-created method to keep the created fractures open after the release of the injection pressure. The greater fracture aperture of CBF is favourable for the flow capacity of the fractures and therefore reservoir productivity, which is verified by the permeability tests with 190.43 times of permeability than that of WBF. Interestingly, the greater fracture width expansion for CBF perpendicular to the major principal pressure promotes the generation of horizontal fractures, even when the injection pressure is smaller than the major principal pressure, and the probability of horizontal fracture for CBF is 4–5 times of that for WBF, which reflect its great potential to create complex fracture network in unconventional gas reservoirs. The test results reveal that CBF is superior to the WBF since it is capable to create much more complex and efficient fracture networks. The findings are beneficial to understand the difference of fracturing behaviours at different in-site stresses between WBF and CBF, and provide experimental guidance for the exploitation of water-sensitive unconventional gas reservoirs by CBF.