Abstract

Conventional hydraulic fracturing may not work well for the effective extraction of shale gas in China. This is due to the fact that the shale rock is much more sensitive to water because of the huge capillary effect and the high content of clay. In this study, we investigate the use of alternative fracturing liquids such as supercritical CO2 or liquid CO2. The initiation and evolution of the fractures are considered as a coupled phenomenon among gas flow, solid deformation and damage. Therefore, a coupled model of shale damage mechanics and gas flow is proposed to simulate the gas fracturing processes. In this model, an equation-of-state of CO2 is included. This inclusion is essential to see the roles of rock-gas interactions on the progressive propagation of fractures driven by the gas pressurization. Through a series of simulation studies, we have demonstrated that due to the low dynamic viscosity, gas penetrates into shale rock easily during gas fracturing processes. This easy penetration changes the effective stresses in the vicinity of the borehole and fractures the elements with a lower strength around the borehole. These fractures propagate progressively as the injection pressure increases. The patterns of fractures induced by supercritical and liquid CO2 are more complex and distribute more widely than those induced by water under the same injection pressure. This is because fluids with a lower dynamic viscosity can flow and transfer the pore pressure into a farther region from the borehole. Similarly, the dynamic viscosity of fracturing fluids also has a great effect on the breakdown pressure. With increasing dynamic viscosity the breakdown pressure increases greatly. These conclusions are consistent with the experimental observations in the literature.

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