Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. During the gas fracturing processes, gas will penetrate into the borehole wall due to the low surface tension and low dynamic viscosity of the fluid and the evolution of the fractures results from the coupled phenomena of gas flow, solid deformation and damage. To represent this, a coupled model of rock damage mechanics and gas flow is presented. We investigate the fracturing processes driven by pressurization of gas within a borehole and compare it with water-based fracturing. Simulation results indicate that gas fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based fracturing. We explore the relation between the fracture initiation pressure, breakdown pressure and complexity of fractures to both the interfacial tension and the dynamic viscosity of the fracturing fluids. It is shown that the fracture initiation pressure and breakdown pressure increase and the complexity of the resulting fracture networks decreases with increasing interfacial tension and dynamic viscosity. These conclusions are consistent with experimental observations.
As an important product from unconventional reservoirs, shale gas has been produced commercially and is increasingly recognized as a "game changer" for the U.S. natural gas and energy markets in the past few years . Shale gas is trapped inside formations of shale, which are fine grained sedimentary rocks with very low permeability, and more than 90% of the shale gas reservoirs need treatments to enhance their productivity, such as fracturing and acidizing .