Impact of water, nitrogen and CO2 fracturing fluids on fracturing initiation pressure and flow pattern in anisotropic shale reservoirs

Water-based fluids are currently main fracturing fluids but have some unavoidable limitation. Alternative options include nitrogen and CO2 in some new fracturing methods. However, the fracturing mechanism with water, CO2 and nitrogen has not been clear so far. This paper proposes a numerical model to investigate the impact of water, CO2 and nitrogen fluids on fracturing initiation pressure and seepage area of a shale reservoir. First, the governing equations for anisotropic deformation and anisotropic seepage are further extended for bedding shale. Second, a moving seepage boundary is used for fracturing fluid permeation. Third, a fracture initiation criterion is established based on the stress intensity factor of mode I crack. Fourth, a complex parameter is suggested to express the combined effect of shale permeability, fluid viscosity and pressurization rate. This numerical model is then verified with two sets of experimental data and numerical simulations available in literature. Finally, the impacts of the complex parameter, Biot's coefficient, confining pressure, and critical state on fracturing initiation pressure and seepage area are investigated. The permeability evolution during fracturing initiation process is discussed. It is found that this numerical model has the capability to simulate the hydraulic fracturing process with water, CO2 and nitrogen. Complex parameter, Biot's coefficient and confining pressure have significant impacts on fracturing initiation pressure and seepage area. The fracturing initiation pressure decreases with complex parameter and Biot's coefficient but increases with confining pressure regardless of fracturing fluid type. Water has the highest fracturing initiation pressure and the smallest seepage area. These differences are from the change of effective stress in seepage area, particularly nearby the permeation front. CO2 and nitrogen are good alternative fluids for hydraulic fracturing under the same condition of complex parameter, Biot's coefficient and confining pressure because they have lower fracturing initiation pressure and larger seepage area compared to water.