Vast system of dense intersecting fractures: A key feature of hydraulic fracturing of gas shale

This contribution presents a multi-physics modeling of fluid-driven propagation of a vast network of fractures and open joints in shale, with a fracture spacing of about 10 cm, as deduced from the observed gas extraction history at wellhead. Because of the vast number of fractures and quasibrittle nature of shale, the fracture of shale is analyzed in a smeared way by the crack band model. One key idea is that, to model lateral fractures branching from a primary fracture wall, fracture pressurization, by viscous Poiseuille type flow, of compressible (proppant-laden) fracturing water must be complemented with the pressurization of a sufficient volume of micropores and microfractures by Darcy-type water diffusion into the shale, which generates tension along the existing fracture walls, overcoming the strength limit of the cohesive-crack or crack-band model. A second key idea is that enforcing the equilibrium of stresses in fractures, pores and water, with the generation of tension in the solid phase, requires a new three-phase medium concept, transitional between Biot's two-phase medium and Terzaghi's effective stress. Finite-element/finite-volume simulations demonstrate the growth of a large hydraulic fracture system. Study of the effects of various parameters could increase the efficacy of fracturing and reduce water injection.