A numerical investigation for the impacts of shale matrix heterogeneity on hydraulic fracturing with a two-dimensional particle assemblage simulation model

A shale gas reservoir has complex mineral components, bedding planes and micro-cracks, thus being of inherent heterogeneity and anisotropy. The roles of bedding planes in hydraulic fracturing performance have been deeply studied, but the effects of rock inherent heterogeneity induced by mineral components are not well understood. This paper establishes a two-dimensional particle assemblage model to investigate the impacts of mineral component heterogeneity on hydraulic fracturing treatments. The impacts of the mass fraction and elastic modulus of low-brittle minerals on fracture initiation pressure, breakdown pressure, fluid injection volume and fracture permeability are comparatively investigated through numerical simulations. It has been found that the mass fraction of low-brittle minerals is closely related to the critical pore water pressure while the elastic modulus of low-brittle minerals plays a significant role in fracture permeability. With the same hydraulic fracturing treatments, the shale gas reservoir with a large mass fraction of clay minerals could decrease the critical pore water pressure, while the shale gas reservoir with a low elastic modulus of clay minerals is conducive to increasing fracture aperture. The shale matrix heterogeneity induced by the fraction coefficient and micro mechanical properties of clay mineral has a critical role in the fracture initiation and propagation as well as the variation of fracture aperture. This understanding of shale mineral composition can benefit the effectiveness evaluation on hydraulic fracturing and the morphology of fracture networks.