The effect of natural fractures on hydraulic fracturing propagation in coal seams

The purpose of hydraulic fracturing is to improve the gas permeability of a coal seam by the high-pressure injection of fracturing fluid into cracks. Some promising results of hydraulic fracturing in a coal seam using isotropic and intact model have been published in our previous study (Wang et al., 2014), based on which further research is necessary for the reason that natural coal is anisotropic, inhomogeneous, inelastic, and characterized by multiple discontinuities, which can be one of the most important factors governing the deformability, strength and permeability. It is difficult to accurately identify and predict the manner in which hydraulic fractures initiate and propagate because of the pre-existing natural fractures. In this paper, five typical coal models-intact coal, layered jointed coal, vertical jointed coal, orthogonal jointed coal, and synthetic jointed coal-are established to simulate hydraulic fracturing in coal seam based on two-dimensional particle flow code (PFC2D). The effect of natural existing fractures on fluid-driven hydraulic fracture is investigated by analyzing the variation of fracture radius, cumulative crack number, and growth rate of porosity versus injection time. It is shown that the existence of natural fractures, which has a significant induced effect on the initiation and propagation of hydraulic fracture, contributes greatly to the increase of crack number and growth rate of porosity. The fracture network is greatly influenced by the interaction between hydraulic fracture and natural fractures. Natural fractures with different structural properties may result in different propagation types of hydraulic fracture, which can be categorized as capturing type, crossing type, and compound type.