Propagation and distribution of shear and tensile hydraulic fractures affected by frictional coal beddings

As a typical sedimentary rock, coal seams are usually rich in bedding planes, which significantly affect the propagation process of hydraulic fracture (HF) network and reservoir permeability. In this study, the profile characteristics and the bedding roughness of the coal seam at depth of 2800 m from Daniudi Gas Field in China is investigated. The failure mechanism of shear-expansion effect under low pressure and dynamic shear under high pressure of coal beddings induced by the dissolution-corrosion effect of fracturing liquid is analyzed. The development process of HF network under different bedding angles, joint roughness coefficients (JRCs), stress differences, and confining pressures is numerically simulated by cohesive element method. Results show that the JRC of coal beddings has significant effects on the morphology of HF networks. When the JRC exceeds 15, a HF network structure with tensile main fractures and shear secondary cracks is developed. The shear fractures account for a considerable proportion (33-66%) multi-layer layered coal seams and should be taken seriously during reservoir reconstruction. As confining pressure increases, the total length of tensile HFs sharply decreases, while the change in the total length of shear HFs is not significant. When the confining pressure increases from 5 to 15 MPa, the length ratio of shear HFs to tensile hydraulic fractures increases from 0.36 to about 1, confirming that increasing confining pressure can promote the transformation of HF failure types from tension to shear. The ratio of secondary crack to main crack length is between 6.31 and 9.78, indicating the secondary fractures occupy an absolute proportion in the hydraulic fracture network. This study is of significance for understanding the hydraulic fracture network development in coal reservoirs.