Effect of rock brittleness on propagation of hydraulic fractures in shale reservoirs with bedding-planes

© 2020 The Authors. Energy Science & Engineering published by the Society of Chemical Industry and John Wiley & Sons Ltd. Hydraulic fracturing forms complex hydraulic fracture networks (HFNs) in shale reservoirs and significantly improves the permeability of shale reservoirs. Although rock brittleness is a major factor in determining whether a shale reservoir can be fractured, the relationship between HFNs and rock brittleness remains unclear. To investigate this relationship in a shale reservoir with bedding planes, this paper presents a series of hydraulic fracturing simulations based on a hydromechanically coupled discrete element model. In addition, we analyzed the sensitivity of the difference in rock brittleness to bedding-plane angle and density. The parameters used in the model were verified by comparing the simulated results with experimental results and a theoretical equation. The results showed that breakdown pressure and injection pressure increased with increasing rock brittleness. The tensile hydraulic fracture of a shale reservoir (THFSR) was always the most abundant type of hydraulic fracture (HF)—almost 2.5 times the sum of the other three types of HFs. The distribution of areas with higher fluid pressure deviated from the direction of the maximum principal stress when the angle between the bedding plane and maximum principal stress directions was large. Upon increasing this bedding-plane angle, the breakdown pressure and rock brittleness index first decreased and then increased. However, regardless of bedding angle, the relative proportions of the various types of HFs remained essentially constant, and the seepage area expanded in the direction of the maximum principal stress. Increased bedding-plane density resulted in a gradual increase in the total number of HFs, with significantly fewer of the THFSR type, and the large seepage areas connected with each other. This study thus provides useful information for preparing strategies for hydraulic fracturing.