Effects of Ductility of Organic-Rich Shale on Hydraulic Fracturing: A Fully Coupled Extended-Finite-Element-Method Analysis Using a Modified Cohesive Zone Model

Summary The popular cohesive zone model (CZM) that only features decreasing cohesive traction along with crack separation might not adequately represent the fracturing behavior in organic-rich shale because of increased ductility. This paper proposes a novel CZM that can realize various traction/separation laws (TSLs) by a unified formulation to better represent the increased ductility of organic-rich shale. This modified CZM was implemented in a fully coupled in-house poroelastic extended-finite-element-method (XFEM) framework that has been comprehensively verified against the latest analytical solutions. The implications of increased ductility in different forms on hydraulic fracturing were studied using the newly designed progressive parametric study. First, the shape of the TSL affects the hydraulic fracturing given the same cohesive crack energy and tensile strength, which further indicates the necessity of the newly proposed TSL. Second, the initial tensile strength, controlling when the cohesive crack starts propagating, has the greatest effect on the hydraulic fracturing among all TSL shape parameters. The effects of TSL parameters become less significant as the fracturing-fluid viscosity increases. Finally, Young's modulus among four common poroelastic parameters most significantly affects the brittleness of rock formation and hydraulic-fracture lengths. The increase in cohesive energy accompanied by the decrease of Young's modulus can greatly reduce the hydraulic-fracture length under the same injection volume.