Numerical Simulation of Hydraulic Fracturing in Coal Reservoirs Based on a Damage–Seepage Coupled Model and Optimization of Perforation Parameters

Abstract

Coal reservoirs exhibit strong heterogeneity and complex in situ stress conditions, and the initiation and propagation of hydraulic fractures are jointly controlled by reservoir geology and perforation design. A damage–seepage coupled numerical model was established on the COMSOL Multiphysics platform (Version 6.2) by combining the maximum tensile stress criterion and the Mohr–Coulomb criterion with stress-dependent porosity–permeability relations, and was verified against the classical Hubbert–Willis analytical solution. The model was used to simulate the whole fracturing process, the effect of natural fractures, horizontal-well multi-perforation fracturing, and the sensitivity of breakdown pressure to perforation parameters under six reservoir conditions. The results show that: the bottomhole pressure evolves in a “buildup–breakdown–drop–re-accumulation” pattern, and the secondary breakdown pressure (about 22 MPa) is approximately 15% lower than the initial one (about 26 MPa), while the near-fracture permeability rises from 0.246 mD to more than 40 mD; natural fractures delay the first breakdown from about 40 s to 46 s but accelerate the post-breakdown propagation and enlarge the high-permeability region; horizontal-well fracturing initiates multiple transverse, feather-like fractures from the perforations, with a first breakdown pressure of 11.4 MPa at 20 s; doubling the initial permeability raises the breakdown pressure by 1.4–1.6 MPa, making initial permeability the most sensitive reservoir factor, followed by in situ stress orientation and rock strength; and the favorable perforation combination under the minimum-breakdown-pressure criterion shifts with reservoir conditions, e.g., from “8 holes/m, 60-degree” to “16 holes/m, 30-degree” when the maximum principal stress rotates from horizontal to vertical. These results provide a quantitative basis for the optimization of perforation parameters in coal-reservoir fracturing.