https://orcid.org/0000-0002-6199-2109
Summary
Pore pressure and strain monitoring around a hydraulic fracture are used to monitor its size and propagation behavior, such as length and height growth, for assessing the hydraulic connectivity between injection and production wellbores in unconventional and geothermal reservoirs. Pore pressure monitoring and fiber-optic distributed acoustic sensing (DAS) usually have used an elastic fracture model without considering coupled poroelastic processes. In this study, a 3D hydromechanical model is developed to study poroelastic phenomena in relation to pore pressure and stress distribution caused by hydraulic fracturing. Fractures and the surrounding poroelastic rock are discretized explicitly, and nonlinear mechanical behaviors of hydraulic fractures are determined through a cohesive law. Fluid pressurization of a fracture reveals that the induced total stresses in the surrounding rock remain approximately constant; however, the induced pore pressure gradually increases due to fluid leakoff and the mean stress increase via the Skempton’s pore pressure coefficient. Strain analysis at locations close to the fracture propagation path demonstrates that the diffusion can lead to the generation of tensile strain, in contrast to an elastic model which predicts a compressive strain in the direction perpendicular to the fracture surface. Importantly, we show that at a monitoring point, the strain variation from tension to compression can also occur due to poroelastic coupling rather than the fracture arrival and departure. Tensile strain is distributed around the fracture edge and ahead of it, and in the close vicinity of the fracture surfaces. The pattern of tensile strain distribution is generally consistent with the pore pressure distribution. In addition, numerical results suggest hydraulic fractures tend to propagate toward regions with relatively lower pore pressure, promoting asymmetric growth, which can lead to the well-known fracture-driven interactions.
