Semi-Analytical Modeling on 3D Stress Redistribution During Hydraulic Fracturing Stimulation and Its Effects on Fault Reactivation

摘要

The hydraulic fracturing technique has been widely applied in many fields, such as the enhanced geothermal systems (EGS), the improvement of injection rates for geologic sequestration of CO2, and for the stimulations of oil and gas reservoirs, especially the unconventional shale reservoirs. The key points for the success of hydraulic fracturing operations in unconventional resources are to accurately estimate the redistribution of pore pressure and stresses around the induced fracture and predict the reactivations of pre-existing faults. The fracture extension as well as pore pressure and stress regime around it are affected by: poroelastic and thermoelastic phenomena as well as by fracture opening under the combined action of applied pressure and in-situ stresses. A couple of numerical studies and analytical methods have been done for analyzing the potential for fault reactivation resulting from pressurization of the hydraulic fracture. In this work, based on our previous studies, a comprehensive semi-analytical model is constructed to estimate the stress and pore pressure distribution around an injection induced fracture from a single well in an infinite reservoir. The model allows the leak-off distribution in the formation to be three-dimensional with the pressure transient moving ellipsoidcally outward into the reservoir with respect to the fracture surface. The pore pressure and the stress changes in three dimensions at any point around the fracture caused by poroelasticity, thermoelasticity and fracture compression are investigated. With Mohr-Coulomb failure criterion, we calculate the fault reactivation potential around the fracture. Then, two case studies of constant water injection into a hydraulic fracture are presented. This work is of interest in interpretation of micro-seismicity in hydraulic fracturing and in assessing permeability variation around a stimulation zone, as well as in estimation of the fracture spacing during hydraulic fracturing operations. In addition, the results from this study can be very helpful for selection of stimulated wells and further design of the re-fracturing operations.