Influence of pressure-dependent porosity and permeability on poroelastic behaviour of reservoirs during hydraulic fracturing operations

The standard constitutive equations of linear poroelasticity, assume constant material parameters. However, in situations such as hydraulic fracturing of unconventional oil/gas reservoirs, porosity and permeability of the reservoir can be pressure and stress dependent and vary in space and time. In this work, we analyze the effect of pressure-dependent porosity and permeability of the reservoir on fault reactivation mechanisms. To accomplish this, we perform finite-element modelling of processes associated with hydraulic fracturing operations. Changes in pore pressure, Coulomb failure stress, and seismicity rate on pre-existing faults are calculated and compared to the model where porosity and permeability are held constant during simulation. The simulation scenarios resemble hydraulic fracturing operations that are performed near Crooked Lake region in Alberta, Canada. Previous studies have suggested the direct pore pressure diffusion and indirect poroelastic effect as the two main mechanisms of induced seismicity in the region. We use specific hydraulic fracturing operational data (i.e. fluid injection volume and rate) and the associated seismicity to investigate the fault reactivation mechanisms and the reservoir poroelastic behaviour due to fluid injection. The numerical results of the model show significant deviation in pore pressure and Coulomb failure stresses compared to the model with constant material parameters. The most pronounced changes in behaviour of the model, occur during the injection operations and are accompanied with faster relaxation once injection ends. On the other hand, the lower permeability reservoirs lead to delayed pore pressure diffusion and poroelastic stressing and can result in post-injection seismicity.