Coupled geomechanics and flow simulation for time-lapse seismic modeling

To accurately predict production in compactible reservoirs, we must use coupled models of fluid flow and mechanical deformation. Staggered-in-time loose coupling of flow and deformation via a high-level numerical interface that repeatedly calls first flow and then mechanics allows us to leverage the decades of work put into individual flow and mechanics simulators while still capturing realistic coupled physics. These two processes are often naturally modeled using different time stepping schemes and different spatial grids—flow should only model the reservoir, whereas mechanics requires a grid that extends to the earth’s surface for overburden loading and may extend further than the reservoir in the lateral directions. Although spatial and temporal variability between flow and mechanics can be difficult to accommodate with full coupling, it is easily handled via loose coupling. We calculate the total stress by adding pore pressures to the effective rock stress. In turn, changes in volume strain induce updates to porosity and permeability and, hence, dynamically alter the flow solution during simulation. Incorporating the resulting time-dependent pressures, saturations, and porosities (from coupled flow and mechanics) into Gassmann’s equations results in seismic wave velocities and densities that can differ markedly from those calculated from flow alone. In a synthetic numerical experiment based on Belridge field, California, incorporation of coupled flow and mechanical deformation into time-lapse calculations produces compressional wave velocities that differ markedly from those produced by flow alone. In fact, it is the closing of the pores themselves (reduction in permeability) in this example which has the greatest impact on fluid pressures and saturations and, hence, elastic wave parameters such as velocity.