The Role of Geomechanics in Pressure Transient Analysis for Reservoir Evaluation

Conventional well test analysis commonly relies on flow-only models that describe pore fluid movement through rock while neglecting the influence of rock deformation on pore pressure. Despite being widely used in field practice, this approach can lead to inaccurate permeability estimates. To assess the extent of this limitation, a coupled flow-geomechanics solution is developed for well test analysis in a layered system comprising a permeable reservoir confined in between impermeable seal rocks with dissimilar mechanical properties. Fluid is produced from a vertical well with uniform inflow across the reservoir thickness. The governing equations for fluid continuity and stress equilibrium are solved analytically using Laplace and Hankel transforms, without making any restrictive assumptions on stress and strain states of rock layers or interlayer tractions. The solution is applied to drawdown and buildup tests. Results show that geomechanical effects are negligible in mechanically homogeneous systems. Conversely, neglecting geomechanics can cause substantial errors in permeability estimates of reservoir rock in the case of mechanically dissimilar layers. These errors consistently yet nonlinearly increase with increase in the stiffness contrast between the permeable and seal rocks.