Anisotropic rate-dependent saturation functions for compositional simulation of sandstone composites

Determination of representative saturation functions for permeable reservoir sandstones requires consideration of mm-cm scale laminations and crossbedding common in these heterogeneous porous media. Modelling selected laminated sandstones from CO2CRC's Otway International Test Centre in Victoria, Australia as bimodal composites of different rock types, we analyse drainage relative permeability parallel and perpendicular to the laminae for a range of driving gradients. The results from these numeric drainage experiments are curve fit across a continuous range of flow rates between the capillary and viscous limits. Our analysis shows that laminae-related small, but spatially correlated grainsize and permeability variations give rise to flow-rate dependent drainage behaviour at the bed scale. The flow direction- and rate-dependent behaviour is captured by a new set of anisotropic extended saturation functions that are implemented and presented in a form that is suitable for compositional simulation in the system CO₂–H₂O–NaCl. For the first time, these new constitutive relationships permit an investigation of feedbacks between the reservoir-scale flow regime and bed-scale phase mobility. To illustrate the application of these functions in reservoir simulation, meter-scale plume spreading simulations are presented for a suite of ten homogenised fluvial-to-estuarine sandstones from the Otway facility.