Observations on the impact of displacement properties on mobility and relative permeability

Abstract

The accuracy of relative permeability and mobility data can strongly impact the uncertainty in predictions of production in reservoir models. It is of particular importance to identify the most practical conditions to perform relative permeability tests in order to mimic the difference between test and subsurface conditions. It is known that the details of a displacement test, such as capillary number, can immensely impact the characteristics of a displacement. Therefore, it is paramount to perform displacement tests representative of the reservoir conditions. To improve the general understanding and design of displacement tests this work presents an exploration of displacement parameters and their impact on results. A multiphase Lattice Boltzmann Method is used to digitally perform displacement tests for a range of relevant conditions. Direct simulations are performed on 3D models of pore spaces built from microCT images of rock. A parameter space consisting of capillary number, wetting condition, viscosity ratio and driving mechanism is explored on multiple sandstone rock images. Results showing the impact multiple displacement properties have on relative permeability and mobility are presented. Particular attention was paid to the role of capillary number on total mobility. A wide range of capillary numbers (1 × 10−7 to 1 × 10−4) was investigated. It was observed that a quantification of mobility, the minimum total mobility of both fluids, is largely constant for the lower most range of capillary numbers for a specific rock image. This plateau behavior holds when capillary number is below a critical threshold where capillary forces fully dominate the 2-phase flow topology. Above this threshold mobility increases monotonically with capillary number. The study also presents relationships for various wetting conditions, viscosity ratios and driving mechanisms. When using an alternate definition of capillary number which relates velocity and fluid properties at the point of minimum total mobility, broad similarity in mobility data was observed for the various viscosity ratios and driving mechanisms investigated. In contrast, the minimum total mobility was consistently higher for oil-wet displacements when compared to equivalent water-wet displacements.