New Upscaling Scheme for Capillary Dominant Displacement

Abstract

Abstract A procedure for upscaling CO2 buoyancy-driven upward migration in finite-difference simulation models is presented in this work. This upscaling procedure allows accounting for effects of capillary and buoyancy forces to enable CO2 upward migration modeling in coarser grids while preserving approximately dominant fine-scaled geological effects. The method has been used in 2D domain simulations with no-flow boundary conditions. The conceptual geological models are built by utilizing sequential Gaussian simulation for different correlation lengths and high level of heterogeneity, Dykstra-Parsons coefficient of 0.7. Multiphase flow upscaling (MPF) has been improved by accounting for spatial connectivity (percolation), which enabled us to obtain more realistic rock-fluid pseudo-functions and capture effects of local capillary trapping at the fine scale (meso-scale trapping). In contrast with other MPF, e.g. Ekrann (1999) and van Duijn et al. (2002), calculation of rock-fluid functions of each cell has been conditioned by accessibility. Therefore, configuration of coinciding fine-scale cells in a coarse cell affects calculation of upscaled rock-fluid functions. In addition, the effective porous medium properties were estimated using mean field theory (the Maxwell approximation), which considers continuity and isolation of geological facies, responsible for macroscopic residual saturation. The upscaling method and estimation of rock-fluid functions were numerically tested and compared with currently accepted single- and multiphase-flow upscaling methods. Our results show that single-phase flow upscaling and Maxwell approximation fail to adequately predict mobility and residual saturation. Those upscaling approximations predict gas travel time 7 fold faster. Therefore multiphase flow upscaling should be employed. Significant improvement in gas travel time (representative of mobility) and trapped CO2 saturation (representative of trapped saturation) are observed when spatial connectivity (percolation) is included. Implementation of percolation, improves prediction of gas travel time and residual saturation 21% and 50% respectively. The developed upscaling scheme lowers simulation execution time 17 fold through upscaling and noticeably enhances the prediction of phases mobilities and macroscopic trapped saturation. This speedup will enable simulating 3D CO2 sequestration simulation scenarios. The scheme can also serve to scale up other subsurface multiphase flow displacement where capillary force is dominant one.