Upscaling miscible CO2 eor processes: Characterisation of physical instabilities

Objectives/Scope: An approach for upscaling of miscible displacements is presented which adequately represents physical instabilities such as viscous and heterogeneity induced fingering on coarser grids using pseudoisation techniques is presented. The approach has been applied to compositional numerical simulations of two-dimensional and three-dimensional reservoir models with a focus on CO2 injection. Methods: The approach is based on the pseudoisation of relative permeability and the application of transport coefficients to upscale viscous and heterogeneity induced fingering in a multi-contact miscible CO2 injection in a black oilwater system. Fine-gridded compositional simulations of 2D cross-sectional and 3D reservoir models were performed for calibration purposes. The cases considered included a homogeneous 2D model and heterogeneous 2D and 3D models. The effects of transmissibility schemes on the performance of the fine-grid 3D miscible displacement process was quantified by considering the minimisation of grid orientation errors by each scheme. The permeability and porosity distribution of the 3D model were extracted from a quarter five-spot of the SPE 10th comparative project. Pseudo-relative permeability curves were computed using various pseudoisation techniques and these were applied in combination with transport coefficients which account for small-scale variations in phase composition and behaviour to upscale the fine-grid simulations to coarser scales. Results and Conclusions: We upscale the fine grid compositional 2D and 3D reservoir models to coarser grid compositional flows. In the fine-grid homogeneous and heterogeneous 2D models, four fingering regimes were observed which we link to very-early time, early time, intermediate time, and late time regimes. These regimes occur both near and far away from the injection well. Our results show that fingering behaviour is adequately captured and represented in the upscaled coarse grids. In the 3D model, a nine-point transmissibility weighting was required to adequately reduce grid orientation effects and was thus employed in the upscaling procedure of the 3D models. The robustness of the upscaled models was assessed by comparing the total volume of fluid produced versus pore volume of fluid injected, and the saturation profiles and history of the upscaled models with those of the fine-grid simulation. The accuracy of the results of the pseudoisation procedures was assessed by applying statistical analysis to compare them to the results of the fine-grid simulations. The results show that the coarse models provide accurate predictions of the miscible displacement process and that the fingering regimes are adequately captured in the coarse models.