An Upscaling Methodology for EOR

Abstract

The outcome of immiscible enhanced oil recovery (EOR) processes such as low salinity water injection and polymer flooding can be very sensitive to geological heterogeneity. In some cases the resulting reduction in macroscopic sweep (versus a waterflood) can be more than the improvement in microscopic displacement efficiency. It is therefore important to be able to model the impact of this heterogeneity during simulation studies. This may require an upscaling step if the geological heterogeneity is smaller than the simulation grid block size or simply to compensate for numerical diffusion on the coarse grid. This paper proposes an upscaling methodology that can be applied to different EOR processes and demonstrates its application to low salinity water injection examples. The methodology involves a hierarchy of upscaling steps. First the absolute permeability is upscaled with the objective of predicting the correct changes in pressure. This may also involve near well bore upscaling. Next pseudo relative permeability curves are generated to capture the shock front behaviour. In this study we compare results obtained from using traditional pore volume weighted pseudos with those obtained using pseudos determined analytically using Buckley-Leverett theory. Finally the simulator models relevant to the EOR process of interest are upscaled. In low salinity waterflooding this is the choice of low and high salinity thresholds. The upscaled models are in better agreement with the fine grid models in terms of pressure, water saturation production and production of either salinity or polymer than are the outputs of coarse grid models without upscaling. The results using the analytical pseudo relative permeabilities are comparable to those obtained from simulations using the pore volume weighted pseudo in many of the cases tested. These models are obviously less time-consuming and complex to generate as they do not need the engineer to run a full fine grid simulation of the EOR process to calculate the pseudo relative permeabilities.