Small-scale Heterogeneities and Upscaling in Field-scale Multiphase Flow

During the lateral movement of a gravity plume underneath a cap rock, the combined effects of aquifer topology and small-scale heterogeneities (e.g. bedding features below seismic resolution of ~10-20m) can cause significant preferential movement of the plume. While it is still unclear as to the full impact of these effects, there are increasing field observations of unexpected plume migration and early breakthrough (e.g. during CO2 injection at Sleipner, In Salah, Frio) which are poorly predicted by conventional modelling approaches. A rigorous approach to tractably simulate large scale plume migration incorporating these small-scale features is to use equivalent, upscaled multiphase flow parameters.

Here, we employ a multi-scale approach combining petrophysical field characterisation, numerical modelling and upscaling to elucidate the impacts of small-scale heterogeneities in field scale flow. We exhaustively characterise the multiphase flow properties of 48 rock cores covering the entire 100m interval of the Captain D sandstone in the Goldeneye field, North Sea, UK. These properties and well-log data are used to geostatistically populate 2D high-resolution simulations with cm-scale heterogeneities. We vary the spatial correlation within the observed bounds to investigate the impacts of heterogeneity under different flow conditions. Isotropic heterogeneity increases the sweep and slows the plume, conventionally assumed to be the key impact of capillarity. In contrast, anisotropic, layered capillary heterogeneities can speed up lateral plume migration by up to 20%, see Fig 1a,b. This capillary controlled enhancement of plume migration is conventionally ignored and may be a significant contributor to field observations of rapid plume movement.

We develop a steady-state, macroscopic invasion percolation method to derive upscaled equivalent relative permeabilities (anisotropic) and capillary pressures. The upscaled models accurately capture plume migration and the impacts of heterogeneities using orders of magnitude reduced computational meshes and time, see Fig 1c. Finally, we use a km-scale model of the Goldeneye field to explore the impacts of small-heterogeneities in a true field-setting, revealing the significant enhancement in plume migration that is possible at the field scale.