Comparison of CO2 Flow Patterns Between Offshore North Sea and Onshore United States

Abstract

Abstract In any flooding process, the flow pattern determines the quality of the macroscopic sweep and expected recovery efficiency. The flow pattern also controls the choice of the flooding strategy. This study compares CO2 flow patterns between two major classes of reservoirs; first, the United States CO2 flooded reservoirs, considered as benchmark for CO2 application elsewhere, and second, the North Sea class of reservoirs considered as future target for CO2 flooding offshore. An inventory of reservoir data was first prepared by inspecting the literature. North Sea reservoirs are characterised with higher temperatures, higher pressures, thicker pays and higher permeabilities. Well spacing is also larger in the North Sea and these reservoirs are depleted faster. Using appropriate correlations, the in-situ CO2 and oil properties were inferred for each individual reservoir knowing its ambient reservoir conditions. Scaling analysis was used to characterise the CO2 displacing oil process in each reservoir by calculating a few key dimensionless numbers. Numerical simulation of CO2 displacing oil in stochastic permeability fields revealed the CO2 flow pattern in each individual reservoir. Although CO2 and oil densities are comparable in North Sea and United States classes of reservoirs, scaling analysis shows that "gravity numbers" for a CO2 displacing oil process are an order of magnitude larger offshore North Sea. This indicates a more gravity dominated CO2 flooding in the North Sea compared to United States reservoirs principally due to thicker pays and significantly higher permeabilities in this province but not due to larger CO2 and oil density contrast. The "mobility number" for a CO2-oil displacement process is also considerably lower (or better) in the North Sea due to lower North Sea oil viscosities. This indicates, in the absence of gravity, the viscous CO2 flooding is expected to be more stable in the North Sea. "Effective aspect ratios", illustrating the degree of cross flow are also lower in the North Sea mainly due to considerably thicker pay reservoirs in these systems. Visual comparison of displacement profiles in different stochastic permeability fields shows that, unlike the majority of United States CO2 flooded reservoirs where the displacement may be characterised with an unstable viscous dominated process, in the North Sea CO2 flow patterns vary mostly between gravity dominated and stable viscous displacements. Better understanding of the CO2 flow pattern can help in the selection of the appropriate CO2 flooding process e.g. selection between horizontal and gravity stable CO2 flooding or the decision to implement WAG instead of continuous CO2 injection for future provinces targeted for CO2-EOR like the North Sea. North Sea reservoirs thus may benefit from a different CO2 flood design than that observed historically in the United States since their CO2 flow patterns is fundamentally different.