CO₂ Flow Regimes Comparison between North Sea and US Classes of Reservoirs

Carbon dioxide (CO₂) enhanced oil recovery (EOR) has long been practiced in the US as an efficient mean for enhancing oil production. Many of the US CO₂-EOR developments have been designed horizontally. This is because of a viscous-dominated CO₂ flow regime that is prevalent in these developments driven by thin and low-permeability reservoirs. Reservoirs and fluid properties are different in the North Sea. Pays are usually thicker with better petrophysical properties. Lighter oils can also be found in North Sea reservoirs. This suggests that a dissimilar flow regime might prevail CO₂ displacements in the North Sea developments, which could favor a dissimilar CO₂-EOR process design. This study thus compares CO₂ flow regimes between several North Sea and US reservoirs. We use scaling analysis to characterize and compare CO₂ flow regimes between these two classes of reservoirs. Scaling analysis characterizes CO₂ displacement in each reservoir system using three dimensionless numbers: gravity, effective aspect ratio, and mobility ratio. Displacement experiments conducted in stochastically generated permeability fields, under exactly matched magnitudes of the derived dimensionless numbers, reveal the prevailing CO₂ flow regime in each reservoir system. Results of scaling analysis indicate that CO₂ flooding in the North Sea reservoirs can be generally characterized with a larger gravity number, smaller effective aspect ratio, and smaller mobility ratio than the average US CO₂ flooded reservoirs. Flow regime analysis indicates that unlike the majority of the US CO₂ flooded reservoirs, CO₂ flow regimes tend to be more gravity-dominated in the North Sea class of reservoirs. CO₂ flow regimes in the North Sea systems are expected to suffer from a higher degree of instability because of thicker North Sea pays, which limit effective crossflow. Understanding the differences and characteristics of CO₂ flow regimes in the North Sea prospects can help operators design their CO₂ flooding more efficiently, which could increase the recovery factor (RF) as well as address CO₂ storage requirements, a necessary consideration for CO₂-EOR deployment in the North Sea.