Effect of well pattern and injection well type on the CO2-assisted gravity drainage enhanced oil recovery

Fundamental understanding and application of process parameters in numerical simulation that leads to optimized gravity drainage oil recovery at field scale is still a major challenge. Reservoir simulations studying the effects of well patterns and type of gas injection wells have not been reported so far. In first ever attempt, the mechanistic benefits of production strategy on gravity drainage oil recovery are identified in this paper. Effects of irregular and regular well patterns and vertical and horizontal gas injection wells are investigated using a fully compositional 3D reservoir model in secondary immiscible and miscible modes under the conditions of voidage balance, constant pressure of injection and production wells and injection rates below the critical rate. Regular well pattern provided longer oil production time at a constant rate until CO2 breakthrough compared to irregular well pattern. It then dropped almost vertically at the same cumulative oil recovery even at higher production rates. However, gravity drainage oil recovery was higher at higher rate combination after CO2 breakthrough. Results also suggested that the regular pattern could result in horizontal CO2 floodfront parallel to the horizontal producers, maintaining reservoir pressure, thus optimizing the oil recovery by additional 2.5% OOIP. Vertical injection and horizontal production wells in both the immiscible and miscible modes provided nearly identical cumulative gravity drainage oil recovery compared to the combination of horizontal injection and production wells in the regular well pattern. This suggests that the type of injection wells may not be a significant factor to impact the CO2-assisted gravity drainage mechanism. Results obtained herein would help in the optimization of CO2-assisted gravity drainage EOR process.

► 3D compositional reservoir simulations of immiscible/miscible CO2-assisted gravity drainage EOR process.
► Optimization/identification of production mechanisms through well patterns and injection well types.
► Well patterns help to form stable/horizontal gas–oil front for effective oil–gravity drainage rate.
► In addition to gravity drainage, oil swelling, extraction/vaporization do contribute in enhancing the oil recovery.
► Miscibility generation may not be a pre-requisite condition for optimum gravity drainage oil recovery.