Modeling of oil production by gravity drainage

Gravity drainage is one of the most effective mechanisms of developing an oil field because of high oil recovery yielded. There have been several models to characterize and model the process of gravity drainage. However the existing models do not work well in many cases. In this study, an analytical model was developed to determine the ultimate oil recovery by free-fall gravity drainage. This model demonstrates clearly the effect of capillary pressure on ultimate oil recovery. The ultimate oil recovery increases with the decrease in entry capillary pressure and the increase in pore size distribution index. The model was modified based on the ultimate recovery model to match and predict oil production. The model was tested against experimental, numerical, and field data of oil production by free-fall gravity drainage. The results demonstrated that the modified oil recovery model could work satisfactorily in the oil–gas cases studied. Initial oil production rate, entry capillary pressure, and average residual oil saturation can be estimated using the oil recovery model. An approach was also developed to infer capillary pressure curves from the oil production data by free-fall gravity drainage.