Prediction of Three-Phase Gravity Drainage from Two-Phase Capillary Pressure Curves

Abstract

Abstract Gravity drainage of oil with gas and the migration of non-aqueous phase liquid (NAPL) spills in the unsaturated zone are two of the most common processes where the three-phase capillary/gravity equilibrium plays an important role. Although three-phase gravity drainage is highly effective for water-wet media, predicting the profiles and ultimate recovery for mixed and oil-wet media is essential as most reservoirs have a mixed-wet nature. The three-phase capillary/gravity equilibrium is different from the two-phase equilibrium, similar to the difference between two-phase and three-phase relative permeability. For water-wet media and spreading oils, the profiles can be predicted from the two-phase pressure-saturation curves. However, the predictions are more complicated for mixed-wet media because of the difference in fluid configuration in pores. We propose a simple method for predicting the three-phase equilibrium saturation profile and test our method using the experimental data. The experimental method used in this study includes performing a series of two-phase capillary pressure measurements and three- phase gravity drainage tests in columns containing fractionally wet sands. The two-phase experiments are compared to experimentally measured three-phase drainage saturation profiles of the same wettability. The bottom of the column is supported by an aquifer such that no oil is produced. The results show that the transition between single phase, two-phase and three-phase zones can be accurately predicted using the two-phase drainage capillary pressures. However, the proposed method is unable to predict the final residual saturation, especially in columns with faction of oil-wet sand exceeding 50%. Because two-phase data for reservoir cores are more accessible than three-phase data, the results of this study are valuable for predicting the saturation distribution in a three- phase gravity drainage process in mixed-wet media, particularly in reservoirs with a wettability gradient. The results of the study may also be applicable to study of CO2 sequestration in aquifers.