The macro and micro analysis on EOR mechanisms during steam and solvent thermal recovery in heavy oil reservoirs

Using only pure steam as a driving medium in heavy oil reservoirs may result in a low oil recovery factor. Solvent is a good oil displacement medium that can be dissolved in heavy oil, which can greatly reduce the viscosity and improve the mobility of heavy oil. Therefore, the method of solvent assisted displacement and drainage has the unique advantage during steam flooding. In this paper, a quantitative method is proposed to choose appropriate solvents. Then a series of experiments are carried out, including oil displacement experiment to measure oil displacement efficiency, 2D visualization experiment to analyze the sweep efficiency and remaining oil distribution, and 3D scaling physical simulation setting with bottom water condition, which are used to analyze production performance, the expanding characteristics of steam chamber and study reservoir fluid distribution. The results show that steam moving upwards to heat and drive heavy oil to production well under the action of displacement and drainage. In a heavy oil reservoir with thin bottom water, if only steam is injected into reservoir, then there is no obvious stage of stable oil production and the oil recovery factor is only 32.7%. However, when solvent and steam are co-injected into reservoir, the stage of stable oil production maintaining about 0.6 PV and the oil recovery factor is 43.2%. During steam and solvent flooding, solvent can greatly reduce the oil viscosity to improve its mobility; meanwhile solvent can promote the expansion of steam chamber. The co-injection of steam and solvent can obviously expand steam chamber and increase oil recovery factor in heavy oil reservoirs (even connecting with bottom water layer). The method of steam and solvent flooding further enriches EOR technologies of thermal recovery in heavy oil reservoirs.