Theoretical Study of Water Blocking in Miscible Flooding

Summary Miscible displacement processes can leave a substantial amount of residual oil behind the displacement front. This phenomenon has two general causes: instabilities caused by local heterogeneities or viscous fingering and water blocking. This paper describes a study of the latter. Numerous laboratory experiments have shown that significant blocking of oil from the solvent by mobile water can occur in water-wet media and at large water saturations. Despite this, water-blocking studies have been limited to either simple correction functions in numerical simulations or microscopic models. To the best of our knowledge, no explicit theoretical model considers the macroscopic bypassing and subsequent interaction of the solvent stream with a trapped hydrocarbon phase. In this study, CO2 is the miscible solvent. A numerical model calculates the mass flux between flowing and stagnant regions separated by a water film. The model considers solvent diffusion and diffusional extraction of oil accompanied by swelling or shrinking of the stagnant hydrocarbon phase. The various process parameters are represented by a set of dimensionless numbers, which allow the problem to be scaled to any size. We show that the conditions for the mobilization of water-trapped oil are determined essentially by two numbers: the ratio of the stagnant water/oil volumes and the ratio of the solvent equilibrium constants at the phase boundaries. We apply the model to published laboratory miscible flood experiments. The water-film thickness is used as a matching parameter to reproduce closely the measured residual oil data for different flood velocities. Results indicate that solvent diffusion and oil swelling mechanisms effectively account for trapped oil on the laboratory scale. Trapping rapidly decreases at field scale if larger water barriers exist.