| Surfactant-enhanced waterflooding is a novel EOR method aimed at improving volumetric sweep efficiencies in reservoirs. The technique depends upon the ability to induce phase changes in surfactant solutions by in-situ mixing of surfactants of opposite charge. One surfactant or salt solution is injected into the reservoir. It is followed later by injection of another surfactant or salt solution. The sequence of injection is arranged so that the two solutions do not mix until they are into the permeable regions well away from the well bore. When they mix at this point, by design they form a precipitate or gel-like coacervate phase, plugging this permeable region, forcing subsequent flow through less permeable regions of the reservoir, and improving the sweep efficiency.; A number of experimental observations are presented here to elucidate the overall mechanism of permeability reduction. New understandings have been obtained in the area of selectivity of the plugging process in the high permeability regions of reservoir sandstones. The depth of plugging and the plug sizes are shown to be controlled by monitoring the pore volumes of brine spacer or the concentration and pore volumes of surfactants injected. Optimum injection strategies are identified and tested whereby the stability of plugs is prolonged and better control over plug placement is obtained.; A general theoretical framework for the process is presented. Some important features of the model include: unlike previous studies, a direct measure of the extent of precipitate is developed based on a specified growth mechanism; the extent of permeability reduction is shown to be directly related to the quantity of precipitate through a semi-empirical model; comparisons are made between the theoretical permeability reductions and those obtained from the experimental observations; permeability reductions are related as functions of injected surfactant concentrations, extent of surfactant adsorption on the reservoir rock, injected surfactant slug volume, and volume of brine spacer injected. A process similator is then applied to conduct a feasibility study of the process. Optimization studies are then carried out based on the forementioned findings.; Since the oil recovery mechanism in surfactant-assisted waterflooding is completely different from that in low-tension surfactant flooding, the new process can operate at lower surfactant concentration and at higher injectivities, thereby both greatly reducing the upfront costs of the new process and increasing its economic feasibility. |
