Pore-scale investigation on microemulsion-based quasi-miscible flooding for EOR in water-wet/oil-wet reservoirs: A 3D study by X-ray microtomography

Abstract

Microemulsion flooding has recently gained attention as an effective chemical-based method for enhanced oil recovery (EOR). The high performance of this technique is attributed to the classical mechanism: plugging effect and divergence of preferential flow path. In this study, miscible behavior was identified as a novel mechanism by which oil recovery was controlled by dissolution process instead of capillary forces. To this end, a new type of solvent-based microemulsion phase was prepared by mixing solvent, water, and surfactant as a flooding fluid. A series of experiments with direct water flooding, microemulsion flooding, and water-alternate-emulsion flooding for EOR were carried out for strong water-wet and oil-wet artificial rock samples. X-ray microtomography was used to visualize the oil configurations in three-dimensional pore spaces. An advanced workflow image processing method was developed for the first time to capture the miscible behavior from a pore-scale view. As suggested in the results, solvent-based microemulsion flooding has a miscible displacement front where the oil phase is gradually solubilized. Therefore, capillary forces can be eliminated because the emulsion-oil interface is invisible but has a compositional gradient, leading to higher oil recovery. Compared with the immiscible waterflooding process, the capillary forces remained dominant, which hinders additional oil recovery. Based on EOR scenarios, we found that the new emulsion flooding could effectively solubilize larger well-connected oil clusters instead of small separate ganglia or singlets. Therefore, considering its miscible behavior, the solvent-based microemulsion flooding can be applied to secondary oil recovery regardless of the reservoirs’ wettability, or to tertiary oil recovery in weakly/strongly oil-wet reservoirs with a well-connected oil phase after waterflooding.