A pore-scale experimental investigation of process-dependent capillary desaturation

Abstract

A series of micro-scale core-flooding experiments were conducted in a miniature water-wet Berea sandstone sample to examine how changes in the desaturation (water-displacing-oil) protocol impact the non-wetting phase recovery and to provide, to the best of our knowledge, the first pore-scale evidences of the associated displacement subtleties. We studied (i) the continuous displacement mode (CDM), which consisted of a series of imbibition tests with increasing capillary numbers and similar initially connected oil, and (ii) the discontinuous displacement mode (DDM), which included a drainage followed by a multi-rate imbibition test. During all experiments, a cutting-edge imaging platform was employed to visualize the fluid occupancies in the pore space.The in-situ saturation measurements showed that, at a given capillary number, the non-wetting phase recovery was more efficient under the continuous mode during which the oil phase was connected prior to each imbibition. This is attributed to the changes in the order by which the pore-scale displacement events take place before entrapment. For the DDM, however, the entrainment of oil clusters is the governing recovery mechanism, and the desaturation process was observed only when a threshold capillary number was reached. These findings are supported by pore-scale fluid occupancy maps demonstrating piston-like and snap-off events during the capillary-driven flow processes as well as trapped cluster entrainment during the viscous-dominated flow tests. Moreover, this investigation presents previously unidentified pore-scale displacement events in natural porous media, such as fragmentation and coalescence of trapped non-wetting phase globules prior to entrainment.These mechanisms took place at intermediate capillary numbers and mostly under the discontinuous displacement mode before the threshold of mobilization was reached. The capillary desaturation process under CDM was found to be gradual and more pronounced compared to that under the DDM mode. It is also demonstrated that even though the pore-scale displacement mechanisms responsible for oil recovery are different for CDM and DDM, the viscous-dominated imbibition tests produce similar distributions of trapped oil clusters under both cases.