Microfluidic Investigation of Low-Salinity Effects During Oil Recovery: A No-Clay and Time-Dependent Mechanism

We present a study of the low-salinity effect during oil recovery using microfluidics experiments in an attempt to narrow the gap between pore-scale observations and porous-media-flow mechanisms, and to explain one type of low-salinity effect with delayed oil recovery and without the presence of clay. A microfluidic toolbox is used, including single-pore-scale microchannels, a pore-network-scale (approximately 10(2) pores) micromodel, and a reservoir-on-a-chip model (approximately 10(4) pores with heterogeneity), all with 2D connectivity. Experiments at the single-pore scale reveal a time-dependent oil dewetting and swelling behavior when a crude-oil droplet is in contact with low-salinity water. An interplay between water chemical potential and oil-phase polar compounds explains this pore-scale observation well. Experiments at the pore-network scale illustrate that the dewetting and swelling of residual oil in the swept region increase the water-flow resistance, modifying the flow field and thus redirecting the flooding liquid into unswept regions. This pore-network-scale effect is re-expressed into a macroscale model as a sweep-efficiency improvement derived from the change of relative permeabilities, which requires time to develop. Finally, experiments on our reservoir-on-a-chip model show significant incremental oil recovery during tertiary low-salinity waterflooding and confirm that late-time sweep-efficiency improvement contributes to most of the incremental oil recovery. On the basis of this microfluidic framework, we emphasize the following three findings: Low-salinity tertiary waterflooding can improve oil recovery by an improvement of sweep efficiency, which is a consequence of residual-oil dewetting and swelling. The low-salinity effect can occur without the existence of clay. The wettability alteration and oil swelling are time-dependent processes and should be expressed as a function of oil/water contact time rather than dimensionless time [pore volume (PV)], which explains some observations from previous coreflood experiments.