The Interface Behavior in Two-Phase Flow: Impacts of Salinity and Wettability at Nano to Micro Scales

Low-salinity water flooding is a promising technique for enhanced oil recovery in sandstone and carbonate reservoirs. Given the complex physical and chemical processes involved, several controlling mechanisms have been proposed to describe oil re-mobilization in the presence of water solution with low salinity. Osmosis and water-in-oil emulsification are among these mechanisms. However, our current knowledge about these processes is limited and their associated time scales are not well understood. In our study, three main research questions have been answered: how do the osmosis and emulsification impact the water transport and the corresponding oil mobilization? What is the effect of low-salinity on the ion transport in a thin aqueous film on a charged solid surface? To solve the first issue, we used three aqueous solutions and two alkanes in a series of microfluidic experiments with hydrophobically coated glass micro-chips for mimicking the low-salinity waterflooding process in an oil-wet rock formation. We created multiple systems of low-salinity water-alkane/high-salinity water in the porous micromodel, and afterward, continuously monitored the domain for 70 hours. We noted that ionic strength and the hydrocarbon chain length both played important roles in water diffusion. A salinity contrast of 1.7 g/L-170 g/L caused a higher water volumetric flux than 50 g/L-170 g/L for both alkanes. There was no simple relationship between the chain length of hydrocarbon and water volumetric flux. Moreover, to investigate the effect of salinity on water behavior in heptane, we conducted molecular dynamic (MD) simulations by considering three different concentrations in the high-salinity water region featuring our experiments. The results indicated that high salinity limited the water diffusion from high-salinity phase into the oil phase and reduced the possibility of water entering the heptane phase. The numerical modelling of ionic diffusive transport through a charged thin film of electrolyte is mathematically and computationally complex due to the strongly coupled hydrodynamics and electrochemical interactions. Due to the highly nonlinear and coupled equations the computational costs are heavy and very often limited to simulations in two-dimensional geometries. We have developed an equivalent one-dimensional electro-diffusive transport model based on mathematical averaging of 2D equations to reduce the computational time. The computational time is improved substantially and simulation of much larger domain sizes which are required to study and interpret the experimental results is shown to be feasible. We have shown the high accuracy of the developed model by comparing the electric potential and concentration profiles of the developed model against the original 2D simulations. Wettability is a crucial factor for pore-filling events in multiphase flow. Different wettabilities of the solid substrate affect the global displacement pattern, the fluid trapping and the hysteretic saturation. Furthermore the effect of wettability needs to be considered in numerical and analytical models to enable the accurate description of the pressure response during such a pore-filling event. The PDMS micro-models are designed with a depth of 100 μm and with one square pore body of 800 μm in width and length which is connected to 4,000 μm long inlet and outlet channels with a height of 150μm. The models are rendered with three surface wettabilities of 40º, 95º and 150º static contact angle (measured with the sessile drop method for a water droplet on PDMS in air). Confocal laser scanning microscopy (CLSM) is applied to recognize the fluid interface and monitor its movement. With an inserted pressure transducer in the upstreaming plastic tube, the pressure change during the events was continuously measured. The collected pressure curves are compared with existing analytical solutions. We correspondingly developed numerical models based on the volume of fluid method which will be validated with the presented experimental data