Abstract
Oil wettability in the water-oil-rock systems is very sensitive to the evolution of surface charges on the rock surfaces induced by the adsorption of ions and other chemical agents in water flooding. Through a set of large-scale molecular dynamics simulations, we reveal the effects of surface charge on the oil contact angles in an ideal water-decane-silicon dioxide system. The results show that the contact angles of oil nano-droplets have a great dependence on the surface charges. As the surface charge density exceeds a critical value of 0.992 e/nm2, the contact angle reaches up to 78.8° and the water-wet state is very apparent. The variation of contact angles can be confirmed from the number density distributions of oil molecules. With increasing the surface charge density, the adsorption of oil molecules weakens and the contact areas between nano-droplets and silicon dioxide surface are reduced. In addition, the number density distributions, RDF distributions, and molecular orientations indicate that the oil molecules are adsorbed on the silicon dioxide surface layer-by-layer with an orientation parallel to the surface. However, the layered structure of oil molecules near the silicon dioxide surface becomes more and more obscure at higher surface charge densities.
Highlights
Enhanced oil recovery (EOR) is becoming more and more important with the growing difficulties in the extraction of crude oil from oil reservoirs [1, 2]
This means that the rock surface alters from oil-wet state to water-wet state as the surface charge increases from negative values to positive values
It is believed that the variation of contact angle with surface charge is caused by the change of the intensions of water-silicon dioxide and oil-silicon dioxide, because the surface charge affects the interactions between water-silicon dioxide and the interactions between oil-silicon dioxide
Summary
Enhanced oil recovery (EOR) is becoming more and more important with the growing difficulties in the extraction of crude oil from oil reservoirs [1, 2]. Among various EOR techniques, water flooding is a dominating approach owing to its low cost and high efficiency. Water flooding is a multiscale process, including either the viscous fingering processes in the porous reservoirs or the two-phase flow processes that the crude oil is displaced by the injected water in microfluidic channels. Owing to the large surface-to-volume ratios of the oil reservoirs and the low capillary numbers of oil displacement process, the wettability of oil and water on the porous rock surfaces has a great impact on the oil-water two-phase flows in oil reservoirs. Wettability of rock surfaces is very crucial for the oil recovery efficiency [3–5]
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