(189cc) Prediction of Calcium Carbonate Wettability By Low Salinity Water Flooding Using Molecular Dynamics Simulations

Low
salinity water (LSW) flooding has been considered by the oil industries in
recent years as an augmented-water flood technique, which presents a
substantial potential to improve the oil recovery factor. Although the LSW
flooding technology was initially applied in sandstone reservoirs, the positive
effects were also observed in those of the carbonate type. In sandstone
reservoirs, low salinity water with less multivalent ions is usually employed,
while sea water with more potential-determining ions such as Ca²⁺,
Mg²⁺ and SO4²⁻ is generally injected into carbonate
reservoirs. Rock wettability defines the tendency of the fluid (e.g. water) to
spread or adsorb on the solid surface of rock grains in the presence of the
other immiscible fluid (e.g. oil).  In
this work, we investigated the adsorption of an oil droplet mixture ( 13% Toluene : 87% Dodecane ) on
a calcite surface wetted with various numbers of water layers (1, 2 and 3
layers) containing different combinations of ionic species (NaCl,
Na₂SO₄ and Na₂CO₃) as shown in
figure 1. In order to determine the calcite wettability and spreading coefficients,
we computed the surface/cohesive energies from MD simulations and fed the
resulting parameters into the Young-DuprŽ equation.

The simulations show, in qualitative agreement with experiments,
that the presence of water on the calcite surface strongly affects the
adsorption of oil on calcite. Without water, oil is strongly adsorbed on the
calcite surface and spreads into a thin layer, forming effectively a zero
wetting angle. (This situation will reduce oil recovery.) A similar behavior is
observed in the presence of a monolayer of water. Note that water is nominally
hydrophilic, so one would expect that the oil should not spread. However, our
previous work on the adhesion of naphthenic acid on calcite shows that the
monolayer of water is strongly adsorbed on the surface of calcite, effectively
making the surface hydrophobic (or conversely oleophilic).
Thus we should expect oil to spread on the monolayer of water. But with more
water molecules, we recover the expected behavior of oil to form a bead (or
lens) on the surface of water.