The ubiquitous behavior of thin film brine residing in mineral pores confined by mineral and oleic phases fascinates many porous media researchers [1–4]. The unusual behavior of an aqueous phase in confining systems is dominated by a broad range of sciences, from drug delivery and catalytic reactions to enhanced oil recovery. So far, extensive efforts have been pursued to unveil the underlying mechanisms behind the stability of thin film brine covering the surface of porous media, mainly by tuning the salinity, which changes the wettability of the porous media surface. In addition, finding an efficient way to eliminate carbon dioxide in the environment has gained attention [5, 6]. Carbonated Smart Water (CSW) has been introduced to achieve these common purposes. In this study, molecular dynamics simulations have been performed to unravel the influence of CO2, salinity, and the polar component of the hydrophobic phase confined in an identical porous medium.
The confined CSW thin film (40000 and 8000 ppm of NaCl) between a calcite substrate and a hydrophobic fluid, consisting of decane, and acid molecules, benzoic acid (BA) or decanoic acid (DA) at high pressure and high temperature (HPHT) conditions (323 K and 10 MPa) has been investigated.
Atomistic simulation reveals that the compacted, well-ordered layers of water in the proximity of the calcite substrate are formed regarding the salinity and the component of the oleic phase. The diffusivity of CO2, acidic molecules, and ions have been changed upon changing the salinity and oil model. The polarity of the BA and DA functional groups, and water ions, makes the interactions more complicated. The higher acidity of DA compared to the BA makes it a better case to move toward the brine phase. At the same time, a higher interaction of carbon dioxide with BA than DA was seen. The higher interaction of BA by CO2 impedes the higher diffusivity of CO2 into the oleic phase compared to the DA. However, the chain structure of DA makes the space accessible for the diffusion of CO2 into the oleic phase and distribution into the bulk oleic phase, which results in higher interaction of decane and lower viscosity for this model oil. Also, the higher diffusion of BA compared to the BA proves the idea of a higher attraction of Na+ ions toward the DA than BA, which in turn would end up with a thicker electrical double layer over the calcite surface. Overall, the attraction of Na+ ions toward the calcite substrate will decrease by introducing DA instead of BA in the system, and it prefers to move toward the oleic interface. In lower salinity, the higher interaction of CO2 with acid molecules has been seen, particularly in the BA case, which indicates that the ions are not enough to move to the interface and enter the interaction with acidic molecules. In summary, the chemical structure of acidic molecules in the oleic phase can change the interaction of CO2 and ion distribution and, in turn, thin film stability.