The
thin brine film that wets rock surfaces governs the wettability
of underground reservoirs. The ionic composition and salinity of this
nanosized brine film influence the wetting preference of the rock
pore space occupied by hydrocarbons. Despite numerous investigations
over the last decades, a unanimous fundamental understanding that
concerns the contribution of ions in the original wetting state of
the reservoir is lacking and hence the mechanisms responsible for
the wettability reversal of the mineral are still unclear. This wettability
reversal is the main consequence of ion-tuned waterflooding. Although
the method is widely accepted in practice, there is no universal consensus
on the underlying mechanisms involved. Molecular dynamics simulation
is an excellent choice to remove such ambiguities. This method can
capture an atomic-level picture of the phenomena that affect reservoir
wettability upon injecting low-salinity water. For the purpose, we
performed simulations of brine films confined between a calcite substrate
and a layer of an oil model. The films were at two different salinities
to represent the initial state of high-salinity connate water and
low-salinity brine. We found the development of ionic aggregates,
mainly comprising Na+ and Cl–, within
the high-salinity thin brine film. These aggregates act as pinning
entities to localize polar oil components within oil/brine interface
and connect the hydrocarbon phase to the calcite surface. This results
in the adhesion of oil components to the rock surface though a high-salinity
thin brine film. Simulation results suggest that the aggregates do
not form after the change of the brine content to low salinity. From
these observations, we concluded that diluting the brine content of
the reservoir leads to the disintegration of such ionic aggregates.
As a consequence, electrical double layers (EDLs) form at both rock/brine
and brine/oil interfaces, which is supposed to be reflected by additional
oil recovery at the macroscopic scale. Furthermore, we pointed out
that EDL at an oil interface is established by negatively charged
oleic polar species and cations around those compounds. Likewise,
the EDL in proximity to calcite is composed of a positive Stern layer
of Na+ cations and a negative diffuse layer of Cl– anions beyond that.
This paper resolve the salinity-dependent interactions of polar components of crude oil at calcite-brine interface in atomic resolution. Molecular dynamics simulations carried out on the present study showed that ordered water monolayers develop immediate to a calcite substrate in contact with a saline solution. Carboxylic compounds, herein represented by benzoic acid (BA), penetrate into those hydration layers and directly linking to the calcite surface. Through a mechanism termed screening effect, development of hydrogen bonding between –COOH functional groups of BA and carbonate groups is inhibited by formation of a positively-charged Na+ layer over CaCO3 surface. Contrary to the common perception, a sodium-depleted solution potentially intensifies surface adsorption of polar hydrocarbons onto carbonate substrates; thus, shifting wetting characteristic to hydrophobic condition. In the context of enhanced oil recovery, an ion-engineered waterflooding would be more effective than injecting a solely diluted saltwater.
This study aims to elucidate the impact of salinity on the interactions governing the adsorption of polar aromatic oil compounds onto calcite. To this end, molecular dynamics simulations were employed to assess adsorption of a model polar organic molecule (deprotonated benzoic acid, benzoate) on the calcite surface in NaCl brines of different concentration levels, namely, deionized water (DW), low-salinity water (LS, 5000 ppm), and sea water (SW; 45,000 ppm). Calcite was found to be completely covered by several wellordered water layers. The top hydration layer is very compact and prevents direct adsorption of benzoates onto the substrate. Instead, Na + ions form a distinct positively charged layer by adhering on the calcite substrate through inner-sphere complexion mode. Cl − ions mostly lodge on top of the adsorbed sodium cations, forming a negatively charged layer. The distribution of ions at the calcite/brine interface thus exhibits the features of an electrical double layer, composed of a Stern-like positive layer followed by a negative one. The positive charged layer attracts benzoates toward the surface. As such, the sodium ions attached onto the calcite can act as adsorption sites to connect benzoates to the surface. By increasing the salinity, more Na + ions adsorb onto the calcite surface, and the density of benzoate molecules at the interface is enhanced as a result of more Na + bridging ions. The monotonic salinity-dependent adsorption of benzoate molecules on calcite offers a mechanism driving additional oil recovery upon injection of diluted brine into subsurface carbonate reservoirs.
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