Calcite–Brine Interface and Its Implications in Oilfield Applications: Insights from Zeta Potential Experiments and Molecular Dynamics Simulations

Abstract

Carbonate reservoirs are made up of predominantly calcite and dolomite minerals and hold significant hydrocarbon reserves globally. However, the production from carbonate reservoirs is limited due to their wettability, which controls the production and fluid distribution. To develop efficient strategies for producing from these formations, it is necessary to understand the underlining mechanisms of carbonate rock wettability. We believe that understanding the native state of the rock mineral in the reservoir environment and how oilfield operations affect the wetting state of minerals is critical to demystifying the change in carbonate rock wettability. Thus, this study extends the understanding of the surface charge development of calcite minerals and provides useful insight into the mineral’s surface charge development. Zeta potential measurements and molecular dynamics (MD) simulations of calcite in different fluids of varying composition and salinity were investigated. We have considered both the mixed brine (seawater and reservoir water) and individual salt brine (i.e., NaCl, MgCl2, and CaCl2). The results show that the calcite mineral surface charge is controlled by the composition and salinity of the surrounding fluids. Indeed, we found that monovalent ions have dominant contributions to the total calcite surface charge. The adsorptions of Na+ and Cl– shape the stern layer structure in the first two calcite hydration monolayers. We found that the interplay between the calcite surface affinity to the brine ions and the hydration-free energies are the two critical parameters shaping the final mineral surface charge. We believe that our study provides essential atomic insights into the calcite–brine interfaces and how ions interact with the surface to control the surface charge, which are vital to the quest for wettability control.