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