Estimation of calcite wettability using surface forces

The shift in the wetting conditions during injection of modified salinity water (MSW) in carbonate reservoirs has been interpreted in several recent works through the DLVO extended theory. Two simplifications are usually adopted when applying the DLVO extended theory: (i) the electrostatic energy interaction is quantified by an analytical solution developed for systems containing only monovalent ions and (ii) the structural forces are independent of the type of brine.

We address those by prioritizing the brine chemistry. We initially calculate the potential at the mineral and oil surfaces using two different surface complexation models implemented in Phreeqc and then we quantify the electrostatic forces by solving numerically the Poisson Boltzmann (PB) equations for non/symmetrical electrolytes.

We observe that not only the identity of the ions, but also, more importantly, the boundary conditions (constant surface charge or constant surface potential) considered for the solution of PB can drastically modify the calculated electrostatic energy profile. We then calculate the total interaction energy and estimate a microscopic contact angle that is consistent with measured values.

Our calculations show that increasing the concentration of salts such as MgCl2, CaCl2, and MgSO4 leads to more water-wet conditions, whereas salts like NaCl, KCl, and Na2SO4 show the opposite effect.