The aim of this paper is to demonstrate the important role of ion composition for successful oil recovery by water injection and it also suggests possible mechanisms by which different ions interact with the carbonate rocks to alter the wettability.
Injected water contains, among other ions, sulfate and magnesium ions. The possible wettability alteration mechanisms of the calcite surface by Mg2+ and SO42- to more water-wet is addressed in light of contact angle measurements, and surface potential.
Several studies have been reported in literature on improving oil recovery by chemical flooding.[1,2] The change in zeta potential during surfactant flooding has been reported in literature, where a change from a positive to a negative potential occurs, which suppresses the adsorption of surfactant as well as desorption of adsorbed surfactant from the solid surface when saline water is used.[3–5]
Recently, an investigation of the influence of sulfate and magnesium on adsorption of fatty acids has shown the important role of pH. They reported the change in surface potential due to the adsorption of sulfate and magnesium ions on the calcite surface and the change in wettability of oil-wet calcite surface. It is also shown in literature by water vapour adsorption isotherm a large increase for calcite aged in sulfate compared to pure calcite.
In this work sulfate and magnesium ions effect on the adsorption of different fatty acids was studied to draw a possible mechanism of the wettability alteration. The work concentrates on alteration of modified calcite surface with fatty acids to water-wet. The degree of alteration is addressed here as function of fatty acid, sulfate and magnesium concentrations.
Solids: In this work depending on the performed experiment, calcite was provided in powder, or cleaved chunk. The calcite powder was suppliedy Norwegian talc AS. The chemical composition of the calcite powder determined at NGU (Norwegians Geological Survey) by XRF method is shown in Table 1. The powder was used for zeta potential measurements. The chunk calcite (Island-spar) from India was supplied by J. Brommeland AS (Norway) for contact angle measurements.
Oil: A model-oil (n-decane) was used in this study and thereafter was termed "oil phase". The n-decane was supplied by Chiron AS at + 99 % purity. Stearic acid (SA) / 18-Phenoloctadecanoic acid8 (PODA), supplied by Chiron AS Company (Norway), were dissolved in the oil phase as additives to modify the originally water-wet calcite to oil-wet. The concentration of 5×10–3 M of stearic acid / PODA in n-decane was used in this work.
Water: In all experiments distilled water (ion-free) was used as reference to the waters including different concentration of ions mainly sulfate and magnesium.