Influence of Halide Anions Cl^-, Br^-, and I^- on the Zeta Potential of Oil-Wet Carbonate Surfaces

The benefits of injecting dynamic water (fluids with designed ionic composition) in carbonate reservoirs have been confirmed in various studies. The underlying mechanisms, however, resulting in enhanced oil recovery, are still not fully understood, which complicates the design of new efficient dynamic water formulations. In the current work, we study the effect of Arabian Gulf seawater and the addition of halide anions (Cl^-, Br^-, and I^-) on carbonate surface alteration. Calcite and carbonate outcrop surfaces were aged with model oils (containing asphaltene, stearic acid, or a mixture of both) to render the rock more oil-wet and afterward conditioned in different dynamic water formulations. The surface charges at different treatment stages were investigated by zeta potential measurements to identify which dynamic water is most effective in altering the aged surfaces back to a more water-wet condition, which is believed to be more beneficial for enhanced oil recovery. Aging in model oils led to an increase in the magnitude of the negative zeta potential values on calcite and carbonate surfaces, indicating the adsorption of surface-active components of the model oils onto the rock surface. When conditioned afterward in the Arabian Gulf seawater with added halide ions, systematic changes of the zeta potential values were observed for the rock surfaces. The iodide system showed the strongest effect among the tested halides in terms of surface charge alteration, which is in line with the Hofmeister theory. Larger, more polarizable ions with low charge density (the chaotropic ions) seem more effective in removing adsorbed surface-active species from the rock surface. Based on these current results, it is highly recommended to use additional iodide in seawater flooding procedures to enhance the oil recovery from carbonate formations.