Impact of Oil Saturation, CO2 Evolution, and Rock Wettability on Acid Efficiencies During Carbonate Acidizing: A Three-Phase Perspective

Abstract Success rates of carbonate acidizing, when upscaling from single, aqueous-phase laboratory analyses to field scale levels have been poor. Analyzing matrix acidizing performances in carbonate reservoirs with two-phase environments. i.e., oil and water, has picked up importance currently in the stimulation industry. A lack of understanding of the mechanisms taking place in this complex subsurface process still exists, particularly with carbon dioxide (CO2) evolution from limestone dissolution, and the presence of a secondary fluid phase in the medium. A detailed investigation via experimental core studies and fluid modeling / characterization has been performed for oil/water environments in the porous media. Moderate permeability carbonate cores, with an average of 14% porosity were used for this study. Six acidizing experiments with 15 wt% hydrochloric (HCl) acid were performed using outcrop Indiana limestone cores, at temperatures of 150°F and initial pore pressures of 600 and 1,200 psi. The presence of a light crude oil in the cores at residual conditions was tested for acid efficiencies in un-aged and aged conditions. Rock wettability measurements via contact angle experiments, and oil/water interfacial tension, σow, measurements were conducted, and the effects of CO2 to alter the rock-wettability, oil/water interfacial tensions, was studied. Characterization of the crude oil was performed using Equation of State based simulations, and the behavior of CO2-crude oil systems were evaluated via simulated minimum miscibility pressure and swelling tests. Cores with residual oil at water-wet conditions yielded the least acid pore volumes to breakthrough (PVbt), at all tested pore pressures. An increased oil-wetness of the rock resulted in greater acid PVbt's, when compared to water-wet systems with both residual oil and fully water saturated cores. CO2's capability to alter rock-wettability did not show any benefits toward improving acid efficiencies in cores at residual oil saturations, Sor. The acid efficiencies in porous media containing oil and water largely depend on the access of sufficient rock surface to the acid species, for which rock-wettabilities are a governing factor.