Three-Phase Sandstone Acidizing: Quantification and Analysis of Evolved CO2 in the Presence of Oil and Water

The proposed paper presents a detailed study on evolving CO2 due to calcite mineral dissolution, and its ensuing activity during the matrix acidizing of sanstone reservoirs. Coreflood experiments were conducted in acidizing, and interpreted via simulation studies using a three-phase, two scale continuum model. Sensitivity studies were then performed on the calibrated simulation model. Acid injection was performed on 6 in.-length, 1.5 in.-diameter Bandera Brown sandstone cores of variable calcite content, using 15 wt% HCl single-phase coreflood experiments at high back pressures were conducted to calibrate and initially test the three-phase, two-scale continuum model. Experimentally measured rock-heterogeneity via computed tomography (CT) scans, relative-permeability and capillary pressures, oil-water interfacial tension and contact-angle parameters were inputs for three-phase, two-scale model-based history matching and sensitivity studies. The three-phase, two-scale continuum model was able to match all performed coreflood experiments with a good level of accuracy. The acid-calcite chemical reaction parameters were fixed in all cases to ensure consistency in analysis. Oil production was observed, with an average of 40% recovery of the residual oil in place at CO2 miscible pressures. CO2 miscibility in oil enhances swelling with time, which was seen as the main mechanism for oil production. A direct symmetry was observed between the oil recovery and average CO2 moles in oil. The recovery curve flattened once surrounding oil reached its full-saturation level with CO2. Reduction in oil-water interfacial-tension increased the recovery factor only by a slight margin, owing to dependency on evolved CO2 volume. Immiscible CO2 conditions yielded no residual oil recovery. The successful application of the three-phase, two-scale continuum model approach sets a new bar in the area of sandstone acidizing. The acid breakthrough criterion has been revised toward application in a three-phase environment. The potential of CO2, a by-product of acidizing, towards its contribution in swelling oil in the presence of a three-phase environment, and towards possible oil recovery in the event of flowing back a well.