Effect of CO2 Flooding on Dolomite Reservior Rock Dever Unit Wasson (San Andres) Field , Texas

This report documents results of a study to determine whether brine and CO2 injection significantly changes total porosity in a dolomite reservoir. Pre- and postpilot cores from closely-spaced wells in the Shell Western E&P Inc. Denver Unit CO, pilot provided the necessary data. We concluded that only minor porosity enhancement resulted from brine dissolution of anhydrite. Detailed petrographic examination of thin sections provided the modal composition (bulk mineralogy, total porosity and pore types) of 112 samples. This data was evaluated by statistical methods (t test) to determine the significance of any change in porosity at a given confidence level. Large vugs, channels or other evidence of high permeability thief zones, created by carbonic acid dissolution of the dolomite, were not observed in the postpilot core located 25 feet from the pilot injection well. However, minor anhydrite dissolution is documented from both chemical analyses of the pilot flood water and thin sections. These results are significant when one considers the large brine and CO2 injection volumes (approximately 90 pore volumes of brine and 30 pore volumes of CO,) that contacted the postpilot core. In addition, minor amounts of solid hydrocarbon were observed to occlude porosity in seven thin sections from the postpilot core. As a result of anhydrite dissolution, total point-counted porosity is slightly higher in the postpilot core but not statistically significant. Our results appear to refute Chevron's (SACROC) concerns that carbonic acid would aggravate reservoir heterogeneities and cause channeling detrimental to area1 sweep.