Effect of CO2 flood on geochemistry of McElroy oil

Geochemical characterization has been completed on a series of oil samples collected over time from the McElroy Field, Texas, where a pilot CO2 flood project was initiated a few years ago. A non-waxy, 31 gravity (oAPI) oil is produced from carbonate reservoirs within the Permian Grayburg Formation. The samples were taken from various wells over a time span of one and one half years for monitoring changes in oil composition induced by CO2 injection over time. The oil samples taken from about 40 wells shortly after the start of CO2 injection showed a marked reduction ( 50%) in the asphaltene content, indicating CO2-induced asphaltene deposition had occurred. Mapping asphaltene reduction could provide information on the flow direction of CO2 in the reservoir. Hydrocarbon profiles of the oils from some wells also showed a significant reduction in the heavy paraffin content (i.e. > C25), suggesting deposition of heavy hydrocarbons within the reservoir. The observation of a slight increase in oil API gravity is also consistent with precipitation of heavy components from the oil phase due to CO2 injection. The organic deposits (or flocs) which can cause restriction of fluid flow in the reservoir by reducing porosity and permeability have probably contributed to reduction in injectivity experienced by some injection wells shortly after the start of CO2 injection. Field observations have been duplicated in laboratory core flood experiments to mimic the field recovery process. The experiments show that most of heavy organics were deposited near the CO2 injection port, suggesting organic deposition in the field occurs near the injection wells rather than producing wells. Despite the continued CO2 flood, some molecular properties such as sterane and hopane distributions remain relatively unchanged. These geochemical parameters are commonly derived from ratios of compounds with similar molecular properties and are generally not affected by the CO2 flood as precipitation of heavy hydrocarbons causes little fractionation among these compounds. Thus, oil samples from fields under the gas flood are still viable for correlation, source evaluation and maturity determination.