ORGANIC COMPOSITION OF PRODUCED WATER FOLLOWING CO2 INJECTION IN THE CRANFIELD OIL FIELD, MISSISSIPPI, USA

As part of SECARB Phase III, a multi-laboratory field experiment funded by DOE and conducted by the Denbury Resources International and Texas Bureau of Economic Geology to investigate the potential for the geologic storage of CO2 in deep saline aquifers under high CO₂ volume conditions, we are examining the organic compounds in the produced brine/oil/gas mixtures. Denbury’s concurrent goal is injection of CO₂ into the Lower Tuscaloosa sandstones in the Cranfield oil field, MS, for enhanced oil recovery (EOR). This field, discovered in 1943, is a simple anticlinal four-way closure that produced ~62 MMbbl of oil and ~670 MMSCF of gas before its abandonment in 1966. Injection of CO₂ by Denbury began in July 2008 with rates of ~500,000 tons per year. Brine, oil and gas samples were collected from seven wells in March and December, 2009; samples were subjected to detailed field and laboratory organic and inorganic chemical and isotope analyses to investigate changes in chemical composition following CO₂ injection. Brine, oil, and gas samples from April 2010 were collected before and after terminating the addition of a corrosion inhibitor to investigate the addition of organics from this source.

Results show the CO₂ content of gas co-produced with oil and high salinity brine (TDS ~150,000 mg/L total dissolved solids) ranged from 5.7 to 90% by volume. The n-alkanes in brine increased in concentration as the percentage of CO₂ in the gas phase increased. Similar general trends were observed in the concentrations of organic acid anions and the parent PAHs and their alkylated homologs with the methyl, dimethyl, and trimethyl PAHs exhibiting the greatest increases in concentration with increases in the percentage of CO₂. The BTEX compounds along with the phenols showed increases in the mid-range (37-49%) of the percent CO₂ with concentrations tapering off as the percent CO₂ increased to 90%. Preliminary results indicate the probability of mobilization and solubility enhancement of some organic compounds, in particular the alkylated PAHs and other high molecular weight organics, following CO₂ injection at the Cranfield test site. Inhibitor absence from the fluid mixture resulted in slightly decreased concentrations of the more volatile components of the system, but in general had little effect on the oil mixtures.