GAS-BRINE-ROCK INTERACTIONS FOLLOWING CO2 INJECTION INTO THE TUSCALOOSA SANDSTONE: RESULTS FROM SECARB PHASE III AT CRANFIELD, MS

The SECARB Phase III test at Cranfield, MS, is a multi-laboratory field experiment funded by DOE to investigate the potential for the geologic storage of large volumes of CO₂ in saline aquifers. The experiment uses a CO₂ injection well and two observation wells, perforated into an 18-m interval of heterogeneous sandstones of the lower Tuscaloosa Formation at ~3000 m depth. The wells are located in the downdip water leg of the antiformal Cranfield oil field that was recently subjected to CO₂ EOR. Injection of CO₂ at 5000 ft³/day began in December 2009, and brine and gas samples were collected during the initial 18 days of CO₂ injection using installed u-tubes and downhole Kuster samplers. Baseline brine, oil, and gas samples were also collected from Denbury-Cranfield production wells in March and December, 2009. Field (pH, EC, alkalinity and fluorescein tracer) and laboratory chemical and isotopic analyses were conducted to characterize geochemical changes in response to CO₂ injection.

Tuscaloosa brine is Na-Ca-Cl type of relatively uniform salinity (~152,000 mg/L TDS) and solute concentrations; the brine is CH₄ saturated (~60 mM), CO₂ is 5-7 wt%, and H₂S is negligible. Geochemical modeling indicates calcite and disordered dolomite are near saturation at subsurface conditions (~125ºC; ~ 350 bars). Brine salinity and monovalent solute concentrations were unchanged following CO₂ breakthrough; however pH decreased (from ~5.7 to 5.0), and alkalinity increased (from ~375 to 500 mg/L as HCO₃). Divalent cations show modest increases-- Ca (8%), Mg (9%), Sr (8%), Fe (28%), and Zn (27%) -- with lower pH and higher pCO₂. Sr/Ca and Mg/Ca ratios are virtually constant as concentrations increase, which strongly suggests congruent carbonate dissolution.

Initial results from this experiment suggest only minimal water-rock interaction, which contrasts sharply with results from the Frio pilot, where pH decreased sharply and alkalinity and Fe concentrations greatly increased. The minor chemical changes at Cranfield may be due to: 1) the experiment design - use of fiberglass-lined casing and non-corrosive well components; 2) slow-reacting host rocks; and 3) advance of CO₂ primarily in high permeability zones that, in the Tuscaloosa, tend to be carbonate poor, and contain minerals coated with non-reactive Fe-chlorite.