Investigation of Brine Geochemistry and Implications for Geologic Sequestration of CO₂ in Deep Sedimentary Basins

Sass, Bruce¹, Gupta, Neeraj¹, Jagucki, Phil¹, Sminchak, Joel¹, Massey-Norton, John², and Spane, Frank³

¹Battelle, Columbus, Ohio

²American Electric Power, Columbus, Ohio

³Pacific Northwest National Laboratory, Richland, WA

Major mechanisms for sequestering CO₂ in geologic formations include containment by caprock, trapping in residual phase, solution in formation brines, and reactions with minerals.  These mechanisms operate at varying spatial and temporal scales, and their relative magnitude/importance varies based on site-specific conditions.  An important aspect of assessing the long-term behavior and storage of CO₂ is the detailed geochemical characterization of in-situ formation fluids.  A 9,190 ft–deep well was drilled to evaluate the CO₂ storage potential of the entire sedimentary sequence in the Appalachian Basin at the American Electric Power (AEP) Mountaineer Plant in New Haven, West Virginia.  Important elements of the AEP borehole characterization program included:  detailed hydrochemical sampling of brine (obtained from two sandstone formations, Rose Run and the basal sandstone), a comprehensive suite of geophysical wireline logging, core analysis, and reservoir hydrologic testing.  Multiple sequences of dense, impermeable dolomite, limestone, and shale overlay and isolate the potential deep storage reservoirs in this area.  Brine samples were collected shortly after borehole  completion and again approximately seven months later during detailed hydrologic testing of selected formation horizons.  Geochemical results obtained from the second sampling event are considered to be more representative of actual in-situ formation conditions, due to the removal of residual drilling fluid (and reversal of local chemical effects) during hydrologic testing activities.  The hydrochemical results indicate that the formation brine waters are consistent with the sparse regional data for these formations, which in this part of the Appalachian Basin are of a Na-Cl or Na-Ca-Cl hydrochemical type.  Brine concentrations for the two deep zones sampled at the AEP borehole also fall within the higher end of the dissolved solids range  (200,000-325,000 mg/L) reported regionally for these formations.  The main implication of high formation brine salinity is that the solubility, and therefore the reactivity of CO₂ with formation minerals, would be reduced.  Therefore, the main storage mechanisms in such mature, deep basin formations would be containment by caprocks and residual saturation.  Results from stable isotope analysis and other hydrochemical measurements indicate that the candidate storage formations at the AEP borehole are  similar geochemically to other deep formations in the region and are isolated from overlying surface and groundwater.  Geochemical modeling results do not indicate any adverse reactions that would have a negative impact on the injection of CO₂ into either of the candidate storage formations.  The work presented here is being conducted under the Ohio River Valley CO₂ Storage Project, funded by U.S. Department of Energy, American Electric Power, BP, The Ohio Coal Development Office, Schlumberger, Battelle, and Pacific Northwest National Laboratory.