NEAR SURFACE AND DEEP SUBSURFACE MONITORING FOR SUCCESSFUL GEOLOGIC STORAGE OF CO2

Geologic storage of CO₂ is now considered one of the necessary options to stabilize atmospheric CO₂ levels and global temperatures at acceptable values. Because the sequestered CO₂ is both buoyant and reactive to minerals and well pipes, some of it may leak upward, possibly contaminating underground sources of drinking water (USDW). We have participated in multi-laboratory field experiments to investigate geochemical parameters and methodologies to monitor the flow of injected CO₂ into deep saline aquifers (Frio Pilot, Texas) and into potable shallow groundwater (ZERT site, Montana). Geochemical results proved powerful tools in: 1- Tracking the successful injection and flow of CO₂ into the Frio C sandstone ; 2- showing that some of the injected CO₂ was detected in the overlying B sandstone that is separated from the C sandstone by 15 m of shale and siltstone; 3- showing mobilization of metals, including Fe ( from 30 to 1100 mg/L), Mn and Pb, and organic compounds (DOC from 5 to 700 mg/L), including BTEX, PAHs, and phenols following CO₂ injection; and 4- showing major changes in chemical and isotopic compositions of formation water , including a dramatic drop in calculated brine pH, (initially from 6.3 to 3.0) and major increases in alkalinity (from 100 to 3000 mg/L as HCO₃). Geochemical modeling, chemical data and Fe isotopes indicate rapid dissolution of minerals, especially calcite and Fe-oxyhydroxides, and that part of the Fe and other metal increases were caused by corrosion of well pipe. The high DOC values likely represent a ‘slug’ of organic matter mobilized by the injected supercritical CO₂, that is a very effective solvent for hydrocarbons.

Significant isotopic and chemical changes, including the lowering of pH, increases in alkalinity, mobilization of metals and detection of BTEX, were also observed in samples obtained from shallow (depth of ~2 m) groundwater following CO₂ injection through a slotted pipe placed horizontally at a depth of ~2 m in the ZERT site, Bozeman, MT. Results from both the deep and shallow field tests show that geochemical methodologies have highly sensitive chemical and isotopic tracers for tracking changes resulting from water-CO₂–sediment interactions. These methodologies, are recommended for CO₂ injection sites to monitor injection performance, and for early detection of any CO₂ and brine leakages.