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Abstract. Carbon dioxide (CO 2 ) injection into deep geologic formations could decrease the atmospheric accumulation of this gas from anthropogenic sources. Furthermore, by co-injecting H 2 S or SO 2 , the products respectively of coal gasification or combustion, with captured CO 2 , problems associated with surface disposal would be mitigated. We developed models that simulate the co-injection of H 2 S or SO 2 with CO 2 into an arkose formation at a depth of about 2 km and 75°C. The hydrogeology and mineralogy of the injected formation are typical of those encountered in Gulf Coast aquifers of the United States. Six numerical simulations of a simplified 1-D radial region surrounding the injection well were performed. The injection of CO 2 alone or co-injection with SO 2 or H 2 S results in a concentrically zoned distribution of secondary minerals surrounding a leached and acidified region adjacent to the injection well. Co-injection of SO 2 with CO 2 results in a larger and more strongly acidified zone, and alteration differs substantially from that caused by the co-injection of H 2 S or injection of CO 2 alone. Precipitation of carbonates occurs within a higher pH (pH > 5) peripheral zone. Significant quantities of CO 2 are sequestered by ankerite, dawsonite, and lesser siderite. The CO 2 mineral-trapping capacity of the formation can attain 40-50 kg/m 3 medium for the selected arkose. In contrast, secondary sulfates precipitate at lower pH (pH < 5) within the acidified zone.Most of the injected SO 2 is transformed and immobilized through alunite precipitation with lesser amounts of anhydrite and minor quantities of pyrite. The dissolved CO 2 increases with time (enhanced solubility trapping). The mineral alteration induced by injection of CO 2 with either SO 2 or H 2 S leads to corresponding changes in porosity. 2Significant increases in porosity occur in the acidified zones where mineral dissolution dominates. With co-injection of SO 2 , the porosity increases from an initial 0.3 to 0.43 after 100 years. However, within the CO 2 mineral-trapping zone, the porosity decreases to about 0.28 for both cases, because of the addition of CO 2 mass as secondary carbonates to the rock matrix. Precipitation of sulfates at the acidification front causes porosity to decrease to 0.23. The limited information currently available on the mineralogy of naturally occurring high-pressure CO 2 reservoirs is generally consistent with our simulations.

Approximately 300 kg/day of food-grade CO 2 was injected through a perforated pipe placed horizontally 2-2.3 m deep during July 9-August 7, 2008 at the MSU-ZERT field test to evaluate atmospheric and near-surface monitoring and detection techniques applicable to the subsurface storage and potential leakage of CO 2 . As part of this multidisciplinary research project, 80 samples of water were collected from 10 shallow monitoring wells (1.5 or 3.0 m deep) installed 1-6 m from the injection pipe, at the southwestern end of the slotted section (zone VI), and from two distant monitoring wells. The samples were collected before, during, and following CO 2 injection. The main objective of study was to investigate changes in the concentrations of major, minor, and trace inorganic and organic compounds during and following CO 2 injection. The ultimate goals were (1) to better understand the potential of groundwater quality impacts related to CO 2 leakage from deep storage operations, (2) to develop geochemical tools that could provide early detection of CO 2 intrusion into underground sources of drinking water (USDW), and (3) to test the predictive capabilities of geochemical codes against field data. Field determinations showed rapid and systematic changes in pH (7.0-5.6), alkalinity (400-1,330 mg/l as HCO 3 ), and electrical conductance (600-1,800 lS/cm) following CO 2 injection in samples collected from the 1.5 m-deep wells. Laboratory results show major increases in the concentrations of Ca (90-240 mg/l), Mg (25-70 mg/l), Fe (5-1,200 ppb), and Mn (5-1,400 ppb) following CO 2 injection. These chemical changes could provide early detection of CO 2 leakage into shallow groundwater from deep storage operations. Dissolution of observed carbonate minerals and desorptionion exchange resulting from lowered pH values following CO 2 injection are the likely geochemical processes responsible for the observed increases in the concentrations of solutes; concentrations generally decreased temporarily following four significant precipitation events. The DOC values obtained are 5 ± 2 mg/l, and the variations do not correlate with CO 2 injection. CO 2 injection, however, is responsible for detection of BTEX (e.g. benzene, 0-0.8 ppb), mobilization of metals, the lowered pH values, and increases in the concentrations of other solutes in groundwater. The trace metal and BTEX concentrations are all significantly below the maximum contaminant levels (MCLs). Sequential leaching of core samples is being carried out to investigate the source of metals and other solutes.