Evolution of hydrogen sulfide in sour saline aquifers during carbon dioxide sequestration

Many deep saline aquifers suitable for carbon dioxide (CO₂) sequestration contain measurable concentrations of hydrogen sulfide (H₂S). These aquifers are described here as sour saline aquifers and the other ones as ordinary saline aquifers. Sour saline aquifers occur wherever even minor amounts of anhydrite or other sulfate sources are present in the formation. In this paper, compositional modeling of CO₂ injection into such aquifers is studied. When CO₂ is injected into a sour saline aquifer, the H₂S initially dissolved in the brine will be exsolved and released into an expanding CO₂ plume. At any time after the start of CO₂ injection, the region swept by the plume consists of two sub-regions. The first of these is an inner subregion extending from the injection well, and is characterized by the absence of H₂S in both aqueous and gaseous phases. The dissolved H₂S in this inner sub-region is nearly completely removed from the brine via an exsolution process. The second sub-region extends from the outer edge of the inner sub-region to the leading edge of the plume. In this outer sub-region, the mole fraction of H₂S in the gas plume gradually increases toward the leading edge and reaches a peak value. While the gas plume is expanding the size of the outer sub-region enlarges. Following the discussion of these phenomena, in the next part of the paper, injection of acid gases (mixtures of H₂S and CO₂) into sour saline aquifers and ordinary saline aquifers is explored. In contrast to sour aquifers, unsaturated water in an ordinary aquifer will strip away H2S from the CO2 stream and consequently the mole fraction of H₂S toward the gas front decreases. The highly toxic nature of H2S gas suggests the need to account for dissolved H₂S in sour saline aquifers when establishing risk assessment, monitoring, and management strategies at CO₂ storage sites.