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

Abstract Many deep saline aquifers suitable for carbon dioxide (CO 2 ) sequestration contain measurable concentrations of hydrogen sulfide (H 2 S). 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 CO 2 injection into such aquifers is studied. When CO 2 is injected into a sour saline aquifer, the H 2 S initially dissolved in the brine will be exsolved and released into an expanding CO 2 plume. At any time after the start of CO 2 injection, the region swept by the plume consists of two sub-regions. The first of these is an inner sub-region extending from the injection well, and is characterized by the absence of H 2 S in both aqueous and gaseous phases. The dissolved H 2 S 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 H 2 S 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 H 2 S and CO 2 ) into sour saline aquifers and ordinary saline aquifers is explored. In contrast to sour aquifers, unsaturated water in an ordinary aquifer will strip away H 2 S from the CO 2 stream and consequently the mole fraction of H 2 S toward the gas front decreases. The highly toxic nature of H 2 S gas suggests the need to account for dissolved H 2 S in sour saline aquifers when establishing risk assessment, monitoring, and management strategies at CO 2 storage sites.