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 sub-region 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. (C) 2010 Elsevier Ltd. All rights reserved.