Numerical Simulation of the Storage of Pure CO2 and CO2-H2S Gas Mixtures in Deep Saline Aquifers

Abstract

Abstract We have studied strategies for maximizing several phenomena beneficial to large-scale subsurface storage of waste gases such as CO2 and H2S. Numerical simulations using a compositional reservoir simulator were carried out for 10,000 years to understand the flow and long-term storage potential of pure CO2 and CO2-H2S mixtures in deep saline aquifers. Hysteresis in the relative permeability curve results in substantial volumes of gas trapping. Aquifer characteristics such as heterogeneity, dip angle and vertical to horizontal permeability ratio were varied to determine their effect on storage potential and injectivity of a CO2-H2S gas mixture. The opportunity for escape of the gases from the aquifer can be minimized by careful design of injection strategies. One such strategy is to use horizontal wells low in the formation so that all of the injected gases are trapped, dissolved or precipitated before they reach geological seals and/or faults. This allows significantly larger volumes of waste gases to be stored in a given aquifer. Preferential solubility of the H2S in brine reduces the distance H2S travels relative to CO2. Simulations with local grid refinement show that fingering due to buoyancy is mitigated by natural heterogeneity in the aquifer petrophysical properties. Thus the amount of gas trapping observed in coarse-grid simulations is likely to be a reasonable estimate of what can be obtained in the field. Three-dimensional simulations of coupled flow and reactive transport showed that the amount of CO2 sequestered as minerals was small relative to gas trapping and dissolution into brine. However, the mineralization further reduces the already small amount of mobile gas over long periods of time.