Hydrodynamic Trapping of CO2 Geosequestered in Saline Aquifers

Abstract Deep saline aquifers are potential reservoirs for commercial scale CO2 geologic sequestration (GS) due to their large storage capacity and wide availability. These reservoirs are regional in scale with no conventional stratigraphic or structural traps. The fate of supercritical CO2 injected in saline aquifers depends on hydrodynamics and phase trapping. This paper examines hydrodynamic trapping, and the question of whether basinward flow of groundwater can counteract updip CO2 movement after injection ends, and render the gas immobile. After injection pressure dissipates, the important flow mechanisms are viscous flow of groundwater (imposed by compaction and gravity) and buoyant flow of CO2 (imposed by the density contrast between injected CO2 and the formation fluid). Hydrodynamic trapping theory is reviewed and its application to GS is discussed. The same methods developed for oil and gas can be used to identify possible hydrodynamic traps for pools of CO2 post injection. Examples are given of deep basin aquifer dynamics and regional groundwater flow gradients as they relate to GS. In a dipping aquifer, groundwater flow and buoyant CO2 flow may be in the same direction (immature compacting basin) or the opposite direction (mature basin). The mature basin scenario is optimum for GS, as long-term updip CO2 movement will be reduced by saline aquifer flow. Analytic expressions are used to investigate trapping potential, and to estimate the rate of buoyant CO2 movement after injection ends in a mature basin example with groundwater flow down the dip. Key inputs such as hydraulic gradient, dip, and density contrast were varied to assess their relative influence. The results demonstrate that, post injection, it is possible for pooled CO2 to be hydrodynamically trapped under field conditions. CO2 could also be displaced downdip in aquifers with high hydraulic gradients (~ 0.02) and low dips (~ 0.25 degrees). In the more likely case of a low to average hydraulic gradient (~ 0.0004 to 0.003), and dip greater than ~ 0.25 degrees, buoyancy dominates, and the CO2 is generally displaced in the updip direction at rates of approximately one to one hundred feet per year. This can lead to significant displacement over the long time scales desired for GS. Therefore, in a mature basin, groundwater flow alone will usually be insufficient to counteract buoyancy, and phase trapping or hydrodynamic trapping will be needed to ultimately immobilize the CO2.