SIMULATING THE IMPACTS OF FAULTS ON CO2 INJECTION

Outcrop-to-simulation studies of faulted Navajo Sandstone of southeastern Utah provide a foundation for exploring how faults might affect CO₂ sequestration in high-porosity quartz sandstones. In places, naturally occurring CO₂ reservoirs are breached and CO₂-charged water discharges to the surface along permeable faults. Two fault types are considered; low-permeability faults dominated by deformation-band networks and high-permeability faults dominated by fracture networks. Simple averaging calculations indicate that equivalent permeability (k) values range from <10^-14 m² for deformation band dominated faults to >10^-12 m² for fracture dominated faults. Water-CO₂ fluid flow simulations model CO₂ injection into high-k sandstone (5x10^-13 m²) with either low-k (5x10^-17 m²) or high-k (5x10^-12 m²) fault zones that correspond to deformation band or fracture dominated faults, respectively. After 500 days, the free CO₂ rises to produce an inverted cone of free and dissolved CO₂ that spreads laterally away from the injection well. Free CO₂ fills less than than 41% of the pore space behind the advancing CO₂ front where dissolved CO₂ is also at or near geochemical saturation. The low-k fault zone exerts the greatest impact the advancing CO₂ front and restricts the bulk of the CO₂ to the region upstream of the fault barrier. In this high-k aquifer, the high-k fault zone exerts only a small influence on the shape of the advancing CO₂ front. In lower-k aquifers, high-k fault zones will become more important as CO₂ pathways. Although high-k fault conduits might lead to reduced sequestration efficiency, aquifer compartmentalization by low-k fault barriers may lead to improved efficiency because lateral CO₂ migration is restricted and the volume of CO₂ emplaced is maximized. Combining insights gained from the numerical simulations with geochemical modeling results suggests that CO₂ injection into a non-reactive sandstone aquifer such as the Navajo Sandstone will lead to geochemical saturation with dissolved CO₂ concentrations that approach the maximum value throughout the region of sequestered CO₂. Meanwhile, the low reactivity of the Navajo Sandstone and a lack of geochemically altered fault rocks suggests that little CO₂ induced change in porosity and permeability should be anticipated in the Navajo Sandstone.