SIMULATION OF THE IMPACTS OF FAULTS ON CO2 INJECTION INTO SANDSTONE AQUIFERS

Numerical flow simulations in the Navajo Sandstone explore how faults affect CO₂ sequestration in high-porosity quartz sandstones. We examine regions where nature offers analogs of “successful” storage, and sites where leakage has occurred via fault breaches. Two end-member fault types are considered: low-permeability faults dominated by deformation-band networks, and high-permeability faults dominated by fracture networks in a damage zone adjacent to a clay-rich gouge zone. Equivalent permeability (k) values can range from less than 10^-14 m² (10 md) for deformation-band-dominated faults to greater than 10^-12 m² (1000 md) for fracture-dominated faults, regardless of the permeability of the unfaulted sandstone. Water-CO₂ fluid-flow simulations model the injection of CO₂ into high-k sandstone (5 x 10^-13 m² or 500 md) with low-k (5 x 10^-17 m² or 0.05 md) or high-k (5 x 10^-12 m² or 5000 md) fault zones that correspond to deformation-band- or fracture-dominated faults, respectively. After 500 days, the CO₂ rises to produce an inverted cone of free and dissolved CO₂ that spreads laterally away from the injection well. Free CO₂ fills ≤ 41% of the pore space behind the advancing CO₂ front, where dissolved CO₂ is at or near geochemical saturation. The low-k fault zone exerts the greatest impact on the shape of the advancing CO₂ front and restricts the dissolved and free CO₂ to the region upstream of the barrier. In this high-k aquifer, the high-k fault zone exerts a small influence on the shape of the advancing CO₂ front. In lower-permeability aquifers, high-permeability fault zones will become more important as pathways for CO₂ to bypass unfaulted sandstone. Although high-permeability fault conduits might lead to reduced sequestration efficiency, aquifer compartmentalization by low-permeability fault barriers may lead to improved efficiency because the barriers restrict lateral CO₂ migration and maximize the volume of CO₂ emplaced in each fault-bound compartment.