FAULT-CONTROLLED CO2 LEAKAGE FROM NATURAL RESERVOIRS IN THE COLORADO PLATEAU, EAST-CENTRAL UTAH

Geologic CO₂ sequestration has been proposed as a desirable option for reducing atmospheric CO₂ emissions, but it can be an expensive failure if stored CO₂ leaks from engineered sites. This study carried out a diffusive soil CO₂ flux survey in conjunction with numerical simulations of CO₂ leakage in the fault zones of the Colorado Plateau in east-central Utah, with the aim of having implications for geologic CO₂ storage. 440 and 140 measurements of soil CO₂ flux were made at 395 and 129 sites respectively in the Little Grand Wash (LGWFZ) and Salt Wash fault zones (SWFZ). CO₂ flux anomalies in the SWFZ (~1,428 g m^-2 d^-1) are comparably smaller to the LGWFZ (~36,259 g m^-2 d^-1). Anomalously high CO₂ fluxes were localized in close proximity of travertines on the northern fault traces (e.g. 5,917 g m^-2 d^-1), joint zones in sandstones (e.g. 120 g m^-2 d^-1), and brine discharge zones (e.g. 5,515 g m^-2 d^-1); however, there were also many fault/fracture zones, which gave no significant CO₂ fluxes, indicating that the faults/fractures selectively behave as conduits for diffusive CO₂ flow. In order to better understand CO₂ and brine leakages processes in the LGWFZ, 2-D numerical simulations were performed with various scenarios for permeabilities of the faults and Navajo Sandstone. Henceforth, the simulation results were validated by matching with field CO₂ flux observations. The results showed that CO₂-brine flow regime depends primarily on the characteristics of fault permeabilities. Only anisotropic low permeabilities of the faults (0.01≤k_h<0.1 and 0.1≤k_v<1 md) led CO₂ flow to be impeded across the faults and pooled in an anticlinal trap, which focuses high CO₂ fluxes on the northern footwall similar to field observations. Nevertheless, the faults enhanced the vertical connectivity among the reservoirs compartmentalized by caprocks so that CO₂ can accumulate in the reservoirs and even flow over a distance of several kilometers towards other fault zones. For a successful sequestration of CO₂, therefore, it must: (1) take precedence to identify faults, joints/fractures, and groundwater discharge zones in a large region, (2) carefully monitor the proximity of injection/monitoring wells to CO₂ plumes, (3) and have a dense spatial monitoring system of soil CO₂ flux after injections.