Advective, Diffusive and Eruptive Leakage of CO2 and Brine within Fault Zone

Abstract:

This study investigated a natural analogue for CO₂ leakage near the Green River, Utah, aiming to understand the influence of various factors on CO₂ leakage and to reliably predict underground CO₂ behavior after injection for geologic CO₂ sequestration. Advective, diffusive, and eruptive characteristics of CO₂ leakage were assessed via a soil CO₂ flux survey and numerical modeling. The field results show anomalous CO₂ fluxes (> 10 g m^-2 d^-1) along the faults, particularly adjacent to CO₂-driven cold springs and geysers (e.g., 36,259 g m^-2 d^-1 at Crystal Geyser), ancient travertines (e.g., 5,917 g m^-2 d^-1), joint zones in sandstone (e.g., 120 g m^-2 d^-1), and brine discharge zones (e.g., 5,515 g m^-2 d^-1). Combined to similar isotopic ratios of gas and progressive evolution of brine chemistry at springs and geysers, a gradual decrease of soil CO₂ flux from the Little Grand Wash (LGW; ~36,259 g m^-2 d^-1) to Salt Wash (SW; ~1,428 g m^-2 d^-1) fault zones reveals the same CO₂ origin and potential southward transport of CO₂ over 10-20 km. The numerical simulations overtly exhibit lateral transport of free CO₂ and CO₂-rich brine from the LGW to SW fault zones through the regional aquifers (e.g., Entrada, Navajo, Kayenta, Wingate, White Rim). CO₂ travels predominantly as an aqueous phase (X_co2=~0.045) as previously suggested, giving rise to the convective instability that further accelerates CO₂ dissolution. While the buoyant free CO₂ always tends to ascend, a fraction of dense CO₂-rich brine flows laterally into the aquifer and mixes with the formation fluids during upward migration along the fault. The fault always enhances advective CO₂ transport regardless of its permeability (k). However, only the low-k fault scenario engenders development of CO₂ anticlinal trap within the shallow aquifers (Entrada and Navajo), concentrating high CO­­­₂ fluxes (~1,273 g m^-2 d^-1) within the northern footwall of the LGW fault similar to the field. Moreover, eruptive CO₂ leakage at a well (Crystal Geyser) solely appears under the presence of anticlinal trap. Thus, it can be concluded that the LGW fault is likely low-permeable, 0.01 md ≤ k_h <0.1 md and 0.5 md ≤ k_v < 1 md, which could be used as a good starting point for other studies and further improved.