Reactive Transport Analysis of Fault ‘Self-sealing’ Associated with CO2 Storage

Abstract:

We present an extensive hydrologic and reactive transport analysis of the Little Grand Wash fault zone (LGWF), a natural analog of fault-associated leakage from an engineered CO₂ repository. Injecting anthropogenic CO₂ into the subsurface is suggested for climate change mitigation. However, leakage of CO₂ from its target storage formation into unintended areas is considered as a major risk involved in CO₂ sequestration. In the event of leakage, permeability in leakage pathways like faults may get sealed (reduced) due to precipitation or enhanced (increased) due to dissolution reactions induced by CO₂-enriched water, thus influencing migration and fate of the CO₂. We hypothesize that faults which act as leakage pathways can seal over time in presence of CO₂-enriched waters. An example of such a fault ‘self-sealing’ is found in the LGWF near Green River, Utah in the Paradox basin, where fault outcrop shows surface and sub-surface fractures filled with calcium carbonate (CaCO₃). The LGWF cuts through multiple reservoirs and seal layers piercing a reservoir of naturally occurring CO₂, allowing it to leak into overlying aquifers. As the CO₂-charged water from shallower aquifers migrates towards atmosphere, a decrease in pCO₂ leads to supersaturation of water with respect to CaCO₃, which precipitates in the fractures of the fault damage zone. In order to test the nature, extent and time-frame of the fault sealing, we developed reactive flow simulations of the LGWF. Model parameters were chosen based on hydrologic measurements from literature. Model geochemistry was constrained by water analysis of the adjacent Crystal Geyser and observations from a scientific drilling test conducted at the site. Precipitation of calcite in the top portion of the fault model led to a decrease in the porosity value of the damage zone, while clay precipitation led to a decrease in the porosity value of the fault core. We found that the results were sensitive to the fault architecture, relative permeability functions, kinetic parameters for mineral reactions and treatment of molecular diffusion. Major conclusions from this analysis are that a failed (leaking) engineered sequestration site may behave very similar to the LGWF and that under similar conditions some faults are likely to seal over time.