(187h) Measurement and Calibration of Self-Sealing Rate of Fractures in Geological CO2 Storage: Case Study of a Natural Analog

Injection of anthropogenic CO₂ into a subsurface reservoir for sequestration will impact the geochemistry, porosity and permeability of the reservoir significantly. If a fault or fracture penetrates the reservoir, CO₂-laden brine may migrate into that fault, eventually sealing it via precipitation or opening it up via dissolution. Having a complete understanding of such effects is crucial for successful deployment of CO₂ sequestration.

We studied the Little Grand Wash Fault-zone (LGWF) in central Utah, where outcrops evidence historical leakage of naturally occurring CO₂ causing self-sealing in fractures along the fault-zone. Moreover, the site also has active leakage of CO₂ and CO₂-enriched groundwater in the form of a cold-water geyser, leading to active precipitation of travertine (CaCO₃) on the surface. Thus, this site serves as an ideal natural analog for leakage along fault-zones from engineered CO₂ systems.

Our hypotheses were that (1) the precipitation reactions would be rapid enough to be measurable within one cycle of the geyser, and that (2) fractures in a fault-zone similar to the LGWF would self-seal with CaCO₃ at the measured precipitation rates. To test these hypotheses, we conducted a 24hr field reactor experiment at the geyser. The mean precipitation rate at the geyser at 24hr was estimated to be 2.9x10^-3­­ mol/m²/s. We then set up a reactive transport model for vertically upward flow through an idealized single fracture with conditions similar to the LGWF. Significant reduction of porosity and permeability was observed in the fracture regions where the precipitation rates matched to those measured in the fields. A key implication from this study is that CO₂-enriched waters that are highly saturated with CaCO₃ can potentially seal a near-surface fracture within tens of years. However, the permanency of such sealing may be dependent on the subsequent flow conditions.