Revealing fate of CO2 leakage pathways in the Little Grand Wash Fault, Green River, Utah

Abstract:

To assure long-term security of geologic carbon sequestration site, evaluation of natural CO₂ leakage should be preceded before actual construction of the CO₂ facility by comparing natural and artificial reservoir systems. The Little Grand Wash fault is located at the northwestern margin of the Paradox Basin and roles on a bypass of deep subsurface CO₂ and brine water onto the surface, e.g., cold water geyser, CO₂ spring, and surface travertine deposits. CO₂ degassed out from brine at the Little Grand Wash fault zone may react with formation water and minerals while migrating through the fault conduit. Leakage observed by soil CO₂ flux on the fault trace shows this ongoing transition of CO₂, from supersaturated condition in deep subsurface to shallow surface equilibria. The present study aims to investigate the reactions induced by changes in hydrological and mineralogical factors inside of the fault zone.

The methodology to develop site-specific geochemical model of the Little Grand Wash Fault combines calculated mechanical movements of each fluid end-member, along with chemical reactions among fluid, free CO₂ gas and rock formations. Reactive transport modeling was conducted to simulate these property changes inside of the fault zone, using chemistry dataset based on 86 effluent samples of CO₂ geysers, springs and in situ formation water from Entrada, Carmel, and Navajo Sandstone. Meanwhile, one- and two-dimensional models were separately developed to delineate features mentioned above.

The results from the 3000-year simulation showed an appearance of self-sealing processes near the surface of the fault conduit. By tracking physicochemical changes at the depth of 15 m on the 2-dimensional model, significant changes induced by fluid mixing were indicated. Calculated rates of precipitation for calcite, illite, and pyrite showed increase in 2.6 x 10^-4, 2.25 x 10^-5, and 3.0 x 10^-6 in mineral volume fraction at the depth of 15m, respectively. Concurrently, permeability and porosity were decreased 4.0 x 10^-18 m² and 3.0 x 10^-4 due to precipitation of minerals. At the middle of the fault conduit (400 m), however, indicates consistent dissolution of minerals in formation which enhances vertical fluid migration.