The effect of CO_2-related diagenesis on geomechanical failure parameters:fracture testing of CO_2-altered reservoir and seal rocks from a natural analog at Crystal Geyser, Utah

摘要

Fracture mechanical tests on samples collected from the Crystal Geyser field site, Utah indicate that sandstones naturally bleached by CO_2-rich fluids have significantly lower fracture toughness values than adjacent, unaltered sandstones whereas altered and unaltered caprock siltstones collected from the site yield similar values. Both the short rod (SR) and double torsion (DT) test methods give comparable results for multiple test specimens from the same samples. CO_2-related alteration at the Crystal Geyser field site is reflected in the bulk mineralogy of siltstone samples collected near fossil CO_2 seeps and primarily characterized by an increase in calcite. Dissolution of Fe-cements,along with the remobilization and precipitation of secondary carbonates and clays noted by petrography, are likely responsible for the geomechanical weakening effect in sandstones. Subcritical crack index, another fracture parameter obtained from the DT test,is also affected by CO_2-related alteration. These results suggest that long-term mechanical behavior of CO_2 reservoir and seal rocks may be affected by interaction with CO_2-rich fluids requiring the consideration of coupled chemical-geomechanical processes and time-dependent geomechanical behavior in CO_2 reservoir simulations.