CO2 Driven Mineral Transformations in Fractured Reservoir

Abstract:

Engineering fracture systems in low permeable formations to increase energy production, accelerate heat extraction, or to enhance injectivity for storing anthropogenic CO₂, is a challenging endeavor. To complicate matters, caprocks, essential components of subsurface reservoirs, need to maintain their sealing integrity in this modified subsurface system. Supercritical CO₂ (scCO₂), a proposed non-aqueous based working fluid, is capable of driving mineral transformations in fracture environments. Water dissolution in scCO₂ significantly impacts the reactivity of this fluid, largely due to the development of thin adsorbed H₂O films on the surfaces of exposed rocks and minerals. Adsorbed H₂O films are geochemically complex microenvironments that host mineral dissolution and precipitation processes that could be tailored to influence overall formation permeability. Furthermore, manipulating the composition of injected CO₂ (e.g., moisture content and/or reactive gases such as O₂, NO_x, or SO_x) could stimulate targeted mineral transformations that enhance or sustain reservoir performance. PNNL has developed specialized experimental techniques that can be used to characterize chemical reactions occurring between minerals and pressurized gases. For example, hydration of a natural shale sample (Woodford Shale) has been characterized by an in situ infrared spectroscopic technique as water partitions from the scCO₂ onto the shale. Mineral dissolution and carbonate precipitation reactions were tracked by monitoring changes of Si-O and C-O stretching bands, respectively Structural changes indicated expandable clays in the shale such as montmorillonite are intercalated with scCO₂, a process not observed with the non-expandable kaolinite component. Extreme scale ab initio molecular dynamics simulations were used in conjunction with model mineral systems to identify the driving force and mechanism of water films. They showed that the film nucleation and formation on minerals is driven by both enthalpic and entropic requirements. Collectively, the synergy between laboratory observations, state-of-the-art atomistic simulations and reservoir modeling has generated important insights for the design and engineering of subsurface reservoirs for CO₂ storage and energy extraction.