Secondary Mineral Formation Examined During CO2/Basalt Interaction

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper CCUS 4423733, “Experimental Investigation of Secondary Mineral Formation During CO2 /Basalt Interaction Under Reservoir Conditions,” by Elizabeth Appiah, SPE, Ernest Owusu, SPE, and Jason Simmons, SPE, New Mexico Institute of Mining and Technology, et al. The paper has not been peer-reviewed. _ Basalt formations are exceptionally reactive CO2-storage candidates because they favor rapid mineral/fluid reactions that promote long-term storage security. Dissolution of basaltic minerals and precipitation of secondary phases alter fluid geochemistry and, potentially, pore structure during CO2 injection and storage. Secondary mineral formation is thought to alter porosity permeability and injectivity, but these effects have not been well-constrained. In this work, a laboratory flow-through experiment designed to quantify dissolution of basaltic primary minerals and conditions favorable for secondary phase formation under deep subsurface storage conditions is presented. Introduction Basalt formations have emerged as promising targets for geological CO2 storage because of abundant reactive silicate minerals and the potential to immobilize CO2 through mineral trapping. Injected CO2 can be transformed into permanent solid mineral phases in basalt, enhancing security of storage. Injection of CO2-bearing fluids results in enhanced dissolution kinetics of olivine, plagioclase, and pyroxene minerals, releasing cations such as Ca2+, Mg2+, and Fe2+ available for secondary carbonate and aluminosilicate precipitation. The implications of these secondary phases on pore-structure evolution and injectivity are currently not well understood because of analytical limitations. While secondary minerals have been identified through X-ray diffraction (XRD) and other analytical methods, most experiments have been time-limited, and high-pressure/high-temperature conditions pose challenges for direct mineral identification. Effluent geochemistry is, therefore, used widely to assess dissolution and infer secondary controls during CO2/rock interaction. A laboratory flow-through experiment was performed to study interactions between CO2 and basalt under reservoir-relevant conditions. Effluent analysis coupled with initial characterization of the solid was used to quantify primary dissolution and determine the potential for secondary phase formation. Linking reaction conditions to evolving geochemistry in basalt provides constraints on CO2-storage mechanisms and pathways for pore-scale evolution. Theory and Methods Basalt Core Preparation and Pre-Experimental Characterization. A basalt core plug (1 in. × 2 in.) was prepared for triaxial flow-through testing. Baseline mineralogical and microstructural characterization was completed before reaction to define initial conditions. XRD identified the primary crystalline phases present in the starting material, including augite, forsterite, and anorthite consistent with a mafic assemblage. Electron-probe microanalysis (EPMA) was used to quantify representative compositions for four targeted phases: pyroxene, olivine, feldspar, and glass. These baseline compositions provided constraints for interpreting dissolution signatures in the effluent chemistry and for selecting mineral endmembers in saturation-index (SI) calculations. Brunauer-Emmett-Teller (BET) surface-area measurements and X-ray computed tomography (XRCT) imaging were performed on the unreacted cores to quantify initial surface and pore structure. XRCT-derived porosity was used alongside laboratory porosity measurements to track changes after testing. XRD results and EPMA-guided backscattered-electron imaging were used to define primary mineral phases before reaction.