PHYSICAL AND CHEMICAL CHANGES FROM BASALT-CO2-RICH FLUID INTERACTION DURING FLOW-THROUGH EXPERIMENTS AT 150°C AND 150 BAR

Basalt is abundant at the Earth’s surface and will readily react when in contact with CO₂-rich fluids. The interaction occurs naturally with alteration of the ocean crust and in geothermal regions with abundant CO₂ as well as in areas where CO₂ is injected into basalt formations, such as during geologic carbon sequestration efforts in southwest Iceland and in Washington in the northwest United States. To constrain physical and chemical processes and changes that arise from alteration of basalt with CO₂-rich fluids, we conducted four reactive flow-through experiments at 150 bar pore-fluid pressure and 150°C, flowing a 1 molal NaCl and 0.65 molal CO₂ solution (initial pH of 3.3) at flow rates of 0.1 ml/min and 0.01 ml/min. Permeability increased at the higher flow rate, but decreased at the lower flow rate. There was a significant difference in the permeability increase for the higher flow rate experiments, due to a difference in the preexisting pore structure. X-ray computed tomography (XRCT) data showed that all experiments caused significant dissolution throughout the cores. Furthermore, Fe, Mg, and Si outlet concentrations were up to 2.5 mmol/kg, 5.9 mmol/kg, and 7.2 mmol/kg, respectively, with reaction limited to ≈5 to 39 minutes. In addition, alkali metals were highly mobile, as up to 29% and 99% of the K and Cs were dissolved from the cores. Secondary electron microscopy imaging revealed secondary phases enriched in Al and Si, and a Fe₂O₃-rich phase was identified on post-experimental cores. While no carbonate was identified, siderite was generally at or above saturation in fluid samples. In addition to imaging dissolution, XRCT data also portrayed secondary precipitation that occurred throughout the cores, indicating a porosity decrease for length scales greater than 25 μm. In contrast, (ultra) small-angle neutron scattering ((U)SANS) suggested a porosity increase from alteration while probing smaller lengths, ranging from 1 nm to 10 μm. (U)SANS analyses also indicated an increase in surface area from reaction, even though they also suggested that the pore structure and mineral surface roughness were maintained. While these datasets illustrate complexity with the many processes involved during reactive transport, they provide observations relevant to basalt-CO₂-rich fluid interaction in geologic systems.