Changes in Permeability and Pore Structure Induced by CO <sub>2</sub> ‐Water‐Basalt Interaction: Insights From Flow‐Through Experiments at 200°C
Naoki Nishiyama, Masao Sorai, Kentaro Masuoka, M. Shiga
Abstract
Abstract The interaction between carbon dioxide (CO 2 ), water, and basalt is a common process in natural and engineered systems, including hydrothermal circulation and CO 2 geological sequestration in basaltic formations. CO 2 –water–basalt interaction can alter basalt pore structure by enhancing mineral dissolution and precipitation, leading to changes in permeability. However, how the interaction changes permeability at high temperatures remains poorly understood. Here, we conducted flow‐through experiments where CO 2 ‐rich water was injected into 200 mm‐long basalt cores at 200°C at varying flow rates for 15–21 days. The injection of CO 2 ‐rich water continuously reduced permeability, by up to approximately two orders of magnitude, but had minor effects on porosity (<1%). The rate of permeability reduction was almost independent of flow rate. Microstructural analyses revealed that the style of CO 2 ‐induced reactions varied along the flow direction. Near the inlet, CO 2 ‐rich water strongly promoted dissolution of primary phases, creating a highly porous reaction zone. The reaction further downstream was characterized by less intense dissolution and concurrent phyllosilicate precipitation, with dissolution producing intragranular microporosity and precipitation forming 1–10‐μm‐thick phyllosilicate coatings on pore surfaces, and a lack of carbonate precipitation; this reaction continued toward the outlet. The phyllosilicate coating led to clogging of the pore throats in the main flow path, which likely controlled the permeability reduction. A comparison with previous data revealed the important roles of temperature and secondary mineralogy in controlling permeability changes caused by CO 2 –water–basalt interaction.