Changes in Pore Geometry and Connectivity in the Basalt Pore Network Adjacent to Fractures in Response to CO₂‐Saturated Fluid

Abstract

Abstract Pores connected to fractures provide an increased surface area for fluid‐rock interactions when reactive fluids, such as CO 2 ‐saturated water, flow within fractures. Diffusion‐controlled transport of ions including dissolved CO 2 , Ca 2+ , Mg 2+ , Fe 2+ , and Si within the connected pore network can lead to local mineral undersaturation or supersaturation and respective local dissolution or precipitation of secondary minerals. In this study, a diffusion‐controlled experiment was conducted on a fractured basalt under subsurface conditions (60°C and 80 bars) to investigate the changes in the pore volume, the connectivity within the pore network, the pore and throat size distribution and to quantify the volume of dissolved and precipitated mineral phases. CO 2 ‐saturated water with a supply of ions from the dissolution of basalt powder was reacted with an artificially fractured basalt sample over 12 weeks. The net pore volume of the sample decreased by 158 mm 3 , equivalent to 7.7% of the initial pore volume (2,041 mm 3 ). The number of pores decreased considerably (15%) while the decline in the number of throats was small (4%) suggesting precipitation primarily occurred in pores. The number of isolated pores declined, which is attributed to mineral dissolution leading to greater connectivity within the pore network. Overall, the results provide insights into the early phase of reactive fluid flow in fractured basalt eventually leading to self‐sealing of fractures and the adjacent pore network.