Carbon Mineralization in Mafic Formations: Fracture Topology and Hydro-Chemo-Mechanical Coupling

ABSTRACT: Coupled hydro-chemo-mechanical processes affect engineered geosystems, including geothermal energy recovery, long-term nuclear waste disposal, and carbon geological storage. In particular, dissolution-transport-precipitation can significantly alter the permeability field and alter the evolution of these coupled processes. Carbon mineralization in mafic rocks is a salient example. Fractures within these rocks, including column-normal and entablature fractures, serve as conduits for fluid transport and host associated chemical processes. This study investigates fracture characteristics in mafic formations, using photographic records from published studies and data collected by the authors during site visits. Different fracturing topologies – from well-developed columnar structures to irregular patterns – exhibit distinct aperture and spacing characteristics. Additionally, new experimental results reveal the intricate interplay between water absorption kinetics and induced strains, which could potentially impact fracture aperture and, subsequently, permeability. The study also highlights the importance of flow rate and residence time in influencing the evolving pore fluid chemistry, including variations in pH and dissolved species. 1. INTRODUCTION Carbon mineralization in mafic and ultramafic rocks offers a promising solution for permanent carbon dioxide (CO2) sequestration (Snæbjörnsdóttir et al. 2020). The injection of CO2 into mafic formations such as basalts releases divalent metal cations (e.g., Ca2+, Mg2+, and Fe2+) from silicate minerals, which react with carbonate ions (CO3-) to form stable carbonate minerals such as calcite (CaCO3), magnesite (MgCO3) and siderite (FeCO3). While pilot projects in Iceland (Carbfix; Matter et al., 2016) and United States (Wallula; McGrail et al., 2017) have shown promising results, significant uncertainties remain for industrial-scale operation (Kelemen et al. 2019). Basalts exhibit distinctive fracture networks, often in the form of vertically aligned polygonal columns, that influence permeability and the extent of hydro-chemomechanical (HCM) processes (Lore et al. 2001; Kelemen et al. 2018). Fractures provide reactive surface area and space for precipitated minerals, which can potentially clog fluid pathways (Adeoye et al. 2017). However, in some cases, a significant decrease in permeability has been reported even with minimal changes in porosity, and the underlying mechanisms remain unclear (Kelemen et al. 2020). The dissolution of basalt, influenced by factors such as mineralogy, fluid chemistry, and flow rate, introduces additional complexity (Gislason and Arnorsson 1993). This study aims to enhance the understanding of fracture networks in mafic formations and their impact on HCM processes.