Coupled Hydraulic-Geochemical Processes in a Faulted Clay-Rich Caprock: Reactive Transport Modelling of an In Situ CO2 Injection Experiment

Clay–rich formations are widely regarded as effective caprocks for geological CO₂ storage; however, the presence of fractures in fault zone introduces significant uncertainty regarding their hydraulic behaviour during injection. In such settings, fluid migration is expected to be largely governed by fracture networks, while coupled hydraulic and geochemical processes associated with CO₂–water–rock interactions may progressively modify hydraulic properties over time. This study presents a reactive transport modelling investigation of the CS–D (Carbon Sequestration – Series D) in situ CO₂ injection experiment, with the aim of quantifying fracture hydraulic behaviour within a faulted clay–rich caprock.

A three-dimensional reactive transport model has been fully implemented using HYTEC to simulate coupled fluid flow and CO₂–water–rock interactions within a fractured fault zone embedded in a low-permeability clay matrix. The modelling framework accounts for aqueous speciation, mineral dissolution and precipitation, as well as advective–diffusive transport, and is configured to reproduce the experimental conditions of the CS–D test. Structural and hydrogeochemical observations derived from the experiment are used to constrain boundary conditions and initial states, while fracture hydraulic properties are treated as key uncertain parameters.

The numerical framework enables a systematic investigation of the sensitivity of pressure evolution and geochemical responses to variations in fracture permeability and reactive surface area. The current geometric representation captures the principal structural characteristics of the faulted zone and retains sufficient flexibility to explore alternative conceptual configurations as the analysis progresses. The present work addresses coupled hydraulic and geochemical processes and is intended to serve as a basis for future extensions towards a coupled thermo-hydro-mechanical-chemical (THMC) framework relevant to subsurface energy applications.

The resulting simulations are expected to provide quantitative constraints on the range of fracture hydraulic properties compatible with the hydraulic and geochemical signals observed during CO₂ injection. Ultimately, this study seeks to improve the process-based understanding of fracture-controlled flow in faulted clay-rich caprocks and to support the interpretation of in situ experiments relevant to the long-term integrity and safety of geological CO₂ storage and related geo-energy technologies.