Time-lapse 3D imaging and modelling of acid-rock interactions within a mudstone: Implications for cap-rock behaviour during carbon storage

• Combine 3D imaging and simulation to investigate caprock-acid interactions. • Fractures close after acid injection and then grow due to calcite dissolution. • Dissolution front and calcite dissolution rates decrease nonlinearly over time. • Prolonged acid in immobile zones causes more calcite dissolution than mobile zones. • Concerns arise about carbonate-rich mudstones as caprock for carbon storage. Assessing caprock integrity is crucial for safe geological carbon storage. This study addresses this by utilising time-lapse micro-CT imaging to capture real-time, dynamic changes in mudstone during acid injection, providing a detailed analysis of the 3D geometry and evolution of minerals and fractures. Additionally, the integration of reactive transport modelling offers insights into the potential changes occurring immediately after acid injection, enhancing our understanding of fluid-rock interactions and their impact on caprock stability. Different acid concentrations were used to mimic a range of possible acid concentrations during CO 2 injection and storage. Changes in mudstones due to acid interaction include initial pre-existing fracture closure, followed by fracture growth and sample swelling. Acid predominantly followed pathways like fractures or laminations. Reactive transport models showed most dissolution occurring near the inlet, driven by a decrease in H + concentration along fluid pathways. Carbonate distribution controlled dissolved areas, with calcite dissolution and acid migration rates decreasing over time. Simulation indicated reduced shear stress after acid injection, especially with low-pH acids. This correlates to decreased fluid velocity, resulting in slower fluid flow. Slower acid movement and prolonged presence in immobile fluid flow zones led to more extensive calcite dissolution compared to mobile zones. This study provides valuable insights into microstructural changes in caprock following CO 2 injection, emphasizing the potential risk of fracture development and leakage.