From pore-scale to macro-scale: Understanding fluid-rock interactions using X-ray Computed Tomography

Fluid-rock interaction represents a common geological process that is highly dynamic and may cause substantial microscale petrophysical and geochemical changes both in a static and syn-deformational environment. Understanding how these local microscale dynamics occur is crucial to comprehend macroscale behaviour of the lithosphere, and for advancing critical subsurface engineering challenges, such as carbon capture and storage; a process that, together with hydrogen storage and geothermal energy recovery, is vital for the energy transition. With the advance of non-destructive imaging techniques (X-ray Computed Tomography - XCT), we can image the evolution of these microscale dynamics and understand how they drive changes in crustal dynamics and subsurface engineering. We present two applications, in which we use XCT to characterise the evolution of reservoir storage properties, such as porosity and permeability, and provide further insights into carbon sequestration. In the first application, we used XCT to investigate the precipitation history of an amygdaloidal basalt now partially filled by calcite as an analogue for CO₂ mineral trapping in a vesicular basalt. We quantified the evolution of basalt porosity and permeability during pore-filling calcite precipitation by applying novel numerical erosion techniques to “back-strip” the calcite from the amygdales and fracture networks. We found that once the precipitation is sufficient to close off all pores, permeability reaches values that are controlled by the micro-fracture network. These results prompt further studies to determine CO₂ mineral trapping mechanisms in amygdaloidal basalts as analogues for CO₂ injections in basalt formations. In the second application, we considered the combined effect of upstream corrosion of the carbon capture and storage (CCS) infrastructure and pre-existing reservoir rock compositions on the evolution of reservoir storage properties. Reactions from the corroded pipelines can change the chemistry of injected brine, which can then react with the adjacent rock formations reservoir, affecting reservoir porosity, permeability and caprock integrity. These are important parameters that determine the injectivity and storage capacities of deep geological sites for long term CO₂ storage. Reservoir rock samples are characterised before corrosion and after carbonation reactions using XCT and other micro-analytical techniques, to assess the changes in the rock storage capacity properties. Our preliminary results prompt further studies into the understanding of fluid-rock interactions for subsurface engineering challenges, with a particular focus to pre-existing microfractures and changes in the injected brine due to corrosion of the upstream pipelines and interaction between CO₂ brine and reservoir rocks.