IAP-25-117

Reactive Fracturing and Carbonation Dynamics in Mafic and Ultramafic Rocks under CO₂ Storage Conditions

Mafic rocks are promising candidates for permanent geologic carbon storage, where injected CO₂ reacts with silicate minerals to form stable carbonates (IEAGHG, 2025). At the same time, these rocks are a valuable resource for critical minerals such as Ni, Co, and Cr (Stanfield et al. 2024). Fractures within mafic rocks (Xiong et al. 2017) are essential as they play a dual role by enhancing reactive surface area and fluid flow but can also evolve through mineral precipitation and crystallisation-driven stress. These feedbacks between geochemical reactions and mechanical processes remain poorly understood and critically influence storage efficiency and long-term reservoir behaviour.
This PhD project will investigate how carbonate mineralisation and the force of crystallisation drive fracture evolution and permeability changes in basaltic systems exposed to CO₂-rich fluids. Laboratory experiments will combine X-ray micro-computed tomography (µCT) and acoustic emission (AE) monitoring to visualise fracture growth, mineral precipitation, and microcracking in real time (Fusseis et al. 2022). These complementary methods will provide a 4D view of how chemical reactions and stress interact to modify rock structure and reactive surface area. These experiments will be conducted under realistic pressure, temperature and stress conditions using either conventional µCT setups at Heriot-Watt and Glasgow Universities or synchrotron-based tomography (Diamond Light Source).
A reactive transport modelling framework will be developed to link observed geochemical gradients with the mechanical effects of crystallisation, improving predictions of how fractures evolve during carbonation. Integrating experimental observations with model simulations will clarify how coupled chemo-mechanical feedbacks control carbonation efficiency and fracture evolution in mafic-ultramafic reservoirs.
Key Research Questions:
1. How does crystallisation-induced stress influence fracture propagation and reactive surface area?
2. What are the coupled geochemical and mechanical controls on carbonation in basalt?
3. How can µCT and AE monitoring be combined to capture and model reactive fracturing processes?
4. What is the potential of mafic rock carbonation in fractured mafic reservoirs for critical mineral liberation?
Expected Outcomes:
• Mechanistic understanding of reactive fracturing during CO₂ mineralisation in basalt
• Integrated experimental–modelling framework for predicting fracture evolution and mineral trapping
• Insights supporting the design and monitoring of secure carbon storage systems in mafic formations

Specimen preparation & baseline characterisation
– Prepare mafic rock core samples with induced or natural fractures; measure initial mineralogy (XRD), texture (SEM–EDS), and porosity/permeability
– Establish baseline 3D fracture geometry via high-resolution microCT; compute aperture and surface-area maps
CO₂–fluid reaction experiments and in-situ monitoring
– Conduct controlled flow-through or batch reactions under CO₂-rich, hydrothermal conditions
– MicroCT time-lapse (4D) and synchronous imaging: periodic scans to quantify carbonate precipitation, fracture infill, topology changes and initiation/propagation of fractures; segment phases and compute volume fractions
– Acoustic emission (AE): continuous monitoring to detect microcracking and temporal clustering to link to crystallisation events
– Hydraulic measurements: track permeability/flow resistance of CO2-charged fluids
Post-experiment multi-scale characterisation
– Map mineral products and textures (e.g. SEM–EDS)
– Quantify fluid/solid compositions (ICP-OES) to obtain carbonate yields and critical minerals extracted
Image-based mechanics
– Apply digital image analysis to derive strain fields and identify zones of crystallisation-induced stress
– Extract fracture network metrics (aperture distributions, connectivity, tortuosity) and link to AE activity and hydraulic data
Reactive transport modelling (chemo-mechanical)
– Build a 1D/2D fracture–matrix model in GeoChemFoam (Maes & Menke, 2021); include silicate dissolution, carbonate nucleation/growth, and kinetic/transport limits

Year 1

• Imaging workflow: microCT acquisition, reconstruction, segmentation
• AE monitoring: sensor setup, calibration, event picking, basic source location
• Pilot experiments: short runs to finalise cell design, CO₂–fluid chemistry, scan cadence, AE thresholds

Year 2

• Advanced microCT: batch segmentation pipelines; quantitative fracture/aperture metrics
• Geochemical analyses: ICP-OES/IC/DIC; reaction budgets; uncertainty propagation
• Reactive transport modelling: dissolution/precipitation kinetics, transport limits
• Intro to chemo-mechanical coupling: permeability/porosity updates, simple crystallisation-pressure term

Year 3

• Experiments focusing on crystallisation-induced fracturing
• strain mapping: link strain, AE, and precipitation
• Hydraulic characterisation
• Reactive transport chemo-mechanical coupling (crystallisation pressure, poroelastic update)
• Sensitivity/uncertainty analysis

Year 3.5

• Integrate sets lab tests and modelling; upscaling to reservoir-relevant metrics; monitoring indicators (AE/hydraulics) for field use
• Thesis writing (paper-based or traditional)

Technical (Year 1 focus)
• Reactive transport & geochemical modelling, especially carbonate & basalt geochemistry: dissolution–precipitation kinetics
• Rock mechanics & fracture flow: permeability/porosity measurement, triaxial basics, fracture aperture metrics
Instrumentation & methods (Yr1–Yr2)
• MicroCT acquisition & reconstruction; image analysis: segmentation, digital volume correlation for 3D strain
• Acoustic emission (AE): sensor installation, calibration, event picking, source location
• Hydraulic testing under HP/HT: flow-through rigs, pressure control, leak testing
• Mineralogy & microanalysis: SEM–EDS, ICP-OES
Computing, data & statistics
• Python for research, HPC & performance
Safety & compliance (front-loaded in Yr1; refreshers yearly)
• HP/HT operations & pressure systems safety
• CO₂ handling and asphyxiation risk management
• X-ray safety (microCT) & electrical safety for sensors
• Chemical safety & waste; lab risk assessment (COSHH)
Professional & research skills
• Scientific writing
• Introduction into machine learning, carbon capture and storage on the field scale
• Poroelasticity & coupled THMC (theory + numerics)
• Project & time management: risk registers, contingency planning
• EDI