Caprock and overburden characterisation: avoiding leakage of CO2 from CCS sites

The aim of the project is to produce unusual data to characterise the caprock and overburden to CCS sites in the UK and beyond.
CCS is an essential piece of the technical jigsaw required to cut greenhouse gas emissions. There is much data available from the North Sea on the storage domain for CCS sites since porous and permeable reservoir rocks have been well-characterised by oil and gas companies (Alcalde et al., 2019). The caprock and overburden to CCS sites are less well characterized since oil and gas accumulations have a rock unit that is relatively impermeable (otherwise oil and gas would not be stored over geological timescales). However, injecting CO2 into the subsurface requires fluid to be injected at high pressure, thus potentially leading to hydraulic fracture and escape of the CO2, and CO2 is potentially a more reactive fluid than oil or gas. We have shown previously that CO2-water mixtures dissolve clay minerals and lead to orders of magnitude increases in permeability (Armitage et al., 2013). The environmental challenge is to develop an understanding of the fundamental controls on the geomechanical stability and the reactivity to CO2 of the caprock and overburden so that realistic parameters can be entered into forward models that simulate the effects of CO2 injected into water-filled porous units in the subsurface (Shariatipour et al., 2016). Caprocks and overburden are predominantly shale and mudstone. Previous CCS projects have been halted due to fears of rupture of the caprock due to stressed rocks resulting from injection of the CO2 (Vasco et al., 2019). This project will develop an overarching understanding of the failure conditions and mineralogical stability of shales and mudstones. The results of this work are planned for a REF impact case study for 2028.

1. The project will involve collecting samples of shale caprocks from a broad range of potential CCS sites. Samples will be selected to cover a range of porosity values and thermal maturity, mineralogy, organic enrichment, and sedimentary structures.
2. The student will characterise the shale core samples in terms of sedimentology via ultra-thin sections and light microscopy and SEM analysis, quantitative mineralogy via XRD and FTIR, inorganic geochemistry via XRF and organic geochemistry.
3. The samples will be analysed by a third party (commercial lab or possibly Leeds University) for pore size distribution using mercury injection porosimetry.
4. The samples will be tested for elastic properties using Vs-Vp acoustic measurements to characterise the responses to elastic deformation and to determine the conditions under which failure occurs.
5. Shale and mudstone samples will be tested for mineral solubility under the acid conditions that result from high pressure dissolution of CO2 in water.
6. The relationships between the downhole log properties, the results of sample characterisation and the results of the elastic and inelastic geomechanical tests will be integrated. Novel and highly publishable links between sedimentology, mineralogy, geochemistry, pore structure and geomechanical properties will be developed.
7. This research will be implemented by the student during internships with BP and Pale Blue Dot by building static models of the CCS sites. Models will incorporate the new geomechanical data and sedimentological, thermal-burial and diagenetic data to help build field-scale static models then used for fluid flow simulations of the effect of CO2 injection.

To apply for this opportunity please visit: https://www.liverpool.ac.uk/study/postgraduate-research/how-to-apply/ and click the ‘Apply online’ button.