Quantifying the role of fault-related shear stress in the permeability of limestone & dolostone

Rationale

The permeability of faulted and fractured limestones and dolostones is key factor in their potential development as a source of geothermal energy (heat or power) in the UK and globally. A popular model for fault related permeability suggests that a ‘critical’ shear stress improves fluid flow at the point of failure (Barton et al., 1995). However, rock deformation experiments on sandstone, granite and mudstone suggest that permeability decreases at the point of failure, but there are no published data for carbonate rocks (Rutter & Mecklenburgh, 2018). This project addresses a fundamental scientific question with a direct application to geothermal energy for the energy transition: What role does fault-related shear stress play in the permeability of limestone and dolostone?

Aims

In this project, the student will aim to deliver:

• Detailed multiscale maps of fault zones in selected limestone and dolostone formations;
• Laboratory experiments designed to explore the role of fault-related shear stress on permeability;
• Microstructural and microchemical analysis of textures in natural and laboratory fault zones;
• A quantitative understanding of the relationship between brittle deformation and the evolution of permeability.

Context

This project will benefit from the wider Geosolutions Leeds project portfolio, including ongoing research into geothermal energy, subsurface storage and critical minerals. Geosolutions Leeds is a core part of the University of Leeds Climate Plan, an ambitious £174M programme designed to achieve Net Zero on campus by 2030 and support decarbonisation of the city and wider region. The University of York Deep Geothermal project is targeting faulted limestones at depths of 4-5 km beneath their Campus East. The School of Earth & Environment (SEE) at Leeds is a major international centre for Earth, Environmental and Sustainability Science that has wide-ranging and positive impacts, and much of our work is cross-cutting, tackling complex global challenges. All SEE Research Institutes have an active seminar series and social events, which PGRs are encouraged to engage with during their time at Leeds.

Methods, Training

Training will be given in field-based structural geology, experimental rock physics, microstructural and microchemical analysis and numerical modelling. Field training will include detailed structural mapping, including the use of digital photogrammetry, scanline surveys and oriented sample collection (with permissions). Experiments will be conducted in the Geosolutions Leeds Geomechanics Laboratory using a state-of-the-art triaxial rock deformation apparatus capable of confining pressures to 250 MPa and temperatures to 200°C. The student will be trained in all aspects of laboratory sample preparation, machine calibration, rock property testing and experimental data analysis. Data collected will include stress-strain, strength, pore volume change, permeability and P- and S-wave velocity changes. Microstructural analyses will be conducted on selected pre- and post-test samples at the Leeds Electron Microscopy and Spectroscopy Centre (LEMAS, part of the Bragg Centre for Materials Research, link). SEM images will be collected in SE, BSE and EBSD modes to map changes in porosity, brittle microcracking and plastic deformation. Numerical modelling will be used to explore the consequences of a coupled process of brittle shear deformation and permeability evolution.

References

Barton, C.A., Zoback, M.D. and Moos, D., 1995. Fluid flow along potentially active faults in crystalline rock. Geology, 23(8), pp.683-686.

Rutter, E.H. and Mecklenburgh, J., 2018. Influence of normal and shear stress on the hydraulic transmissivity of thin cracks in a tight quartz sandstone, a granite, and a shale. Journal of Geophysical Research: Solid Earth, 123(2), pp.1262-1285.