2024-L14 Pore fabric controls on rock strength and fracture growth

Project highlights

Develop expertise in finite element modelling
Contribute to our understanding of fracture behaviour in enhanced geothermal reservoirs and sites for mineral storage of CO2
Gain experience in geomechanics laboratory work and XCT scans

Overview

Rock fractures are known to decrease rock strength and increase rock permeability, with significant impact on both at all scales. Fracture formation and properties are therefore critical in a number of resource industries, including geothermal and carbon capture & storage (CCS). Volcanic rocks are a common target in these industries and yet the fracture behaviour of these rocks has been relatively understudied compared to sedimentary rocks. This is particularly true of rocks that contain millimetre- to centimetre- scale bubbles, known as vesicles or pores, as they are extremely challenging to study in laboratory experiments. Numerical computer modelling offers an alternative to relying solely on laboratory experiments and will be used to make advances in our understanding.

It is well established that spherical pores in rocks reduce strength and increase the potential for rock failure during applied loading. Estimates of these reduced rock strength values feature prominently in upscaled calculations of bulk rock strength, from fluid reservoir models to volcano stability models. Recent work has shown that rocks containing anisotropic pore fabrics that are shaped as aligned flattened discs show a large departure from predicted spherical-pore rock strength. The degree of departure from predicted is dependent on the direction in which the rock is compressed. Fracture behaviour, distribution, and resulting permeability can therefore also vary with direction.

This project will use numerical modelling with the commercial software COMSOL to expand our understanding of the effects of anisotropic pore fabrics in controlling rock fracture. It will quantitatively relate pore fabrics to anisotropic rock strength and fracture behaviour by running numerical simulations at the pore-scale, using both idealised pore fabrics and natural pore fabrics from X-ray computer tomography scans of rocks captured between stepped uniaxial compression experiments in a geomechanics laboratory.

Figure 1: Finite Element mesh rendered from XCT scan of pore space in vesicular basalt.

Host

University of Leicester

Theme

Climate and Environmental Sustainability
Dynamic Earth

Supervisors

Project investigator

Prof Simon Gill, University of Leicester, spg3@leicester.ac.uk

Co-investigators

Dr Catherine Greenfield, University of Leicester

How to apply

Each host has a slightly different application process.
Find out how to apply for this studentship.
All applications must include the CENTA application form.
Choose your application route

Methodology

The successful student will perform stepped uniaxial compressive tests on basalt rock cores and produce x-ray computer tomographic scans of the cores at each compressive step. The scan data will be processed and analysed in image processing software to extract a visualisation of the pore space and location of brittle damage growth. The pore visualisations will then be used to create finite element meshes suitable for import into COMSOL. The COMSOL software will be programmed to model the predicted damage growth and the results compared with the experimental compressive test data to validate the COMSOL code. Once validated, the code will be run on a suite of pore fabrics of known statistical distributions to gain insight into the role of pore fabric in rock fracture behaviour.

Training and skills

Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.

You will become proficient in the numerical modelling of elastic and brittle rock deformation using the state-of-the-art commercial finite element software COMSOL. The coding and statistical analysis techniques that are used are highly desirable transferable skills for the commercial scientific industries and for academia. You will gain skills in producing XCT scans and in performing laboratory geomechanics tests such as uniaxial compression and acoustic emissions monitoring.

Further details

Further details on how to contact the supervisor for this project and how to apply for this project can be found here:

For any enquiries related to this project please contact Catherine Greenfield ([email protected]).

To apply to this project:

You must include a CENTA studentship application form, downloadable from: CENTA Studentship Application Form 2024.
You must include a CV with the names of at least two referees (preferably three) who can comment on your academic abilities.
Please submit your application and complete the host institution application process via: https://le.ac.uk/study/research-degrees/funded-opportunities/centa-phd-studentships. Please scroll to the bottom of the page and click on the “Apply for NERC CENTA Studentship” button. Your CV can uploaded to the Experience section of the online form, the CENTA application form 2024 can be uploaded to the Personal Statement section of the online form. Please quote CENTA 2024-L14-CENTA2-SGGE7-GILL when completing the application form.

Applications must be submitted by 23:59 GMT on Wednesday 10th January 2024.

Possible timeline

Year 1

Gain training and experience with COMSOL. Undertake preliminary XCT scans of larger numbers of cores and image process to visualise the pore space in each core. Use statistical techniques to describe the pore fabric and use this data to select a suitable sub-set of cores for compression tests. Attend a national conference.

Year 2

Develop the current 2D COMSOL code into a 3D version. Run stepped uniaxial compression tests on the selected cores and take XCT scans at each step. Image process each core to produce a mesh and import into COMSOL. Model the predicted damage and compare to the physical experiment data. Presentation of results at large national and international conference.

Year 3

Create synthetic meshes by varying the statistical descriptions of pore fabrics and investigate how these changes affect the damage growth. Production of a model for damage growth in the presence of anisotropic pore fabrics. Publication of papers. Presentation at large national and international conference.