How underground lithologies respond to thermo-mechanical coupling during energy extraction/storage.

On our quest to decarbonise our energy resources, underground heat energy storage is a key player. However, the impact of frequent cyclic thermo-mechanical (TM) stress changes over prolonged periods remains poorly understood and may threatened the longevity of the systems. This project aims to fill this gap by performing laboratory and numerical experiments under relevant cyclic TM loading conditions to investigate the stability of targeted lithologies in such systems. To address the energy transition challenges, new subsurface solutions focus either on new resources exploitation (geothermal) or storage (radioactive waste, heat and/or gas -underground gas storage, compressed-air energy storage, H, CO2). All these applications have in common to induce new, shallow, periodic, local thermo-mechanical stress changes.
The scope of this PhD project is to use different-scale observations to model and predict the stability of targeted lithologies in underground complex systems when those are subjected to cyclic TM stress changes over prolonged periods. This research work seeks to understand how grain-scale deformation can contribute to the global response of the underground systems and how this response can be controlled to reduce any accompanied induced hazards. An innovative methodology, combining laboratory and numerical experiments, will be applied to extend the understanding of the thermo-sensitive brittle deformation processes in porous rocks. The data will provide information to support field-scale operational conditions involving periodic TM stress changes as well as shed light on potential cascading shallow geohazards.
Objectives:
O1: Examine the relationship between microstructural deformation and TM stresses.
O2: Collect, analyse and model TM data together.
O3: Transfer and improve existing DEM code from UDEC to open source and undertake grain size sensitivity analysis.
O4: Provide relevant data to inform on risks associated to TM stresses in underground geological storage conditions.
Laboratory experiments will be performed at different scales (from grain to sample scales). Core samples will be x-ray scanned at HWU to understand their internal 3D microstructure and assess their transport properties (porosity and permeability) at pre- and post-TM experiments (O1). TM experiments will be performed at BGS to simulate elevated environmental conditions. Several deformation scenarios will be investigated to cover different industrial field operations. This lab-scale global sensing data will be combined to petrographical analysis (O2) to correlate the spatiotemporal distribution of the lab-induced damage within the tested materials with the microstructural evolution. Moreover, similar grain-scale experiments with syn-deformation monitoring (x-rays) are possibly planned to unravel the micro-scale processes. Numerical modelling and machine learning techniques are nowadays used more frequently to predict the subsurfacesystem behavior. TM coupling calibrated Voronoi Grain Based Modelling (GBM) can capture micro-cracking as a mechanism of progressive damage, reproducing the stress-strain behavior of laboratory tests. Developing such models will help to understand how TM brittle damage develops across the scales, from grain-size cracking to rock mass fracturing, and its time dependency. This PhD project will build on the DEM developed by Woodman et al. (2021) to undertake notably a grain size and distribution sensitivity analyses in thermo-mechanical simulations (O3) to further assess the up scaling of laboratory data to field scale (O4).