Modelling the thermal, hydraulic and mechanical controlling processes on the stability of shallow mine water heat systems

Abstract

The drive for net zero requires alternative, renewable heat sources and storage solutions to transition away from traditional gas heating. Decarbonisation of the heating sector is more complex than for electricity and heating accounts for around 50% of energy use in the UK. Therefore, a number of different options are required to fulfil the UK government’s Net Zero Strategy.

The use of abandoned mines as a heat source and store has been receiving increased attention. It is a favourable option due to the proximity of mines to population centres and their higher temperatures and permeabilities relative to standard shallow aquifer geothermal heat sources. However, there are several risk factors in the development of mine water heat schemes, one of which is the potential for stability issues due to injecting and extracting heat from shallow abandoned coal workings.

The hydraulic, thermal and geomechanical processes governing heat storage and extraction are complex. Understanding these processes is critical to safe heat extraction and injection into mine water systems which have undergone anthropogenically induced changes such as dewatering. One key knowledge gap in the development of mine water heating and cooling systems is what impact they could have on existing mine infrastructure in general and on the stability of the whole mine system in particular. In order to address this gap, this thesis outlines the development of a fully coupled thermal-hydraulic-mechanical model representing shallow abandoned flooded coal mine workings to gain an understanding of the key controlling processes in mine water heating and cooling systems.

The first stage of model development was a fully saturated 2D model simulating mine water rebound in Midlothian, Scotland. The modelled surface uplift to water level rise ratio of 1.4mm/m is of the same order of magnitude as that observed through InSAR data in the coalfield due to mine water rebound. This shows that the model created has suitable fidelity for identification of the key governing processes.

To improve the applicability of the model, thermal processes were added to further understand the key underlying controlling mechanisms. This second stage model focussed on one layer of room and pillar workings. A methodology to determine mechanical stability was also developed to understand the relative importance of different parameters and scenarios on the model results. It was found that the cyclical injection and extraction of heat does have an impact on both the modelled displacements and mechanical stability of the system. Due to the assumptions in the model and the fact it is an initial attempt to construct a THM model of these systems, the absolute values are of less importance than the relative impacts, i.e. the model is a useful research tool for process comparison.

In a mine water heat system, the components that can be controlled operationally, i.e. the injection temperature and the water level changes, have more of an impact than the underlying geological conditions. This is highly significant because it means that the future location of mine water heating schemes should take into consideration the likely scale of water level changes. Equally significant, the risk of impact reduces with temperature, so shallow systems with a low temperature change are likely to be less risky than those with higher temperature and pressure changes. The results suggest that mine water heat scheme regulations could include threshold values for temperature and water level changes which would require an additional stability assessment of the system. It is important to characterise the rock properties at a specific site, particularly whether the surrounding rocks are hard or soft, to understand how the stresses are likely to transfer and whether they will build up around the workings. Consideration should also be given the injection of colder water as a way to manage the build-up of thermal stresses over time. Modelling shows that thermally balanced systems are preferable to exclusive heat injection in managing mechanical stability over time.

In conclusion, this research has successfully developed a methodology for assessing the controlling processes in mine water heat injection and extraction. In so doing, it has provided initial input into one of the key knowledge gaps on the geomechanical stability of these systems, with the ultimate aim of contributing to a better informed regulatory framework around shallow mine water heat schemes in both the UK and beyond.