The role of fractures in flow partitioning during minewater geothermal energy extraction

<p>The UKGEOS Glasgow research field site comprises a network of 12 boreholes into flooded coal mines, and is designed to observe how warm water moves around the abandoned mine workings over time <em>(Monaghan et al., 2018)</em>. Minewater geothermal projects involve the redevelopment of abandoned mining areas into large volume, low temperature resources and use heat pumps to drive heating for homes, industry or agriculture. This technique has proven potential as a renewable, decarbonised heat source providing reliable heating, cooling and heat storage with stable pricing to former mining areas.</p><p>Flow through minewater systems is partitioned between flow through the mine voids, through fractured media, and through porous media. This heterogeneity in flow is crucial to the development of models to predict the efficacy of minewater geothermal systems, as water flowing through the fractured material should absorb more heat than that flowing directly through the mine voids. This heat exchange then goes on to control the rate at which heat can be sustainably extracted from the minewater system.</p><p>The majority of fluid flow has generally been assumed to be through the mine voids. However, the proportion of fluid flow through the porous wallrocks is very sensitive to the fracture populations that they contain, due to the shallow nature of these mine workings leaving them under low stress.  Geothermal tests at the Gaspé mines in Québec demonstrate this clearly, with high wallrock conductivities (10<sup>-6</sup>-10<sup>-4</sup> m.s<sup>-1</sup>) attributed to mine-blasting <em>(Raymond & Therrien, 2008)</em>. Coal mining in the Glasgow area was predominantly carried out using the Pillar & Stoop or Longwall methods, which lead to very different damage states in the wallrocks, and so the effect of these fracture populations is expected to have a large effect on flow partitioning.</p><p>Here, relationships between in-situ stress, fracture population and permeability were determined from well-core samples of the Glasgow Main Coal and underlying mudstone and sandstone strata, in order to characterise how flow may be partitioned within different regions of these mine-workings.</p><p>Stress-dependent permeability and storativity were measured using the osciallting pore-pressure method, and elastic tensors were determined using an array of ultrasonic transducers. Axial fractures were then generated within these samples under low triaxial stress states, and the change in permeability with induced fractures then measured at a single stress state, with the newly developed fracture population characterised through the changes in the elastic tensor.</p><p><em>Monaghan, A. A., Starcher, V., Dochartaigh, B. É. Ó., Shorter, K., & Burkin, J. (2018)</em>. <strong>UK Geoenergy Observatories : Glasgow Geothermal Energy Research Field Site - Science infrastructure.</strong> http://nora.nerc.ac.uk/id/eprint/521444/%0A</p><p><em>Raymond, J., & Therrien, R. (2008)</em>. L<strong>ow-temperature geothermal potential of the flooded Gaspé Mines, Québec, Canada.</strong> Geothermics. https://doi.org/10.1016/j.geothermics.2007.10.001</p>

Analysis and Prediction of Gas Recovery from Abandoned Underground Coal Mines in China

Mine closures are likely to become especially widespread in eastern China. However, because of relatively high residual coal-bed methane content, the abandoned coal mine methane (ACMM) reserves of China are huge, and from a greenhouse gas–control perspective it is preferable that they be developed and utilized rather than allowed to vent to the atmosphere. The exploitation and development of ACMM in China is still in its infancy, with theory and practice undergoing rapid development. Four factors are particularly influential in the design of ACMM recovery strategies. The first factor is what may be termed the “enrichment space,” which reflects the final state of the strata after completion of longwall extraction and subsequent strata settlement and is here defined as the region between the outer limit of stress relief and the limit of the extracted panels. Quantitative analysis of the components of gas mixtures recovered from the enrichment space can be tracked using stable carbon isotope techniques. The second factor is the permeability field surrounding the abandoned mine voids. The thick mudstones that commonly overlie the coal seams serve to confine the water and gas within the enrichment space and old mine voids. The geometry of these confining layers can be confirmed by seismic reflection or other geophysical methods, which can reveal the extent of the zone affected by fracture development. On this basis, models of methane movement in abandoned mines can be constrained, allowing valuable predictions of availability of ACMM resources under different mining and post-closure drainage conditions.