In order to establish sustainable heat loading (heat removal and storage) in abandoned flooded mine workings it is important to understand the geomechanical impact of the cyclical heat loading caused by fluid injection and extraction. This is particularly important where significantly more thermal loading is planned than naturally occurs. A simple calculation shows that the sustainable geothermal heat flux from abandoned coal mines can provide <1/10th of Scotland's annual domestic heating demand. Any heat removal greater than the natural heat flux will lead to heat mining unless heat storage options are also considered.
As a first step, a steady state, fully saturated, 2D coupled hydro-mechanical model of a generalised section of pillar-and-stall workings has been created. Mine water rebound was modelled by increasing the hydrostatic pressure sequentially, in line with monitored mine water level data from Midlothian, Scotland. The modelled uplift to water level rise ratio of 1.4 mm/m is of the same order of magnitude (1 mm/m) as that observed through InSAR data in the coalfield due to mine water rebound. The modelled magnitude of shear stress at the pillar corners, as a result of horizontal and vertical displacement, is shown to increase linearly with water level. Mine heat systems are expected to cause smaller changes in pressure than those modelled but the results provide initial implications on the potential geo-mechanical impacts of mine water heat schemes which abstract or inject water and heat into pillar-and-stall coal mine workings.
Scientific editing by Heather Stewart