| Abstract |
The United Downs geothermal project is owned and operated by Geothermal Engineering Ltd (GEL) and is the first geothermal electricity project in the UK. The site was selected in 2010 for its geology (strongly faulted, radiogenic granite), surface infrastructure, grid connection and potential community acceptance. The deep drilling programme started in 2018, with the production well (UD-1) drilled to 5,275m MD and deviating from 3,390m to a final inclination of 33.5o, representing the deepest onshore well in the UK. The injection well (UD-2) was then drilled to 2,393m MD, deviating from 1,020m to a final inclination of 40o. The drilling of a deep production well and shallow injection well into a natural fault zone is a novel concept developed by GEL, based on some of the results from research at the Hot Dry Rocks project during the 1980s at Rosemanowes in Cornwall. The production well has successfully encountered temperatures of 180oC, high lithium concentrations and significant permeability horizons within the fault zone. From September 2020 to July 2021, both wells underwent a significant period of testing and stimulation to provide an understanding of and improve the deep reservoir conditions. Hydraulic stimulation was undertaken on UD-1 over a number of months to help improve permeability and reservoir volume prior to the specification and installation of a power plant. Cyclic injection of more than 6,200m3 of fresh water was undertaken, with gradual changes in fluid volume and injection rates made on a daily basis. During testing, pressure, flow rate and induced microseismicity were closely monitored by a panel of experienced geothermal specialists to ensure any changes could be assessed and adjustments made in real-time, with microseismic events responded to quickly and appropriately. This iterative approach to testing was developed by GEL to ensure technical operational success as well as continued community acceptance. Analysis of pressure changes through time in UD-1 show that stimulation at low pressures successfully improved the hydraulic conditions of the reservoir. Correlation of this hydraulic data with seismic events also shows the gradual expansion of the reservoir above and below the openhole section throughout stimulation, as natural fractures are opened and the zone around the well is depressurised. From this data, it is also estimated that the area of the reservoir affected by stimulation to date is more than 50,900,000m3. |