Geothermal exploration and reservoir modelling of the United Downs deep geothermal project, Cornwall (UK)

Two deep geothermal wells, one of which is the deepest onshore UK well, have been successfully drilled to intersect the Porthtowan Fault zone in the early Permian high heat flow Carnmenellis Granite, part of the major post-Variscan Cornubian batholith which underlies SW England. The drilling target was prognosed at 4 to 5 km depth from existing surface data and limited geophysics. It is a late Variscan complex strike-slip fault zone with an episodic reactivation history, possibly as late as the Tertiary, and is associated with fluid flow on a geological timescale including present day geothermal springs in mines. Logging and initial hydraulic testing have been carried out to help define the reservoir volume, where temperatures at 5 km depth are around 180 °C. Initial reservoir modelling based on the fracture data and the induced micro-seismicity during drilling and injection testing afterwards indicates that open fractures aligned close to the maximum horizontal stress at >4 km depth are in critical extension or shear and a subset of them are hydraulically active. These early results are encouraging for this project, which aims to produce power and heat, as well as for other similar geothermal ventures in the region.