With the ambition to increase the share of clean energy in United Kingdom’s energy portfolio, Cornwall has been the focus of deep geothermal resource studies starting from 1980s with the Rosemanowes Hot Dry Rock project. Today our associated partner the United Downs Deep Geothermal Power Project (UDDGP), the first industrial deep geothermal power project of UK, has proven its importance following its promising results obtained in the first two phases of the project. Successful completion of drilling of United Downs wells (i.e. two deep wells of the UDDGP project finished in April 2019 (UD-1, 5058 m total vertical depth (TVD), 5275 m measured depth (MD)) and in June 2019 (UD-2, 2214 m TVD, 2393 m MD) both intersecting the targeted Porthtowan Fault Zone, as planned.) have triggered geothermal interest in the region.
At TUDa, our aim is to provide a comprehensive petrophysical characterization of the Cornish granites that can be used for more accurate parametrization in numerical modelling studies within the MEET EU project. To do that, representative samples from the Cornubian Batholith were taken from the outcropping Land’s End, St. Austell and Carnmenellis plutons in June 2019 (Figure 1, left). 47 granitic samples (Figure 1, middle) with a total weight of 1 ton, were collected from fractured and/or hydrothermally altered areas of 23 main outcrops representative of the granitic varieties as well as the fracture and fault zones within the United Downs wells.
Figure 1: Left: Littlejohn’s pit (operated by Imerys’) an active quarry located on St. Austell pluton (Cornwall, UK);
Middle: Granite at the beginning of orthose alteration, albites are altered, sample with tourmaline vein;
Right: 64 mm diameter core prepared for laboratory analysis.
338 cores with different diameters and lengths have been drilled in the HydroThermikum Research and Teaching Laboratory of Technical University Darmstadt as preparation for the investigation of their properties (Figure 1, right). 42 thin sections have been prepared for a detailed petrographic analysis including the degree of weathering or (hydrothermal) alteration and fluid inclusions.
As of October 2020, measurement of the petrophysical (grain density, permeability, bulk density and porosity, compressive and shear velocity, thermal conductivity, thermal diffusivity, heat capacity and radiogenic heat production) and rock mechanical properties (uniaxial compressive strength, Poisson’s ratio, Young’s modulus, bulk modulus and compressibility as well as tensile strength, shear strength, cohesion, coefficient of friction and shear modulus) on outcrop analogue samples were completed in the laboratories of the Technische Universität Darmstadt. Present work extends the previously published database (Deliverable D5.5) by the contribution of the cuttings and sidewall cores gathered from the depth of 4 to 5 km for the first time. This database will also ease the future geothermal investors’ decision-making process and will enable them to start the different phases of projects with less uncertainty due to the collected exhaustive data set.
Xenia Kirschstein one of the student assistants that work on Cornwall outcrop analogue samples (shown on the picture), which have been drilled for lab specimens in Technische Universität Darmstadt.
Our above-mentioned research on the Cornubian Batholith, within MEET EU funded project, has been the focus of 1 PhD, 2 MSc and 1 BSc student theses so far and has enabled 4 student assistants to earn extensive lab skills.