A hydrothermal doublet-system in a fault-related granitic reservoir has been drilled in Redruth, Cornwall from November 2019 until June 2020. At 5 058 m depth (TVD, 5275 m MD) up to 190 °C have been reached in the dedicated production well UD-1. The dedicated injection well UD-2 is aligned vertically above UD-1 and reaches a depth of 2393 m MD. Target of the hydrothermal doublet-system is the Porthtowan Fault Zone (PTF), which transects the Carnmenellis granite, one of the onshore plutons of the Cornubian Batholith in Cornwall, SW England.
As part of the design process for a potential chemical stimulation of the open-hole sections of the hydrothermal doublet, lab-scale acidification experiments of outcrop analogue samples from the Cornubian Batholith, which include mineralised veins, have been performed. The experimental set-up comprised autoclave experiments on sample powder and plugs and core flooding tests on sample plugs to investigate to what degree the permeability of natural and artificial (saw-cut) fractures can be enhanced. A set of different analytical methods, such as XRF, XRD, AAS, ICP-MS, thermoscanning, helium-pycnometry, helium-permeametry, water-permeametry and sonic velocity, has been used to characterise the samples petrologically and petrophysically before and after the acidification experiments to to track all changes resulting from the acidification. Based on the comparison of the mineralogical composition of the OAS samples with the drill cuttings of UD-1, the results can be transferred to the hydrothermally altered zones around the faults and fractures of the PTF. Core Flooding Tests and Autoclave Experiments result in permeability enhancement factors of 4 to >20 and 0.1 to 40, respectively. The results are documented in three tables: petrophysical and petrological characterisation of outcrop analogue samples before and after acidification (table 1), chemical characterisation of spent acids after autoclave experiments (table 2), permeability tracking during Core Floodings Tests (table 3). The experimental set-up as well as the results are presented and discussed in Schulz et al. (2021).
Acknowledgments: The present work has been elaborated at the Institute of Applied Geosciences, Technical University of Darmstadt in the framework of the projects MEET (grant agreement No. 792037) and UDDGP. We would like to thank the supporting staff from the Department of Geo-thermal Science and Technology, namely G. Schubert, S. Schmidt, R. Seehaus, D. Scheuvens and R. Petschik from Goethe University of Frankfurt for the XRD measurements as well as our project partners related to MEET/UDDGP, to mention A. Genter, A. Turan, J. Reinecker, J. Gutmanis, R. Shail, B. Simons, C. Dalby.
