Lab analysis of permeability enhancement by chemical treatment of fractured granite samples (Cornubian Batholith) in the context of the United Downs Deep Geothermal Power Project

Lab analysis of permeability enhancement by chemical treatment of fractured granite samples (Cornubian Batholith) in the context of the United Downs Deep Geothermal Power Project

Katja Schulz1, Kristian Bär1, Nils Recalde Lummer2, Ingo Sass1

Organisation(en): 1: Technische Universität Darmstadt, Deutschland; 2: Fangmann Energy Services GmbH & Co. KG

In the framework of the EU-H2020 project MEET (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials, grant agreement No. 792037) and the ERDF-project UDDGP (United Downs Deep Geothermal Power), we conducted a lab analysis of permeability enhancement by chemical treatment of fractured granite samples.

In Redruth, Cornwall a hydrothermal doublet-system with a depth of 5 058 m TVD (production well, bottom temperature approximately 190 °C) and 2 214 m TVD (injection well) was completed in 2019. Geological target of the project is the NNW-SSE striking, 200 to 500 m wide-, more than 15 km long-Porthtowan strike-slip fault zone (PTF), which transects the Carnmenellis granite and links the two wells vertically. The Carnmenellis granite is one of the onshore plutons of the Cornubian Batholith, which is characterised by a strong geothermal anomaly.

The analysed samples are outcrop analogue samples taken from six different locations of the Cornubian Batholith. They have a high geochemical and mineralogical conformity with the Carnmenellis Granite and include mineralised veins, which allow analogies with hydrothermally altered zones around the faults damage zones of the PTF encountered in the production well.

The enhancement of permeability of the rock matrix and fracture systems by acidification was analysed by applying a set of analytical methods. XRF, XRD, ICP-MS, thermoscanning, helium-pycnometry, helium-permeametry, water-permeametry and sonic velocity were used to characterise the samples petrologically and petrophysically before and after chemical treatment in autoclave-experiments and Core Flooding Tests at reservoir temperatures and pressures. In the chemical treatment experiments, the samples are placed in autoclaves together with acids at 150 °C for 24 hours. The acids used are 15 % HCl and ‘white acid’, consisting of 12 % HCl and 3 % HF, which resembles Regular Mud Acid. The spent acids have been analysed after the experiments, using AAS, IC and Photometry to allow a quantification of the effectivity of the acids. In the Core Flooding Tests, which have been conducted by Fangmann Energy Services, the chemical blends SSB-007 and SFB-007 are circulated through the cores at a temperature of 150 °C and a confining pressure of 172 bar. During the flow rate-controlled test, the differential pressure is logged to calculate the permeability development over time. SSB-007 and SFB-007 have relatively similar characteristics as the fluids used in the autoclave-experiments.

The Core Flooding Tests on these samples resulted in an increase of the core permeability by a factor ranging from 4 to >20. The autoclave-experiments result in permeability variation by a factor between 0.1 and 40. Precipitates on and within the samples of the autoclave experiments, which decrease permeability were analysed using electron microscopy.

Potential target zones in the production well UD-1 have been identified by ultrasonic borehole log interpretation (UXPL), spectral gamma ray logs, fluid losses, gas intrusions and anomalies of the geothermal gradient. Drill cutting samples from these zones are analysed mineralogically to transfer the geochemical changes during autoclave or CFT experiments directly to the theoretical scenario of a chemical treatment of UD-1.

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