Detailed report on reservoir performance in terms of sustainability Application to the geothermal well GPK-4 , Soultz-sous-Forêts Deliverable D 4 . 3 WP 4 : Demonstration of combined hydraulic-thermal-chemical treatments in sandstones , carbonatic rocks and granites

This report is done in the framework of EU project Destress related to sustainability of geothermal exploitation. It is divided in three distinct parts. Part A done by UoS provides an overview of the experimental pre-stimulation tests done on an analog two micas granite to assess the permeability evolution of the Soultz GPK4 well during long term acid treatments with or without thermal treatments. A complementary modelling of acid stimulation has been performed using the KIRMAT code that uses single and double porosity models and account for a wide range of mineralogy. The model evaluates changes in porosity and permeability in the vicinity of the geothermal wells (up to 6m) based on changes in the amount of primary and secondary minerals and as a function of time. Part B done by ESG provides an update of the monitoring and investigations made onsite and especially on the injection well GPK-4 from the Soultz-sous-Forêts power plant (France), confirming MS28. The well hydraulic monitoring is presented. The injectivity index varies between 0.54 kg/s/bar and 0.65 kg/s/bar during the reporting period, from early 2017 to February 2020. In parallel to the hydraulic performance during exploitation, a chemical monitoring of the geothermal brine has been carried out since January 2018 by collecting geochemical data from the production well GPK-2 and the second reinjection well GPK-3. Several geochemical analyses have been done and confirm the high TDS and the gas content of the native brine. Part C done by GFZ and ESG, describes the soft chemical stimulation of the Soultz-sous-Forêts injection well GPK-4 from the concept to the operation (executed in December 2019), with observed environmental and hydraulic consequences and results. DESTRESS Demonstration of soft stimulation treatments of geothermal reservoirs 2 28.02.2020 Authors Part A from UoS: Patrick Baud, Jamie Farquharson, Alexandra Kushnir, Yann Lucas, Viet V. Ngo, Alain Clément, Bertrand Fritz, Gerhard Schäfer, Jean Schmittbuhl Part B & C from ESG: Régis Hehn, Nicolas Cuenot, Justine Mouchot, Vincent Maurer, Albert Genter, Olivier Seibel, Abigaelle Peterschmitt, Benoit Imbs Part C from GFZ: Thomas Reinsch, Ernst Huenges Lead Beneficiary UoS Type R report, document etc. OTHER software, technical diagram etc. DEM demonstrator, pilot etc. E ethics DEC website, patent filing etc. Status Draft WP manager accepted Project coordinator accepted Dissemination level PU Public CO Confidential: only for members of the consortium Contributors 1-GFZ 5-GES 9-GTL 13-SNU 2-ENB 6-TNO 10-UoS 14-KIC 3-ESG 7-ETH 11-TUD 15-ECW 4-UoG 8-GTN 12-NEX 16-WES Creation date 1.10.2019 Last change 25.02.2020 Version final Due date 29.02.2020 Submission date 28.02.2020 DESTRESS Demonstration of soft stimulation treatments of geothermal reservoirs Context and objectives DESTRESS is aimed at creating EGS (Enhanced Geothermal Systems) reservoirs with sufficient permeability, fracture orientation and spacing for economic use of underground heat. The concepts are based on experience in previous projects, on scientific progress and developments in other fields, mainly the oil & gas sector. Recently developed stimulation methods are adapted to geothermal needs, applied to new geothermal sites and prepared for the market uptake. The DESTRESS concept takes into account the common and specific issues of different sites, representative for large parts of Europe, and will provide a generally applicable workflow for productivity enhancement measures. The project mainly focuses on stimulation treatments with minimized environmental hazard (“soft stimulation”), to enhance the reservoir in several geological settings covering granites, sandstones, and other rock types. This deliverable was written in the framework of Task 4.3 (“Realization of chemical injection tests during long-term circulation (GPK-2/GPK-4 Soultz-sous-Forêts) & verification of injectivity index and permeability enhancement with low seismic nuisance (GRT-1 Rittershoffen)) of WP4 (“Demonstration of combined hydraulic-thermal-chemical treatments in sandstones, carbonate rocks and granites”). It compiles all the work done for the Soultz-sous-Forêts site with a focus on the chemical stimulation executed in December 2019. DESTRESS Demonstration of soft stimulation treatments of geothermal reservoirs A. Soultz-sous-Forêts: pre-stimulation laboratory tests and modeling 1 Laboratory-scale acid stimulation of Soultz-sous-Forêts granite 1.1 Background and sample selection The GPK-4 well was completed in 2004 and has been used variously for both production and injection. It reaches a vertical depth of 4982 m, but strong deviation means that the overall length of the well is ~5260 m. The open-hole section of the well is estimated to be around 200 °C, and intersects a finegrained two mica granite. The material recovered from the drilling operation is insufficient for the purposes of our experiments; instead, blocks of granite were collected from a site near Forbach in Germany, where the surface expression of the granite basement is accessible. The variably-altered granites contain muscovite and biotite—determined by X-ray powder diffraction (XRD) and energydispersive X-ray microanalysis (EDX), and appear to be an ideal analogue for the deep reservoir material at Soultz-sous-Forêts. Many of the granites were found to host macroscopic fractures. Although previous studies have identified fractures in the Soultz basement to host illite and calcite, we were unable to confirm the presence of either mineral through XRD or EDX analyses. Samples were divided into three suites for experimentation (shown in Figure 1): a fine-grained leucocratic 2-mica granite (G2M-A): a slightly hydrothermally altered granite containing additional secondary minerals such as apatite (G2M-B): and an unaltered granite identical to G2M-A, but containing abundant macroscopic fractures (G2M-F). DESTRESS Demonstration of soft stimulation treatments of geothermal reservoirs Figure 1: Examples of three classes of granite collected at Schwarzenbach. [above] sample photographs. [below] Scanning Electron Microscope images. Mineral phases identified using XRD and EDX: Qz = quartz; or = orthoclase; ab = albite; bt = biotite; ms = muscovite; ap = apatite. 1.2 Equipment and experiment design 1.2.1 Acid permeameter In order to explore the influence of aggressive permeants treatments on the two mica granite, over a range of relevant pressures and temperatures, a new permeameter has been custom-designed and constructed at Université de Strasbourg. The apparatus is comprised of a pressure vessel connected to a two-cylinder fluid pump. Confining pressure is applied by silicone thermofluid using a two-stage manual hand pump, up to a maximum of 70 MPa. The pressure vessel is wrapped in high temperature AMOXTM fabric tapes, and the whole ensemble is enclosed in a bespoke clamshell jacket. This jacket is composed of layers of Tempmat (mechanically bonded glass fiber matting), fiberglass cloth, and silicone-impregnated fiberglass cloth, which insulates the pressure vessel effectively due to the low thermal conductivity of the constituent materials. The AMOXTM tapes are connected to a programmable PID temperature controller. The controller reads temperature from a thermocouple embedded beneath the insulating jacket, and the power output to the heat tapes is adjusted accordingly in order to heat the pressure vessel to the desired (user-set) temperature. Within the pressure vessel, thermofluid surrounds a flanged seal, custom-moulded from temperatureand acidresistant rubber. The sample and two spacers are inserted into the seal. Another k-type thermocouple is in contact with the sample, in this case a mineral insulated k-type probe coated in Halar®, an acidresistant copolymer of ethylene and chlorotrifluoroethylene. Temperatures of the sample and vessel exterior are recorded using a National Instruments voltage input module integrated into a custombuilt data acquisition hub. The acquisition hub communicates with a programme written in LabVIEW, which is also used to send commands and receive flowrate, pore pressure, and fluid volume data from the Quizix pump. Figure 2 presents the details of our new permeameter designed and built for this project. DESTRESS Demonstration of soft stimulation treatments of geothermal reservoirs Figure 2: Schematic of acid permeameter. [a] Technical diagram of permeameter circuit. [b] Sketch of primary components. [c] Diagram of pressure vessel interior. (1): pressure vessel; (2): confining pressure inlet/outlet; (3) pore fluid outlet; (4): vent valve; (5): pore fluid inlet/outlet ports; (6) Quizix pore fluid pump; (7): permanent reservoir; (8) pressure gauge; (9): analogue pressure transducer; (10): confining pressure outlet valve; (11): confining pressure pump/oil reservoir; (12): effluent reservoir. (a): upstream endcap; (b): downstream endcap; (c): silicone gasket; (d): confining pressure inlet/outlet; (e) silicone gasket; (d) confining pressure inlet/outlet; (e) steel spacers; (f): sample; (g): annular sleeve. 1.2.2 Batch reaction tests Alongside the development of the acid permeameter, sample suites were immersed in acid solutions for varying periods of time. For the purposes of this study, two different concentrations of hydrochloric acid were prepared (0.2 N and 2.0 N HCl), by combining concentrated HCl with distilled and deionised water. Mass, porosity, and permeability of all samples were measured prior to immersion. Periodically, samples were removed and re-characterised in order to monitor the evolution of these physical properties. Additionally, some samples underwent thermal stressing in a furnace, whereby they were heated at 1 °C min-1 until a target temperature, left to dwell at that temperature for 2 hours, then cooled again at the same rate. A control suite of granites was set aside for mechanical testing (uniaxial compressive strength), against which to compare