Productivity Increase Of Egs Plants By Means Of Colder Fluid Re-Injection And Orc Use

Abstract

All EGS plants operating in Europe are mainly located in the Upper Rhine Graben. The geothermal plants are discharging a brine on surface at temperature higher than 150°C, take the geothermal energy via heat exchangers for producing heat (Rittershoffen) or electricity (Soultz-sous-Forêts, Insheim). The geothermal water is usually reinjected at temperature lower than 80°C in the granitic reservoir. We here present an ambitious research and development project financed in the framework of an H2020 grant aiming to develop and investigate EGS in different geological conditions at European territory scale (Trullenque et al. 2018). The MEET (Multidisciplinary and multi-context demonstration of EGS exploration and Exploitation Techniques and potentials) project will notably focus on the use of hot fluid from mature oil fields and increase the productivity of existing geothermal plants. The Soultz sous Forêts demonstration site was chosen in order to test a new generation of small scale ORC units, which are particularly suitable for the conversion of heat sources of very low temperature. Since these ORCs have hardly been used in the field geothermal, our aim is to test their applicability even under extreme conditions on a site typically considered as a reference in terms of successful EGS plant. Our demonstration consists in re-injecting the geothermal water at 50°C in the granitic fractured reservoir and using the 20°C difference temperature for electricity production. An ORC with a power output of 40 kW, incorporating a high performance kinetic turbine technology, will be used for this demonstration site. These temperature ranges are also the ones typically encountered in mature oil fields. Several challenges will be investigated since pumping of cold water at great depth in crystalline hot rocks was never tested during long-term exploitation: • Thermal contraction could possibly damage the well casing and thermo-mechanical effects in the surrounding granite might induce some induced seismicity. A permanent multi-disciplinary monitoring including fibre optic technology deployed in a neighbouring deep observation well will be performed in order to adapt if necessary the re-injection strategy (temperature, flow rate). In parallel to the reinjection, surface hydraulic monitoring will be carried out in order to evaluate the thermal impact on the production well. • Re-heating of the brine during reinjection could possibly induce some mineral precipitation (scaling) that could plug the fracture network which in turn will reduce well injectivity. The same scaling, could concentrate radioactive elements like Ra of Pb from the surrounding granitic rocks. However, the use of inhibitors might be considered in order to avoid the scaling phenomena. • Given the very high salinity of the brine, ORC’s heat exchanger could possibly be corroded reducing drastically its efficiency. On site corrosion tests with material coupons will be performed in geothermal fluid (brine) at low and high temperature. In this project, the material selection of the heat exchanger will mainly be based on knowledge from previous work and coupon exposure test in in-situ geothermal fluids performed in a pressure vessel at Reykjanes geothermal power plant. This test will include stainless steel and nickel materials. After the examination and evaluation, the most suitable material selection will be chosen for long term testing of an ORC unit for 6 months.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018