Enhanced supercritical geothermal systems: toward stimulation design
(Helmholtz Centre for Environmental Research - UFZ)
Jueves 15 de marzo 2018 a las 12:15 h
Departamento de Ingeniería Civil y Ambienta, Modulo D2-Aula CIHS, Planta Baja
In the last decade, the possibility of extracting energy from supercritical geothermal systems (SCGS) has drawn increasing attention from the scientific community and is strongly emerging as a new technology. Albeit their definition is not yet agreed upon universally (sometimes called superhot geothermal systems), supercritical geothermal systems are geothermal systems in which the circulating fluid is close to or above the supercritical conditions (374°C and 22.1 MPa for pure water). Although power production from supercritical fluids could be enhanced by up to one order of magnitude compared to more traditional geothermal systems, drilling and operating under such high temperature conditions pose great technological challenges, several of which are yet to be overcome. Current research projects are located in active volcanic areas such as Kakkonda in Japan, Taupo Volcanic Area in New Zealand, Larderello in Italy, Krafla and Reykjanes in Iceland, The Geysers and Salton Sea in USA and Los Humeros in Mexico. Research activity at Los Humeros site is inserted in the GEMex international research project, a cooperation project between European Union and Mexico. A geothermal power plant is currently operated in Los Humeros Volcanic Complex, in which temperatures exceeding 400°C have been registered in wells at 2.5 km depths. Such depth is, at the time speaking, the deepest wellbores have been drilled. One of the goals of the GEMex project is to evaluate the possibility of extending the current field and reach the roots of the geothermal system, possibly at 4 or 5km depth. At such depth, permeability of the host rock might not be high enough to allow for hydrothermal fluid circulation: the host rock is most probably beyond the brittle-ductile transition conditions. In this case, stimulation techniques to create a network of more permeable fractures are necessary: this leads to a new kind of technology named Enhanced Supercritical Geothermal Systems (ESCGS). This seminar aims at presenting the latest research challenges and results relative to mechanical stimulation in supercritical geothermal reservoirs, with rocks beyond the brittle ductile transition. To tackle said challenges, we have developed models of fracture propagation (damage, phase-field and lower interface elements) and models for the rheology of rocks that are extended beyond the brittle-ductile transition conditions. The seminar is divided into three parts: in part I, a non-local plastic damage model is presented and validated against a set of fracture mechanics experiments on granite; in part II, three different approaches to simulate hydraulic fracture propagation have been compared and their performance
assessed against available analytical solutions; in part III, a rheological model for brittle-ductile transition of rocks has been developed and validated against observations. Finally, the talk is concluded with a brief discussion on future research perspectives and outlooks.