L. Rybach, GEOWATT AG Zurich (email@example.com)
The EU project ENGINE was a major step forward in moving EGS ahead. It assembled a large, knowledgeable group of wide specialization; the results will represent a milestone in EGS R&D. Yet it must be realized that only as an intermediate position has been established on a still long way to reach the ultimate EGS goal: development of a technology to produce electricity and/or heat from a basically ubiquitous resource, in a manner relatively independent of site conditions. There are still major knowledge gaps, unanswered questions, and unavailable solutions to work on. Here only a selection is listed.
· Although the minimum requirements for an economically viable EGS reservoir are set since quite some time (size, heat exchange surface, flow impedance, thermal and stress-field properties), their realization in a tailor-made manner to comply with differing site conditions is not yet demonstrated.
· The creation, characterization and operation of an EGS reservoir at depth needs techniques of remote sensing and remote control; here we are still far from having “best practices”.
· Many questions of rock mechanics like the degree of anisotropy, stress propagation -again under different site conditions- are unanswered; connectivity throughout a planned reservoir cannot yet be engineered.
· How to control induced seismicity? Most economic and long-term production models assume uniform reservoir permeability and heat exchange surfaces, consisting of numerous, well-distributed fractures. If stimulation results in larger seismic events the created, correspondingly extended single fractures could easily form short-circuiting pathways.
· There is no experience about possible changes of an EGS heat exchanger with time. Permeability enhancement (e.g. new fractures generated by cooling cracks) could increase the recovery factor while permeability reduction (e.g. by mineral reactions) or short-circuiting could reduce recovery.
· So far the envisaged electric power capacity of EGS systems is limited at a few MWe. But in order to play a significant role in electricity supply a system capacity of at least several tens of MWe would be essential. One of the main future R&D goals will be to see whether and how the EGS power plant size could be upscaled.
· For prospective investors a well-designed cost-risk analysis and corresponding measures still remain to be developed.
The competence and working spirit of the ENGINE team should by all means remain focussed and united. Already existing, important but not yet realized R&D action plans like GEISER should be rapidly implemented. And most importantly: as many kilowatt-hours of electricity as possible needs to be produced from EGS sources, as soon as possible.