| Title | Reassessing Stimulation for EGS Reservoirs |
|---|---|
| Authors | Rob JEFFREY, Xi ZHANG, Reinhard JUNG |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | stimulation, fracture mechanics |
| Abstract | For the past 30 years the common principle for stimulation of EGS reservoirs is that massive fluid-injection into fractured crystalline rock formations induces shear dilation of natural fractures and results in the formation of a complex and highly conductive fracture network. The resulting stimulated fracture systems that occupy a rock volume of several hundred million cubic-metres may then be used for long-term heat extraction and power production on industrial scale by circulating water between wells separated by a distance of 0.5 to 1 km. The process is called hydraulic stimulation in order to distinguish it from conventional hydraulic fracturing. The evidence for such a process is based mainly on the mechanism and spatial distribution of the micro-seismic events associated with fluid-injection, the onset of micro-seismicity at a fluid-pressure far below the minimum principal stress, and the observation that fluid is generally leaving or entering the stimulated wells via natural fractures. We review observations and results from major EGS-projects reported in the literature and analyse them by using fracture mechanics and hydraulic fracture mechanics methods to assess the fracture stimulation process. Our study indicates that large through going single fractures have been formed at a fluid-pressure below or close to the minimum principal stress by inducing shear on small scale natural fractures and linking them through the formation of wing-cracks. The normal to these fractures is inclined to the direction of the minimum principal stress. Similarly we analyse the results of hydraulic-fracturing tests at intermediate scale ( less than 100m) in fractured hard rock where the created fractures were mapped during and after mining. The results of theses tests show that conventional hydraulic-fractures are being created when fluid is injected at a fluid pressure higher than the minimum principal stress. These fractures are oriented perpendicular to the direction of the minimum principal stress with offsets developing as they cross some natural fractures. Growth of fractures by both mechanisms is simulated numerically using a fully coupled hydraulic fracturing model. The simulation results confirm the observations that both modes of fracture propagation are selective processes resulting in a single through-going fracture per stimulation test. This makes clear that multiple fracture systems cannot be created under practical injection conditions by performing a single massive stimulation test even if the open-hole section is very long and contains a large number of natural fractures. The desired number of fractures can only be achieved by a corresponding number of stimulation tests in isolated borehole sections, as was envisaged in the pioneering Hot-Dry-Rock-projects and as is currently done in shale gas formations. Depending on the stress regime present in the reservoir, horizontal wells may be required to allow multistage fracturing. Alternatively, vertical wells can be used to place multiple horizontal fractures if the minimum stress is vertical. In either case, the wells must be completed with a cemented casing string across the target interval. With these changes, wells can be stimulated to effectively connect injection and production wells with 10 or more conductive discrete fractures. Such an approach will allow for more complete engineering of the EGS reservoir to produce a conductivity that allows commercial circulation rates to be achieved. |