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4.5.2 Lessons learned at Hijiori Review of EGS and Related Technology – Status and Achievements Chapter 4 423 The Hijiori project provided useful data for future projects to build on. Added to the Fenton Hill and Rosemanowes experience, it became clear that it is better to drill one well, stimulate, map the acoustic emissions during stimulation, and then drill into the acoustic emissions cloud, than to try to drill two or more wells and attempt to connect them with stimulated fractures (Okabe et al., 2000). • The reservoir continued to grow during the circulation test. • If natural fractures already connect the wellbores, stimulation may result in an improved connection that causes short circuiting, particularly if the wellbore separation distances are small. • The acoustic emissions (AE) locations from the deep circulation test suggest that the stimulated fractures or the stress field change direction away from the well. • The lineation found from the AE locations in the shallow reservoir parallel the direction of the lineation found during the stimulation, but the acoustic emissions farther from the well during the circulation test trend in a new direction. • If the stress direction changes from one part of the reservoir to another, it may be almost impossible to predict how the stimulated fractures will be oriented and where they will grow and be most permeable. • With current technology, it is difficult – if not impossible – to predict what the stress field will be in the wellbore prior to drilling; and it is even more difficult to know the stress field away from the borehole. • It is much easier to drill into the zone that is mapped as the fracture zone from AE locations and establish a connection than to attempt to connect two existing wells. • While attempts to control the stress field by modifying pressure in wells not being directly stimulated did not accomplish what was hoped for at Hijiori, this concept still holds promise. Further work is needed. • At Hijiori, it was clear that while stimulation by injecting at high pressures for short periods had some effect on the permeability of the naturally fractured reservoir, injecting at low pressures for long time periods had an even more beneficial effect. • The reservoir grew and connectivity improved more during circulation tests than during efforts to stimulate at high pressures. • The data suggest that cool water shortcircuited the modeled path length, either because fracture growth during injection testing connected the deep reservoir with the shallow one, or because the deep and shallow reservoirs were connected through one of the wellbores penetrating both zones. • Well spacing needs to be as large as possible while still making a connection. The Hijiori project also showed the value of understanding not only the stress field but also the natural fracture system. Both the Fenton Hill project and the Hijiori project were on the edges of a volcanic caldera. While very high temperature gradients can provide access to a large reservoir of thermal energy and can make the project economics better since one only needs to drill relatively shallow wells to reach very high temperatures, the geology, stress conditions, and fracture history of rocks in such areas can be extremely complex. This can make project design, construction, and management very challenging.PDF Image | Review of EGS and Related Technology
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