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446 Chapter 4 Review of EGS and Related Technology – Status and Achievements • Monitoring of acoustic emissions is our best tool for understanding the system. Mapping of acoustic events is one of the most important tools we have for understanding the reservoir. In hydrothermal systems, we know from well tests and tracer tests that water is circulating and in contact with large areas of rock. We can assess stimulated fractures in the same way, once we have two or more wells in hydraulic connection to allow for circulation tests. We can map the location of acoustic emissions generated during stimulation and during circulation extremely accurately, i.e., +/ 1030 m. While we are not completely sure what the presence or absence of acoustic emissions means in terms of fluid flow paths or reservoir connectivity, knowledge of the location and intensity of these events is certainly important. This information helps define targets for future wells. If we drill into a zone that has already been stimulated and shows a large number of acoustic emissions events, it is commonly assumed that the well is connected to the active reservoir. However, this fact does not always result in a good system for heat extraction. For example, at Soultz, GPK4 was drilled into an area that was within the volume of mapped acoustic emissions, but it did not produce a connected fracture system between the production and injection wells, even after repeated stimulations. Mapping of acoustic emissions has improved, so that we can locate acoustic emissions and determine the focal mechanism for these events more accurately than in the past. As a result, we can better understand the stress field away from the wellbore and how our stimulation affects it. Methods for mapping fractures in the borehole have been developed, and the upper limit for temperatures at which they can operate is being extended. Ultrasonic borehole televiewers, microresistivity fracture imaging, and wellbore stress tests have all proved very useful in understanding the stress state, nature of existing fractures, and the fluid flow paths (before and after stimulation). Correlating the image logs with highresolution temperature surveys and with lithology from core and cuttings allows a better determination of which fractures might be productive. • Rockfluid interactions may have a longterm effect on reservoir operation. While studies of the interaction of the reservoir rock with the injected fluid have been made at most of the sites where EGS has been tested, there is still a good deal to learn about how the injected fluid will interact with the rock over the long term. The most conductive fractures often show evidence of fluid flow in earlier geologic time such as hydrothermal alteration and mineral deposition. This is encouraging in that it suggests that the most connected pathways will already have experienced some reaction between water and the rock fracture surface. Fresh rock surfaces will not have the protection of a layer of deposited minerals or alteration products. We also do not know how much surface water (which cannot be in equilibrium with the reservoir rock) we will need to add to the system over the long term. Our longest field tests have seen some evidence for dissolution of rock leading to development of preferred pathways and short circuits. Regardless, we will cool the produced fluid through our surface equipment, possibly resulting in precipitation of scale or corrosion (Vuatarez, 2000). Although not analyzed in this study, the use of carbon dioxide (CO2) as the circulating heat transfer fluid in an EGS reservoir has been proposed (Pruess, 2006). Brown (2000) has developed a conceptual model for such a system, based on the Fenton Hill Hot Dry Rock reservoir. The argument is made that supercritical CO2 holds certain thermodynamic advantages over water in EGS applications and could be used to sequester this important greenhouse gas. We also address this topic in Section 8.3.3.PDF Image | Review of EGS and Related Technology
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