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Review of EGS and Related Technology – Status and Achievements Chapter 4 445 whether these will form largescale flow paths and connect over large volumes of the reservoir. This means that the fracture spacing in the final reservoir is governed by the initial, natural fracture spacing. The number of fractures in a wellbore that will take fluid is, therefore, important to assess in each well. The total heat that can be recovered is governed by the fracture spacing, because the temperature drops rapidly away from the fracture face that is in contact with the injected cool fluid. • We currently do not have a reliable openhole packer to isolate some zones for stimulation. This is routine in the oil and gas industry; but in the geothermal industry, hightemperature packers for the open hole are not reliable, so we stimulate the entire open interval. Logging shows that the first set of open fractures is the one most improved. If we want to stimulate some zones more than others – or if we want to create new fractures – we will need a good, reliable, hightemperature openhole packer. Although earlier testing at Soultz using a cement inflatable aluminum packer have been encouraging, more development work remains to be done to improve reliability and increase temperature capability. • Hydraulic stimulation is most effective in the nearwellbore region. The nearwellbore region experiences the highest pressure drop, so stimulation of this region is important. But we also need connectivity in the far field away from the wells to maintain circulation and accomplish heat mining. We can effectively use a variety of techniques both from the oil and gas industry and from geothermal experience to improve nearwellbore permeability. Hydraulic stimulation through pumping large volumes of cool fluid over long time periods, and acidizing with large volumes of cool fluid and acid (of low concentration), have been most effective. Use of highviscosity fluids, proppants, and high rate highpressure stimulation has been tried with mixed success and may still have potential in some settings, particularly in sedimentary reservoirs with high temperatures. However, there are limits to the temperature that packers, proppants, and fracturing fluids can withstand, so some of these techniques are impossible or very costly in a geothermal setting. In crystalline rocks with pre existing fractures, oil and gas stimulation techniques have failed to result in connection to other fractures and may form short circuits that damage the reservoir. Our current efforts to stimulate geothermal wells and EGS wells, in particular, are limited to pumping large volumes of cold water from the wellhead. This means that the fractures that take fluid most readily anyway are stimulated the most. Only a small portion of the natural fractures seen in the wellbore support flow. Because these more open fractures may also be the ones that connect our producers to our injectors, this may not be a disadvantage. However, there may be a large number of fractures observed in the wellbore, and an ability to identify and target the best ones for stimulation is limited because of a lack of research. • The first well needs to be drilled and stimulated in order to design the entire system. Early efforts to create reservoirs through stimulation relied on drilling two wells, oriented such that there appeared to be a good chance of connecting them, given the stress fields observed in the wellbore and the regional stress patterns. However, at Fenton Hill, Rosemanowes, Hijiori, and Ogachi, this method did not yield a connected reservoir. Stress orientation changes with depth, or with the crossing of structural boundaries, and the presence of natural fractures already connected (and at least somewhat permeable) makes evaluating the stimulated volume difficult. It seems much easier to drill the first well, then stimulate it to create as large a volume as possible of fractured rock, then drill into what we think is the most likely place, and stimulate again. Because of this, we can design wells as either producers or injectors, whereas it would be better if we could design wells for both production and injection. This emphasis on the first well demands that it be properly sited with respect to the local stress conditions. Careful scientific exploration is needed to characterize the region as to the stress field, preexisting fractures, rock lithology, etc.PDF Image | Review of EGS and Related Technology
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