**Abstract**

Engineered or enhanced geothermal systems (EGS) differ from conventional hydrother‐ mal reservoirs in that supplementary hydraulic stimulation is required to create surface area needed for heat exchange, and to allow adequate fluid production. Historically, geo‐ thermal wells have been straight hole or inclined and usually employ barefoot comple‐ tions. If horizontal drilling and hydraulic fracturing experience, refined to some extent with recent shale gas and shale oil stimulation campaigns, can be adapted for geothermal applications, it may be possible to improve the chances for successful EGS. One central issue for vertical, inclined, extended reach or horizontally drilled wells is whether there is merit in landing and cementing casing. This would allow discrete zones to be frac‐ tured, isolate thief zones or low temperature zones, allow future remediation and facili‐ tate generation of multiple fracture systems.

Most experienced geothermal operators balk at perforated and cemented completions. The arguments can be legitimate. There are supplementary costs associated with this completion, and the temperatures can make cementing and perforating challenging. Plugging of existing fracture systems from casing and cement is also proposed as a problem – which is easily overcome by the supplementary stimulation required. On the other hand, simple calculations suggest that proximal and interconnected fracture systems, natural or otherwise, are required for economic viability in all but the hottest scenarios. To effectively develop multiple fracture systems, wellbore isolation seems to be a natural requirement. One legitimate method to accomplish this is diversion, but the question remains as to how many intersected fractures can be stimulated. Another option is cementing and perforating. A comparative and realistic analysis is done to assess the impact of perforation skin, tortuosity associated with shear

© 2013 Glauser et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

fractures intersecting the wellbore and relative economics associated with perforating and cementing geothermal wells.

neered geothermal prospects – using hydraulic fracturing. Pritchett, 2012 [1], cautions that the practicality of these scenarios depends on fractures that are conductive enough to support the required high geothermal flow rates. "Performance will be significantly impaired if the average fracture separation is greater than 50 meters or so. … The crea‐ tion of such extensive and pervasive artificial fracture networks at costs that will prove acceptable … is the fundamental challenge for EGS. … New stimulation paradigms may be required." These fractures need to be close enough together and appropriately orient‐

Do Perforated Completions Have Value for Engineered Geothermal Systems

http://dx.doi.org/10.5772/56211

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Following Pritchett's logic, for a very specialized generic reservoir analysis, assume 100 kg/s are required for an economic system. To adequately delay thermal breakthrough (on the order of 30 years), required fracture spacing varies from 20 to 70 m.The time to breakthrough decreases as the fracture spacing becomes larger. Wu et al., 2012 [59] demonstrate fracture

What non-specialists don't always realize is the throughput that is required to ensure an economic geothermal prospect. If there is a single producer, the criterion for economic throughput is colloquially expressed as 100 kg/s by some; 2000 gal/minute by others. In any case, at 200°C this is between 62,000 and 69,000 BWPD at the sand face. To accommodate such high rates with nominal friction, large diameter casing is conventionally used with large diameter barefoot sections. This philosophy may be acceptable in conventional systems. If EGS is planned, large contact between one or more fractures and the wellbore is essential. Without effective diversion, multiple fracturing in an open hole is extremely difficult. Extended fracture contact with the well and/or multiple fracture intersections seem to be essential for EGS. This brings up the contentious topic of whether cased and perforated completions would be

Even barefoot completions are cased and cemented over a substantial portion of their length. For example, deep geothermal wells in Australia have the casing set below 4000 m or so and are open hole below that for a length of 500 m. Similar situations exist at Raft River in the United States. Nevertheless, cemented completions across thermally-productive zones will allow isolation and multiple zones can be stimulated. For example, the advantages of a

**• Potential for isolating fluid thief zones.** If low-pressure, shallow thief zones are present,

**• Potential for isolating zones that bring in low temperature fluid.** There are many anecdotal examples of cooler fluids entering the wellbore uphole in uncased environments, jeopard‐ izing overall economics. Even if these zones have been perforated, the possibility for plugging, while always difficult, is improved by squeezing perforations rather than an

ed to encourage heat sweep and thermal energy recovery.

spacing issues numerically.

acceptable in geothermal environments.

cemented completion include:

openhole section.

**3. Cemented versus barefoot completions**

crossflow can be avoided by casing across those zones.

**Keywords:** geothermal, perforations, openhole, multiple fractures
