Do perforated completions have value for engineered geothermal systems

Abstract

Engineered or enhanced geothermal systems (EGS) differ from conventional hydrothermal reservoirs in that supplementary hydraulic stimulation is required to create surface area needed for heat exchange, and to allow adequate fluid production. Historically, geothermal wells have been straight hole or inclined and usually employ barefoot completions. 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 fractured, isolate thief zones or low temperature zones, allow future remediation and facilitate 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 fractures intersecting the wellbore and relative economics associated with perforating and cementing geothermal wells.