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

Comprehensive data set of in-situ hydraulic stimulation experiments for geothermal purposes at the Äspö Hard Rock Laboratory (Sweden)

“…The core idea of multistage fracturing is adopted from shale gas production perforated completion design (parallel and isolated fractures), which is applied to economically produce gas from horizontal wells. When adapted to an EGS, multistage fracturing will economically produce heat by increasing the size of heat exchangers in a controlled way; one heat exchanger per perforation (Glauser et al, 2013;Fleckenstein et al, 2022). Planned in 2014 and executed in 2015, the Äspö underground experiment described in this study, was a mine-scale test campaign in naturally fractured granitic rock demonstrating the safe development of sub-parallel, zonal-isolated multi-fractures as potential heat exchangers for geothermal purposes .…”