Fervo Energy is developing a commercial next-generation geothermal project in northern Nevada, adopting many unconventional technologies, such as horizontal drilling, plug-and-perf stimulation, and reservoir diagnostics with distributed fiber optic sensing (DFOS). We successfully installed permanent fiber optic cables cemented behind casing in three wells. The recorded DFOS data include in-well and cross-well distributed temperature (DTS), acoustic (DAS), and strain (DSS) sensing data. We evaluated the adaptability of DFOS to geothermal applications and showcased that DFOS is a beneficial tool for optimizing multi-stage completions, characterizing the stimulated reservoir volume, and determining well placement in geothermal reservoirs.
Beginning in January 2022, we executed a three-well drilling campaign, including a vertical monitoring well and a pair of horizontal wells that form a geothermal injection and production well doublet system. The wells targeted a high-temperature (350 °F to 375 °F), low-permeability geothermal reservoir in a mixed metasedimentary and granitic formation. In-well and cross-well (in vertical well) DFOS data were acquired during the stimulation treatment performed on the first horizontal well. These data allowed for characterizing the plug-and-perf completion design, evaluating fracture initiation and flow allocation, and guided the decision on the second horizontal well placement (producer) to establish a flow path between the wells via an induced fracture network.
First, DTS data from the vertical well were used to validate a thermal model to place the first horizontal well. During stimulation of the first horizontal well, in-well DAS and DTS data provided valuable insights into fracture initiation, slurry and proppant distributions at the cluster level, and thermal warmback behavior. This information helped validate and improve our completion design for the following stimulation. Integrated analysis of cross-well strain data recorded in a vertical well and two-well DAS-based microseismic constrained stimulated reservoir volume (SRV) height and length and guided our decision on the second horizontal (producer) well placement. Finally, cross-well DSS data recorded in the producer was used to understand the flow path between two wells during an injection test of the injector.
The 16-stage plug-and-perf stimulation treatment described in this study was the first of its kind in a high-temperature mixed metasedimentary and granitic formation in a fully horizontal geothermal well. To our knowledge, we recorded the first cross-well strain data during the stimulation of a geothermal well. The DFOS data acquired with three fiber-instrumented wells prove the applicability of unconventional approaches and their value for optimizing completion designs and well placement strategies in geothermal development programs. The in-well DAS data indicate that all clusters were opened during fracture initiation, and the treatment uniformity was high. Also, we found that strain change signals from induced fractures can be detected over large distances (> 1,500 ft). The DSS response recorded during the injector injection test confirmed the hydraulic communication between the injector and producer doublet before producer stimulation.
