Flowback Test Analyses at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) Site

Abstract

In 2017 and 2019, injection testing was carried out in three zones in a vertical well in granite at the Frontier Observatory for Research in Geothermal Energy site near Milford, Utah, USA. In several injection cycles, flowback was implemented rather than shut-in. The goal was to explore an alternative to prolonged shut-in periods for inferring closure stress, formation compressibility, and formation permeability (permeability thickness product). The flowback procedures involved a cyclic flowback/shut-in, while pressure decreased. The flowback data are presented, and analyses are shown. The inferred closure stress(es) from flowback analyses are lower than for equivalent injection cycles that were strictly shut-in. Relatively high formation compressibility obtained from the flowback analyses indicates an extensive, fractured system. This study also includes numerical simulation of the flowback events. The numerical model shows that the rebound pressure is not necessarily the lower bound of the minimum principal stress. The signature of stiffness change can be identified as the process when the depletion mainly transitions from hydraulic fracture to natural fractures from numerical analysis. Overall, flowback potentially has advantages over shut-in because of the reduced time to closure.

Hydraulic stimulation strategies in enhanced geothermal systems (EGS): a review

“…In this part, we summarised representative EGS sites globally and selected six of them for a detailed review of the hydraulic stimulation strategy, permeability enhancement performance, and induced seismicity. However, there are some other recent important field projects, such as the Espoo project in Finland (Hillers et al 2020;Kwiatek et al 2019;Leonhardt et al 2021), Eden project in the UK (Abesser et al 2020;Baumgärtner 2022;Fink et al 2022) and projects under the Frontier Observatory for Research in Geothermal Energy (FORGE) in the ◂ USA (Moore et al 2018(Moore et al , 2019Xing et al 2022). More details concerning these projects can be found in the references, and we expect that many interesting results and lessons learned will emerge from these projects.…”

In-Situ Stress Measurements at the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) Site

“…Flowback measurements are a desirable substitute for measurements that require unreasonably long shut-in periods as part of a DFIT. Xing et al [7,34] provided a detailed analysis of the flowback tests at the FORGE site. The estimated closure stress for Zone 2 determined from flowback tests is 14.7-15.4 MPa/km, which is much lower than the values of 17.2-21.5 MPa/km estimated from step rate and extended shut-in tests.…”

“…The in-situ stresses obtained by different methods are summarized and will serve as a reference for future hydraulic fracture design. Pump-in/flowback tests [7] and temperature signatures [8] provide alternative methods to infer the in-situ stresses, which is beyond the scope of this investigation.…”