During April and early May 2019, injection testing was carried out in three granitic zones in a vertical well at the FORGE site near Milford Utah. The deepest zone was in an uncased openhole region that had also been treated in September 2017. Two cased and perforated intervals farther uphole were also evaluated. In a number of the injection cycles, flowback was implemented rather than shut-in, with the goal of finding an alternative to prolonged shut-in periods for inferring closure stress and formation permeability (transmissibility).

The flowback data from the FORGE program involved a progressive increase in the choke size, or cyclic flowback/shut-in while pressure decreased. The flowback data are presented, and analyses are shown. The predictions are compared with equivalent injections that were strictly shut in. Closure signatures are considered, and after flow evaluations – for permeability (transmissibility) determination – are carried out. Flowback potentially has advantages over shut-in because of the reduced time to closure.

1. INTRODUCTION

Enhanced Geothermal Systems (EGS) offer the potential to bring low-cost geothermal energy to locations that lack natural permeability through hydraulic stimulation (Moore et al., 2019). The U.S. Department of Energy selected a location near Milford, Utah, as the site for the Frontier Observatory for Research in Geothermal Energy (FORGE). The goal of the FORGE program is to develop the techniques required for creating, sustaining, and monitoring EGS reservoirs.

In Sept 2017, an injection program was carried out in the openhole toe of Well 58-32 at the Utah FORGE site (see, for example, Balamir et al., 2018). Well 58-32 is approximately 7500 feet deep with 147 feet of open hole below the production casing shoe. A follow-on injection program was carried out in this same well in April and May, 2019. One of the aims of the 2019 testing program was to evaluate the repeatability of injection into the barefoot section along with the potential for pumping into cased and perforated zones farther uphole. Post-injection measurements were undertaken under shut-in conditions or while flowing back the well. The flowback measurements assessed using previously proposed technology as a substitute for unreasonably long shut-in periods as part of Diagnostic Fracture Injection Testing (DFIT).

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