The use of pulse interference tests (PITs) to measure reservoir hydraulic parameters has been explored for nearly five decades, with recent utilization in Enhanced Geothermal Systems (EGS). In a PIT, a finite volume of fluid is injected and either pumped back or flowed back under reservoir pressure. This process may be repeated multiple times with different volumes of fluid being introduced. PITs, therefore, have an advantage over recirculation tests for establishing hydraulic structure of formations, in that the volume of hydraulic interrogation can be varied with fluid volume. The test may be tuned, for instance, to evaluate formation near the injection wellbore or farther from the wellbore depending on the injection volume. PITs were conducted immediately following a 30-day recirculation test at the Utah-FORGE site (Milford, Utah). Injection and backflow were induced in well 16A(78)-32 while pressures were measured in wells 16A(78)-32 and 16B(78)-32 (hereafter referred to as 16A and 16B) during a single day of testing. During the tests, 16B was shut in so variation in formation pressure was measured. The PITs included three pressure pulse periods of 20, 40, and 120 minutes repeated four times each. The recorded periodic hydraulic responses in 16B were miniscule but were resolved through filtering processes at known frequencies of pulses. The ability to extract hydraulic signals from noise using time-series analysis is a significant practical advantage of periodic PITs as small injection volumes can be used. We used the generalized radial flow (GRF) model (Barker, 1988) to interpret the tests. The GRF allowed for an interpretation of the hydraulic responses without assuming the dimensionality of flow within the fracture networks and can be readily adopted to periodic signals. In our presentation here, however, we assume only radial flow to and from the injection well. We found that the effective hydraulic diffusivity estimated from these tests generally decreased with increasing injection volume.

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