Tracer Recovery and Mixing from Two Geothermal Injection-Backflow Studies

Abstract

Injection-backflow tracer testing on a single well is not a commonly used procedure for geothermal reservoir evaluation, and, consequently, there is little published informat i o n on the character or interpretation of tracer recovery curves. Two field experiments were conducted to develop chemical tracer procedures for use with injection - backflow test in g, one on the fracture - permeability Raft River reservoir and the other on the matrix-permeability East Mesa reservoir. Results from tests conducted with incremental increases in the injection volume at both East Mesa and Raft River suggests that, for both reservoirs, permeability remained uniform with increasing distance from the well bore. Increased mixing during quiescent periods, between inject i o n and backflow, at Raft River suggest an area near the well bore that has a hydrologic character different from the f a r well bore environment. Increased flow rates for East Mesa test in g resulted in a general decrease in mixing. Comparison of recovery curves from the Raft River reservoir with those from the East Mesa r e servoir suggests that mixing is greatest, and therefore permeability is greatest, in the fractured reservoir. These test results indicate that injection-backflow testing with tracers can be used successfully to characterize flow in the near-well bore environment.