Short-Term Fluid, Heat, and Solute Transport in Deep Georeservoirs Likely to Become EGS: Some Challenges to ICDP Hydrogeologists Who Might Like Using Artificial Tracers

Abstract

Single-well ‘push-then-pull’ (SW) tracer methods appear as attractive for a number of reasons: less uncertainty in dimensioning, and lower tracer quantities required, than for inter-well (IW) tests; stronger tracer signals, enabling easier and cheaper detection, than in IW tests; shorter fluid sampling duration required, reaching higher tracer mass recovery, than with IW tests. However, SW tracer tests face one major issue: the ‘push-then-pull’ design weakens the correlation between tracer residence time distribution (RTD) and georeservoir transport parameters (in particular, it destroys the equivalence between mean RT and georeservoir size), inducing insensitivity or ambiguity of tracer signal inversion against some of the most essential parameters supposed to be the target of tracer tests par excellence (since not determinable from hydraulic or geophysical methods): pore velocity, transport-effective porosity, fracture aperture and spacing or fracture network density (where applicable), fluid/solid or fluid/fluid phase interface density. Two new ways using reactive alongside with physico-chemically stable tracers are proposed for overcoming some of the limitations to parameter invertibility associated with the SW test design.