| Abstract |
The EU project FracRisk (June 2015 - May 2018) set out to provide decision support tools for dealing with fracking-related hazards [McDermott 2014]. It relies on top-down approaches to FEP screening in order to reduce the risk assessment job to a small set of relevant scenarios, and on efficient, FEP-overriding source/pathway/target (SPT) compartmentalizing [Sauter 2012]. Though long-established in the realm of radioactive waste management, FEP concepts prove of limited value when applied to shale-gas (SG) exploitation, whose main environmental footprint stems from low intrinsic productivity (need to frequently re-enter the wells), rather than from so-called hazards. Thus viewed, SG development is neither a significant-risk technology, nor the game changer. Artificial and natural tracers may serve several purposes in the SG context: besides assessing un-/desired transport connectivity between S and T, or within S and P, and quantifying fluid/solute/mineral mixing/dilution/transfer within/between S and P, a more significant contribution is expected from tracers in measuring productivity-critical transport parameters within S (porosity, retardation, dispersion). However, parameter inversion from measured tracer signals poses great challenges in single-well settings (as used for SG), where flow-field reversal destroys the equivalence between tracer residence times and fluid transport parameters. Recent tracer-technological innovations address inflow profiling, proppant imaging, proppant-embedded tracers; yet, to date, not a single example is known of a tracer-based estimation of a frac-related parameter. We present tracer BTC type-curve sets for assessing parameter sensitivity, and, where applicable, inverting tracer signals in terms of production-critical parameters (frac volume, fissure aperture and spacing, fluid-rock interface area), for liquid-/gas-phase tracers detectable uphole during backflow/production (no downhole instrumentation required); most of these findings prove applicable to geothermal reservoirs as well, especially the EGS type involving multiple artificial fractures. Of all target parameters, matrix porosity appears most recalcitrant to tracer-based estimation, unless reactive tracers with very special properties would become available (steerable-delay in-situ release was realized for casing-/proppant-embedded, yet not for matrix-pervading tracers). |