Sorption, Matrix Diffusion, … Need Not Make a Major Difference for Frac Characterization from Short-Term Tracer Signals

Abstract

The use of equilibrium adsorptive-desorptive or ion-exchanging (alongside with conservative, including contrasting-diffusivity) liquid-phase tracers for characterizing a single, large-area hydrofrac created at a pilot unconventional-reservoir site in the NW-German Sedimentary Basin is examined prospectively by numerical simulations. The purpose of the exercise is twofold: (I) to assist in the process of tracer species selection for upcoming tests at this particular site, and (II) to derive more general recommendations for the early-stage use of tracers in unconventional-reservoir development, from the S[ource] and P[athway] characterization perspective adopted within the EU-H2020 project FracRisk (grant no. 636811). In terms of S and P compartments [cf. fig. 5 in McDermott et al. 2014], the take-away from our scoping simulations can be summarized qualitatively: (i) overall tracer recovery ratios correlate with transmissivity ratios between hydraulically competing P, S compartments; (ii) tracer arrival times correlate with the effective aperture or porosity of the critical (spill-controlling) P compartment; (iii) mid-/late-time tracer signal slope changes may indicate geomechanical effects within P or S compartments. The pilot unconventional-reservoir site in the NW-German Sedimentary Basin offers the unique advantage of well-steerable forced-gradient flow conditions, thereby enabling to more reliably estimate in-situ adsorption-desorption properties for a number of tracer species currently regarded as promising candidates for reservoir stimulation applications. On the other hand, processes like tracer adsorption-desorption and/or matrix diffusion need not make a major difference for frac characterization from short-term tracer signals.