Unconventional reservoirs have emerged as significant hydrocarbon supplies all over the world. Although horizontal drilling combined with multistage hydraulic-fracturing techniques has enabled commercial hydrocarbon production from unconventional reservoirs, fracture characterization in such complex reservoirs remains challenging. In this study, we propose an analytical model to characterize fracture using the early-time post-stimulation tracer flowback profile. The tracer flowback analysis is a quick method used to evaluate load recovery of hydraulic liquids, productivity of each stage and fracture characterization. However, tracer flowback analysis combined with microseismic data is still at qualitative appraisal stage and non-Newtonian effect of hydraulic fluids is not considered. Therefore, based on convection-diffusion transport theory, a tracer injection flowback model is developed and solved by Laplace transform and image reflection method, and then fracture parameters characterization is proposed considering the non-Newtonian hydraulic fluids flow. The results demonstrate that the calculated tracer profile of the presented model can well explain the main part of tracer flowback profile except the long trail. That is because the model only considers the process of tracer transports in hydraulic fractures while the long tail of tracer flowback profile reflects the mass interchange between hydraulic fractures and tight matrix. Parameters sensitivity study shows that the peak of tracer flowback profile decrease with the increasing dispersion coefficient and retardation; tracer dispersion induces the increment of tracer slug extent. The differernce of tracer injection and flowback rate severly affects the shape of tracer flowback profile. Fracture parameters are characterized and validated with field data. The presented model for describing the behavior the tracer profile during the flow back process provides an alternative approach for reservoir characterization. This study encourages the industry to manage the flowback operations carefully.
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