Early-flowback tracer signals for fracture characterization in an EGS developed in deep crystalline and sedimentary formations: a parametric study

Abstract Artificial-fracture design and fracture characterization is a central aspect of many Enhanced Geothermal System (EGS) projects. The use of single well (SW) short-term tracer signals to characterize fractures at the Gros-Schonebeck EGS pilot site is explored in this paper. A certain degree of parameter interdependence in short-term flowback signals leads to ambiguity in fracture parameter inversion from measured single-tracer signals. This ambiguity can, to some extent, be overcome by (a) combining different sources of information, and/or (b) using different types of tracers, such as conservative tracer pairs with different diffusivities, or tracer pairs with contrasting sorptivities on target surfaces. Fracture height is likely to be controlled by lithostratigraphy while fracture length can be determined from hydraulic monitoring (pressure signals). Since the flowback rate is known during an individual-fracture test, the unknown parameters to be inferred from tracer tests are (i) transport-effective aperture in a water fracture or (ii) fracture thickness and porosity for a gel-proppant fracture. Tracers with different sorptivity on proppant coatings and matrix rock surfaces for gel-proppant fractures, and tracers with contrasting-diffusivity or -sorptivity for a water fracture were considered. An advantage of this approach is that it requires only a very small chaser injection volume (about half of fracture volume).