Single-Well Dual-Tracer Spikings During EGS Creation in Northern-German Sedimentary Layers

Abstract

Dual-tracer spikings were conducted during or immediately after reservoir stimulation (chemical and/or hydraulic faulting, fracturing or fissuring) in single-well configurations in two different geological settings in the Northern-German sedimentary basin. The first one was a single-well flow-path spiking between two sandstone layers in and between which faulting and some limited-radius fracturing or fissuring had previously been induced: the massive water injection at the lower-layer well-screen created a divergent flow field; after opening the well-screen at the upper layer, the flow field became (more or less) focused to the latter. At the other site, the spiking configuration was that of a single-well injection-withdrawal (push-pull) into and back from one insulated layer out of several stimulated sandstone and volcanic layers: the flow field first diverging away from, then converging towards the same well-screen (natural background flow being negligible in all cases). In both configurations, the difference between the return signals of two tracers with differing 'diffusion coefficients' (or rates of exchange between im-/mobile fluid zones) was supposed to enable estimating the encountered fluid-rock contact-surface area, more or less equivalent to the heat exchange area that would become a crucial parameter for the envisaged EGS at each site. Besides an inherent limitation of their reservoir penetration scale, considerable ambiguity persists with the inversion of transport parameters from both tracer test types described. It also appears that this ambiguity can hardly be reduced by adding hydraulic information.