Travel time based thermal tracer tomography

Abstract

Active thermal tracer testing is a technique to get information about the flow and transport properties of an aquifer. In this paper we propose an innovative methodology using active thermal tracers in a tomographic setup to reconstruct cross-well hydraulic conductivity profiles. This is facilitated by assuming that the propagation of the injected thermal tracer is mainly controlled by advection. To reduce the effects of density and viscosity changes and thermal diffusion, early-time diagnostics are used and specific travel times of the tracer breakthrough curves are extracted. These travel times are inverted with an eikonal solver using the staggered grid method to reduce constraints from the predefined grid geometry and to improve the resolution. Finally, non-reliable pixels are removed from the derived hydraulic conductivity tomograms. The method is applied to successfully reconstruct cross-well profiles as well as a 3-D block of a high-resolution fluvio-aeolian aquifer analog data set. Sensitivity analysis reveals a negligible role of the injection temperature, but more attention has to be drawn to other technical parameters such as the injection rate. This is investigated in more detail through model-based testing using diverse hydraulic and thermal conditions in order to delineate the feasible range of applications for the new tomographic approach.Published by Copernicus Publications on behalf of the European Geosciences Union.

Synthetic fracture network characterization with transdimensional inversion

“…The two upper injections from the left side did not produce any breakthroughs at the observation points. Observations where no breakthrough occurs frequently appear in the results of tomographic tracer tests, but they are often ignored in the inversion [ Vasco and Datta‐Gupta , ; Brauchler et al ., ; Somogyvári et al ., ]. If no breakthrough is observed within the simulated time, it is concluded that there is no (relevant) advective pathway leading from one point to the other.…”

“…Conservative tracers do not decay or react with the flowing fluid or with the rock matrix. For tracer tomography, multilevel tracer injection is used (which is obtained by repeated tracer tests or by using multiple tracers at the same time) and the breakthrough curves are recorded at multiple depth points in downgradient wells (receivers) [ Illman et al ., ; Jiménez et al ., ; Somogyvári et al ., ]. In fractured media, local injection can be achieved by using packer systems to isolate specific fractures.…”

“…It is also possible to use moments or breakthrough times for comparison instead of the full curve in this step. This could be advantageous when the quality of the breakthrough curves is poor or the flow field is altered by the used tracer (e.g., density effects [ Somogyvári et al ., ]).…”

Field validation of thermal tracer tomography for reconstruction of aquifer heterogeneity

“…Third, despite the strong influence of heat diffusion and density effects, thermal tracer testing in heterogeneous formations allows for the delineation of preferential pathways, where heat transport is dominated by advection [ Irvine et al ., ; Wagner et al ., ; Djibrilla Saley et al ., ]. In our earlier theoretical study [ Somogyvári et al ., ], we have shown that by using a travel time‐based inversion technique, the influence of density changes, viscosity variations, and diffusion effects is minimized and a wide application window for thermal tracer tomography is available.…”

“…Alternatively, computationally efficient travel time‐based inversion has been suggested by Somogyvári et al . []. Travel time‐based tomographic inversion methods exploit the similarities between the propagation of tracer fronts and seismic pressure waves [ Vasco and Datta‐Gupta , ; Brauchler et al ., ].…”