| Title | Serendipitous Petrothermal-Reservoir Development in a Hydrothermal Par-Excellence Play Setting |
|---|---|
| Authors | Horst BEHRENS, Julia GHERGUT, Martin SAUTER |
| Year | 2023 |
| Conference | Stanford Geothermal Workshop |
| Keywords | temperature decline, thermal drawdown, tracer test, inter-well, matrix diffusion, joint inversion, single-fracture model, petrothermal, Malm aquifer, Molasse basin, Alpine foreland |
| Abstract | We deal with a particular well doublet in a fractured-porous formation that had been assessed as very promising and expected to provide for hydrothermal-reservoir operation at commercial levels for at least four decades, but whose conspicuous decline of production temperature has raised suspicion of a flow-path shortcut between injection and production locations. Various hypotheses on the hydrogeological nature of possible shortcut features, and on their geometry and size have been examined by Behrens et al. 2020, 2021 (SGP-TR-216, SGP-TR-218). Finally, measured tracer signals to date (2022) yield unambiguous evidence for the presence of a misaligned large-scale fault zone (which has remained undetected by prior geophysical exploration) connecting the injection and the production well, and enable to quantify its transport-effective aperture, 15–20 cm, and fluid-swept area, ~10 million m². Such large sizes are incompatible with the previously considered scenario of a misaligned fracture induced by early-time stress-field rotation (a poroelastically coupled effect of fluid circulation onset, with the well doublet behaving like a strike-slip source mechanism). Owing to the petrothermal behavior thus ascertained, production temperature decline rates are likely to significantly dwindle over the next decade; production temperatures would not drop below 95°C for at least 35 years under the current operation scheme. It looks somewhat spectacular to find such markedly petrothermal behavior in a reservoir play setting known as hydrothermal-par-excellence (Germany’s best hydrothermal region, by the current state of knowledge). Moreover, this unintended petrothermal-reservoir development appears to be the most successful (!) among all petrothermal systems whose development was attempted in Germany so far. Along with certain non-standard findings from other sites in the molasse basin, like Geretsried, St. Gallen (CH), and Kirchweidach in North-Eastern Bavaria, it sheds a new light on carbonate-petrothermal prospects which might become worth dedicated exploration in the future. |