| Abstract |
Geothermal heat in the Netherlands is produced via balanced circulation between two or more wells, targeting porous sandstone reservoirs at depths of 1.5 – 3 km. The cooled brine is reinjected and is expected to progressively cool the reservoir. Knowledge of how this cooling evolves in space and time is important as it has an impact on geothermal license area boundary definition, project lifetime assessment, as well as fault reactivation, and seismic hazard- and caprock integrity assessment. In turn, these assessments guide limits on injection pressures and temperatures. According to predictive models, the rock volume around the geothermal reinjection well gradually cools down over time. However, since field validation of such models was lacking there are substantial uncertainties in these models. Recently, several replacement geothermal wells were drilled close to former injection wells. This provided a unique opportunity to probe the cooled reservoir rock in the vicinity of the injector. Here, we present various observations and temperature logs acquired in these wells at different geothermal sites within the TKI Geo4All research program. The observations showed that 1) cooling was less than expected from the models, and 2) cooling was far from uniform over the reservoir interval. Possible mechanisms behind the observed cooling profiles are discussed, including the effect of facies and heterogeneity on flow, and density-driven flow. The heterogeneous cooling is expected to have significant impact on, amongst others, the seismic hazard calculations and caprock integrity assessment. |