The proposed Den Haag Zuidwest district heating system of the city of The Hague consists of a deep doublet in a Jurassic sandstone layer that is designed for a production temperature of 75°C and a reinjection temperature of 40°C at a flow rate of 150 m h-3. The prediction of reservoir temperature and production behavior is crucial for success of the porposed geothermal doublet. In this project, Geophysica was asked by Eon Benelux and IF Technology (Arnhem) for a study of the important geothermal and geohydrological issues for the doublet design. In the first phase of the study, the influence of the three-dimensional (3D) structures of anticlines and synclines on the temperature field were examined. A comprehensive petrophysical investigation was performed, to build a large scale 3D-model of the reservoir. Several bottomhole temperatures (BHTs), as well as petrophysical logs were used to calibrate the model using thermal conductivity measurements on 50 samples from boreholes in different lithological units in the study area. Profiles and cross sections extracted from the calculated temperature field were used to study the temperature in the surrounding areas of the planned doublet.
In the second phase of the project, a detailed 3D numerical reservoir model was set up, with the aim of predicting the evolution of the producer and injector temperatures, and the extent of the cooled area around the injector. The temperature model from the first phase provided the boundary conditions for the reservoir model. Hydraulic parameters for the target horizons, such as porosity and permeability, were taken from data available from the nearby exploration wells. The simulation results are encouraging as no significant thermal breakthrough is predicted. For the originally planned location of the producer, the extracted water temperature is predicted to be around 79°C, with an almost negligible cooling in the first 50 years of production. When the producer is shallower parts of the reservoir, the yield water temperatures is lower, starting at 76°C and decreasing to 74°C after 50 years of operation. This comparatively larger decrease in temperature with time is caused by the structural feature of the reservoir, namely a higher dip causes the cooler water to easily move downward. In view of the poor reservoir data, the reservoir simulation model is constructed to allow iterative updates using data assimilation during planned drilling, testing, and production phases. Measurements during an eight hour pumping test carried out in late 2010 suggest that a flow rate of 150 m h-3 is achievable. Fluid temperatures of 76.5 °C were measured, which is very close to the predicted value. Further informationen are available from BBR - Fachmagazin für Brunnen- und Leitungsbau.