Erik Simmelink MSc
With increasing fossil fuel prices, geothermal energy is an attractive alternative energy source for district heating and for heating industrial greenhouses. An important benefit of geothermal energy is that it has almost zero greenhouse-gas emissions, making it a sustainable energy source.
The Dutch horticultural firm A+G van den Bosch, in the greenhouse area in Bleiswijk, near The Hague, has shown entrepreneurial courage by developing the first geothermal heating plant ever in the Netherlands. The enterprise currently heats 7.2 ha of tomato greenhouses with warm water coming from a successfully drilled geothermal doublet that reaches into Lower Cretaceous aquifers at 1,700 m depth. TNO was closely involved in the pre-drill geological uncertainty assessment that led to the final drilling plan for the doublet. The aquifer would have to meet a number of constraints: the temperature of the aquifer water had to be as high as possible; the aquifer had to have a considerable thickness and good reservoir properties (porosity and permeability); ideally, it had to be tectonically undisturbed; and, last but not least, it should not interfere with producing oilfields.
The most promising aquifers, satisfying all constraints in Bleiswijk were the Rijswijk and Berkel Sandstone of Early Cretaceous age. It was concluded that the producer and injector wells would have to be placed in the aquifers as far south as possible from the horticulturist's premises, in order to reduce the geological risk. The assessment showed that the risk of producing insufficient water volumes was 'fairly small but not negligible'. This conclusion was used by the horticulturist to obtain a guarantee from the Government that he would be at least partially compensated for his investment in the first well if it did not produce geothermal water (or did so in clearly insufficient volumes). TNO's geological assessment was used by PMGi (drilling consultants) and Visser & Smit Hanab (contractors) for the well planning, and by the end of 2006 a drilling contract had been awarded to the drilling company Daldrup AG. The drilling, completion and testing of the two deviated wells took almost a year.
The drilling of the two wells yielded good results: the actual depths of the formation tops varied by only a few percent from the projected depths. The thicknesses of the Berkel and Rijswijk sandstones exceeded our expectations. The permeabilities seem higher than anticipated: the doublet now produces and re-injects water at flow rates between 130 m3/hr and 160 m3/hr. The produced water temperature, however, was somewhat lower than expected: about 60º C at 1,700 m depth. Nevertheless, the excellent reservoir conditions cause the COP to be about 30, which is reasonably in line with our scenario calculations. The geothermal doublet will prevent the previously required burning of 3 million m3 of natural gas per year and hence will save the enterprise some 700,000 Euros per year. Expected pay out time of the investments is about 7 years. Because of the positive results, the horticulturist has accelerated his plans for expanding the greenhouse area to about 14 ha. It is estimated that the geothermal heat can cover up to 60% of the energy demand for the additional 7 hectares of tomato greenhouses currently being built.