Geothermal Reserves and Sustainability in the Greater Copenhagen Area

Abstract

The partners in the Greater Copenhagen geothermal license in Denmark have completed a study of the geothermal reserves in the area.
Three main sandstone reservoirs have been identified and described in 462 blocks each of an area of 4 km2. The decrease of production temperature with time at a geothermal plant with moderate, but commercial production rates has been modeled over 500 years. The results show, that the production temperature decreases relatively slowly due to a significant flow of heat from the layers above and below the reservoirs partly reheating the injected water. The production can thus be continued for many decades when commercial at a lower production temperature than the initial reservoir temperature.
The modeling was also used to establish a generalized correlation between production temperature as fraction of initial temperature and produced fraction of heat in gross reservoir. Another computer program estimating geothermal heat production costs was used to establish a generalized relation between transmissivity and a lower temperature limit for commercial production.
These two generalized relations were then combined with an assessment of the transmissivity and the initial temperature and heat in place in each of the 462 blocks to calculate the commercially producible heat from each block.
The thus producible heat at a chosen commercial cut off heat price in the Greater Copenhagen area reaches 60.000 PJ. Compared to the district heat consumption for the area of 40 PJ/year, the underground is seen to have a capacity to supply whatever heat is needed for thousands of years.
A simulation of the reheating of reservoirs to estimate the temperature distribution with depth after 5000 years shows that the reheating of the reservoirs by then has given a substantial contribution to the reserves.