Examining the possibilities of establishing thermal storage in the chalk/limestone aquifer in the greater Copenhagen area. Phase 1 of project HTES (High Temperature Energy Storage). Part of an EUDP Project (ID 64016-0014)

GEUS reviewed existing literature on thermal energy storage and evaluated the available geophysical, geological, hydrological and reservoir data. Only little information about heat storage in chalks and limestones is published, however. The evaluation of the geophysical and geological data led to a better understanding of subsurface geology, including stratigraphy and location of fault systems. Our screening of the aquifer systems points out a number of water protection areas within the Copenhagen area, and this aspect has to be considered prior to pointing out well locations. The data compilation resulted in two geological well prognoses accompanied by reservoir parameters: a prognosis for the Vestforbrænding site and a site located within a fault zone associated with the Carlsberg Fault Zone. A key conclusion is that the storage chalk in itself is characterised by high porosity but very low matrix permeability.

Work on establishing two numerical groundwater and reservoir models is carried out. The objectives are to model heat and water transport, and to estimate potential production rates. The modelling runs point to rather low rates, unless using specific well configurations based on an array of wells, for example. Application of well stimulation technologies will surely enhance flow conditions, leading to enhanced productivity and higher rates. Supplementary work on well stimulation does not form part of Phase-1, but should be included in a forthcoming Phase-2. GEUS constructed a number of numerical and groundwater simulation models aiming at modelling a series of water injection and withdrawal scenarios are simulated using Eclipse 100 and FEFLOW. The presumed interaction between storage and groundwater zones is also considered, but the simulations carried out so far indicate, however, that there is no conflict between storage and groundwater interests. Most likely, the chalk package in between the groundwater and storage zones acts as a sort of seal that prevents upward fluid flow.

GEUS recommends stimulating the wells in order to increase the effective permeability of the chalk, as the challenge is to ensure and maintain reasonably high production and injection rates in a low permeability chalk reservoir. It is thus recommended to conduct a Phase-2 including the drilling of a pilot well with a sufficient data acquisition programme (logging, coring and testing). Phase-2 should also comprise core analysis and deployment of various well stimulation techniques, if appropriate. Water treatment should also be considered.