Prerequisites Hydrology

Geological suitability

When planning a geothermal heat storage project one of the essential things to investigate is the geological suitability of your site. The main things to take into account are

the geology; sediment composition and thermal properties,

hydrology; ground water and drinking water aquifers, and

hydrogeology; migrating ground water and free moving water. Below we will discuss these three subjects in more detail

Hydrology

In Denmark, all drinking water resources come from ground water aquifers. Determining the impact of the heat storage is therefore seen as immensely important when doing shallow geothermal projects. The main issues are leakage of water with antifreeze, cross-connecting different aquifers, seepage of surface water along the borehole, drilling into artesian aquifers and unwanted thermal effects on the aquifers. To prevent any of these highly undesirable events from occurring there is a lot of legislation on the area, where safety distances to other ground source heat exchangers and to extraction wells for drinking water are specified. The legislation also specifies which antifreeze agents can be used in the ground loop – only non-toxic and easily biodegradable fluids are allowed. 

However, not all ground water is drinking water aquifers and high water content has a great positive effect on the thermal conductivity of sediments (table A). This means that a high site ground water table is usually preferred, since that will put the majority of the storage below the water table. Determination of the ground water table at specific sites can be done by using the Jupiter database or by drilling the site and measuring directly.