Prerequisites Geology

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

Geology

Knowledge about the sediment composition of the selected heat storage site is necessary to determine the dimensions of the reservoir, and to place the heat transferring hoses in the right horizons. Typically, a sediment profile from terrain to the deepest layer of hoses consists of several types of sediments. Each sediment type has specific thermal properties (table A) associated with the heat capacity and thermal conductivity of the system. The thermal conductivity of geological materials depends on their mineral composition, texture, and water content. Above the water table, where air is present in the pore spaces, sediments generally have a low thermal conductivity. Hence information about the position of the water table in the borehole is important when planning a new site.

Since the thermal properties of the site-specific sediments directly correlate to the amount of heat that can be transferred into and stored in the heat storage, they are invaluable for deciding system size and hose location. 

To determine site lithology (sediment composition) you need borehole data. In Denmark the Geological Survey of Denmark and Greenland (GEUS) have been collecting such data, including information on location, construction, geology, water table and groundwater chemistry, since 1926. The data is stored in the national borehole database (Jupiter [link]) and is freely available on the Internet. This means that an estimate of site lithology can be done fairly easy just using the data from Jupiter. 

Another and more precise option is drilling the actual site from the terrain to the bottom of the planed storage. In the process, it is possible to sample the different sediments and make a borehole log [link]. Determination of the thermal proporties can then be done in either of these ways:

•     Using Jupiter data or the actual borehole log to determine the type and thickness of each sediment layer and then  calculating average heat capacity and thermal conductivity values, based on generic table values for the sediment type.

•     Do an on-site thermal response test of the specific layers.