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
- 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.
Prerequisites
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.
Prerequisites
Hydrogeology
The transfer of heat into the sediments is dominated by heat conduction, but where the heat storage is below the ground water table advective transport also plays a role. How much depends on the permeability, hydraulic conductivity and transmissivity, which determine how fast the water can move through the sediments. The hydraulic conductivity and transmissivity can be measured by test drilling and pumping.
How fast migrating ground water can move through the system also depends on the above mentioned sediment properties, and fast moving water can be showstoppers because of the excessive heat loss they can cause.
To avoid this it is important that the heat storage is not implemented in an area where there is significant movement of water at a depth near or above the bottom level of the boreholes. If the worst should happen and significant water movement is registered at the chosen heat storage site, the storage must be placed above the level of the moving water. If the ground water is stationary or with negligible flow, the storage can be implemented but the ground water level must be lowered during the construction phase until the hoses are installed and the boreholes grouted.
