Prerequisites hydrogeology

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.

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.