The modelled thicknesses of the aquifers are gross values. This means that the depth of the base has been subtracted from that of the top. Although only layers known to contain a significant amount of sand(stone have been modelled, all aquifers also contain to a certain extent of layers with a permeability so low that they contribute little or nothing to water production. These layers often consist of clay(stone) or shaly sand. For a correct calculation of the geothermal potential, the gross thickness of the aquifer must be corrected for the layers that do not contribute to production. This is done by multiplying the gross thickness by the percentage of the total layer thickness that does contribute to production. This percentage is called the net-to-gross ratio or N/G. It is assumed here that the average aquifer permeability is also calculated over the net reservoir thickness. An equal result is otherwise obtained if the gross thickness is used in combination with the average aquifer permeability of the gross reservoir thickness.
Net-gross ratios were calculated per borehole and per aquifer based on values of the gamma ray log, using a machine learning algorithm trained with petrophysically determined net-gross ratios (Vrijlandt et al. 2022).
Maps of the per borehole and per aquifer determined net-gross ratios were created using Kriging.