The Upper Permian Rotliegend sandstone in the Netherlands is a very well-explored reservoir. That is because a lot of gas fields are reservoired in this aeolian and fluvial succession that attains thicknesses up to 300 m in places.
But the fact that the Rotliegend has been drilled and logged so much does not mean that legacy gas exploration data can be used 1:1 for geothermal purposes. And that’s not only because air permeability measurements have to be transformed to brine permeability measurements. It is also because the aeolian facies across the Rotliegend succession in the country seem to have a predictive pattern, with different ramifications for gas or geothermal developments.
Why is that?
The recent Amstelland-01 geothermal exploration well drilled by EBN in Amsterdam proved a particularly interesting characteristic of the Rotliegend succession. Even though the gamma-ray log suggested that the formation is very clean, sandy and homogeneous, the reality was different.
Based on a production logging test, it could be shown that about 85% of the produced water from the entire 111 m thick Rotliegend came from two highly permeable zones near the base of the reservoir succession. Comparing these zones against the cores from this interval, it could be shown that the sands are probably from the basal parts of aeolian dunes. The much less productive zones in the reservoir, representing the bulk of the interval, are composed of a mix of parallel-laminated dune slip faces and intra-dune or sand flat deposits.
This pattern of high-quality zones near the base of the Rotliegend succession is also observed in other geothermal projects in the country. Whether this is a coincidence or not, it does mean that geothermal projects in the Rotliegend mainly draw their brines from the base of the reservoir succession.
That is in contrast to what many gas fields in the Rotliegend will or have experienced, at least those gas fields which produce from a Rotliegend succession that is also of a primarily aeolian origin. Namely, the gas-water contact for these fields will be somewhere within the Rotliegend succession in most cases, unless the Rotliegend is quite thin and the contact extends into the underlying Carboniferous succession. In other words, the gas mostly resides in the top part of the reservoirs section. And for that reason, core data will also be mainly from the upper part of the reservoir as well, where permeabilities can be lower due to the factors mentioned above.
On that basis, it is important to think twice before applying the results from core and well data from one part of a reservoir section to another, even though it is the same stratigraphic interval. The nature of the extractive activity, and the predictive way in which facies organize will need to be taken into account.
