Determining the geologic history of the Delft area is empirical when planning a geothermal doublet. The homo- or heterogeneity of the geology determines how effective the reservoir will be. Therefore, extensive research has been done beforehand, but will continue as more data is available when the wells are drilled.
Coring is an essential component for most of the research performed subsequently to well installation. Cores offer the advantage of having intact physical samples, which can be tested in the laboratory, logged using small scale petrophysical instruments and scanned using microscopy techniques. In addition to cores, small rock samples and chips are also collected as they come up during drilling. While cores give a very detailed image of certain depths, these small samples give a generalised image over the entire length of the well. These data combined, give a good description of what formations are present at which depths and how they look in the well.
Planned cores in DEL-GT-01 and DEL-GT-02
In the geothermal doublet, cores are targeted in the reservoir section, the immediate over- and underburden and at several locations through the overburden. As with most projects that produce fluids from a reservoir the cores from within the reservoir allow most insight into the physical processes occurring during production and therefore have the most direct impact on geothermal production. Coring from the overburden offers two key advantages: (i) it allows overburden (mostly mechanical and seismic) properties to be investigated which can improve the interpretation of seismic signals and surface effects, e.g. subsidence, and (ii) these layers due to a lack of past interest have not been previously cored in the Netherlands, and therefore offer wide scientific interest and can inform future engineering projects. The shallow subsurface cores also offer the potential for the exploration for heat storage and other waste storage.
Cores are taken from three different locations: the producer (DEL-GT-01), the injector (DEL-GT-02) and the monitoring borehole DAPGEO-02. This way open hole time is limited in the individual well sections and to enable prioritisation of the coring.
Planned cores in DEL-GT-01/02