To develop high enthalpy geothermal systems in the Netherlands, reservoirs with a depth greater than 4000 m need to be targeted. The Lower Carboniferous limestone is proposed as the main target due to its depth and wide occurrence. To investigate the potential of this formation, the physical rock properties are studied in a variety of laboratory experiments. This study consists of two parts: (1) the geomechanical and hydraulic characterization of the intact rock and (2) the permeability evolution of a fracture altered rock.
Analogous rock samples from a quarry were used to determine tensile, compressive and shear strength and the elastic moduli. Further rock properties such as porosity, microstructure and mineralogy were measured by a gas pycnometer, mercury intrusion porosimetry and electron microscope analyser. The permeability was determined by flow-through experiments at different effective pressures. Subsequently, shear fractures were generated by triaxial deformation and a punch-through shear test to investigate the effect on the permeability.
A high degree of heterogeneity between the samples and a clear transverse anisotropy was observed. The maximum intact rock permeability is 150 μD at low effective pressures (2 MPa) and at reservoir conditions the permeability is below 1 μD. This results in an insufficient matrix conductivity and enhancement of the reservoir is needed to improve the reservoir connectivity. Both shear experiments show an increase in permeability of one to four orders of magnitude. The fractures show to be sustainable in terms of time and pressure variations. In combination with its physical rock properties, it can be concluded that the application of hydraulic stimulation within the Lower Carboniferous limestone is a reliable technique to improve the reservoir permeability.