Slates: a potential rock type to extract geothermal energy from the underground?

Commonly used host rock reservoirs for Enhanced Geothermal Systems (EGS) are composed of granite, as they display highly conductive and sustainable fracture networks after stimulation. However, considering the large amount of metamorphic rocks in Europe’s underground, these rock types may also show a large potential to extract geothermal energy from the subsurface. Within the framework of the European Union’s Horizon 2020 initiative ‘MEET (Multi-Sites EGS Demonstration)’, we are conducting fracture permeability experiments at elevated confining pressures, p_c, temperatures, T, and differential stresses, 𝜎, representing in situ conditions, to investigate the suitability of shale-type reservoir rocks for (unconventional) EGS.

The investigated samples are prepared with a saw-cut of given roughness allowing to observe the evolution of fracture permeability with time under pre-defined p_c-T conditions. The studied Wissenbach slates are of Variscan age and recovered from the ‘Hahnenklee’ drill site, which is located in the Harz mountains (GER). Microstructural observations using high resolution scanning electron microscopy (SEM) on polished thin sections in addition to Energy dispersive X-ray diffraction (EDX) reveal a fine grained, anisotropic slate matrix with a typical grain size of d ≤ 50 µm, mainly composed of mechanically weak phyllosilicates, intermediate strong carbonates and strong quartz minerals. He-pycnometry yielded average porosity values of 𝜙_He = 1-2 %. Results of constant strain rate deformation experiments conducted at p_c = 50 MPa, T = 100°C prior to fracture conductivity test displayed mainly brittle deformation behavior of the slate matrix with substantially higher strength if samples were loaded perpendicular to the visible bedding orientation rather than parallel to it.

With increasing p_c and 𝜎 (acting perpendicular to the fracture surfaces), fracture permeability, k, of saw-cut bearing sample assemblies is decreasing. The data suggest that k approaches a minimum value at high p_c (> 45 MPa) and 𝜎 (> 20 MPa), potentially induced by a change from fluid flow over the whole fracture surface towards flow through localized channels at high p_c-𝜎 conditions. Comparing the initial and final roughness of the fracture surfaces after deformation reveals that the amount of surface asperities is reduced. The influence of increasing temperatures is relatively low as k remains nearly constant up to 100°C after an initial drop from 20 to 40°C.

Our results suggest that slates may be considered as a potential host rocks for (unconventional) EGS as the influence of mechanical in situ boundary conditions is relatively low. Additional test will be performed, focusing on the influence of fracture roughness, proppant embedment behavior, shear deformation and fluid composition on fracture permeability evolution.