Criteria and geological setting for the generic geothermal underground research laboratory, GEOLAB

Abstract

High flow rate injection and related hydromechanical interaction are the most important factors in reservoir development of Enhanced Geothermal Systems (EGS). GeoLaB, a new generic geothermal underground research laboratory (URL), is proposed for controlled high flow rate experiments (CHFE) to address limited comprehension of coupled processes connected to EGS reservoir flow conditions. As analogue for typical EGS development, CHFE require specific hydromechanical conditions including a connected fracture network in crystalline basement rock, sufficient hydraulic fracture transmissivities, a strike-slip to normal faulting tectonic regime, controllable hydraulic boundary conditions, and hydrothermal alteration fracture fillings that improve conditions for hydromechanical interaction. With the aim to identify most appropriate areas for future site selection, four criteria have been established based on the EGS reference site of Soultz. Two URLs in crystalline basement worldwide approximate the requirements of a new generic GeoLaB and may be used for accompanying experimentation. Besides favourable geological, hydraulic, and stress conditions, the vicinity to long-term EGS production favours the southern Black Forest as potential region for GeoLaB. Therefore, an exemplary site assessment has been carried out at "Wilhelminenstollen" in the southern Black Forest (Germany). New remote sensing, hydrochemical, and geophysical analyses as well as reactivation potential, and stress modelling were added to complement existing geological and hydrogeological information. At this site, reactivation potential analysis reveals two local maxima prone for shear reactivation as strike-slip faults. The highest lineament density is observed for the N110°E strike direction that is associated with both slip and dilation tendency maxima. Clay minerals occur in fractures and the matrix. Local, partly water-bearing fractures, when partly filled with ore minerals, were connected to veins in the tunnel using shallow geophysical methods. Hydrochemical data reveal infiltration of the tunnel water from at least 500 m above the tunnel. The results suggest a crystalline basement with a fracture network that is regionally connected and water-conducting. Hydraulic conductivity in the southern Black Forest granite is estimated to amount to about 4.5·10 −8 m s −1 at 500 m depth. The hydraulic boundary conditions exclude unknown drainage. Analyses of the influence of topography on orientation and magnitude of the maximum stress indicate a minimum overburden of about 500 m for regional reactivation to be valid. In conclusion, the southern Black Forest and in particular "Wilhelminenstollen" offers favourable condition for CHFE. Final decision on the GeoLaB site is to be drawn from forthcoming exploration wells.

Experiments and Analysis of the Hydraulic Fracture Propagation Behaviors of the Granite with Structural Planes in the Gonghe Basin

“…According to the Eq. ( 6), the tensile strength of quartz is generally greater than that of the granite matrix (Schill et al, 2016). Meanwhile, the QVs are the intrusive rocks in the granite; therefore, the cementation strength of the QVs and the granite matrix is relatively low.…”

“…The influence of the rock property difference on both sides of the LI has not been fully considered. Schill et al (2016) pointed out in their research that QVs are very common SPs in granite. It can be seen from Fig.…”

Integrated Research as Key to the Development of a Sustainable Geothermal Energy Technology

“…With respect to the development of exploration approaches for EGS in the central European crystalline basement, further MT studies were performed in the granitic basement in the Southern Black Forest, where the basement is outcropping at the surface. This area is also one of the possible sites for GeoLaB (see Section 4.2). Differences in 3 H and 14 C contents for Ca−Na−HCO 3 ‐type water in the springs in the Southern Black Forest, compared with Na−SO 4 −HCO 3 −Cl‐type water in northern Switzerland, led to the interpretation that fluids in the crystalline basement of northern Switzerland were recharged in the Black Forest .…”

“…GeoLaB is representative of the world's most widespread geothermal reservoir rock, the crystalline basement. The first‐order suitability of a site for GeoLaB located in the Black Forest or the Odenwald is proven …”

“…The first-order suitability of as ite for GeoLaB located in the Black Forest or the Odenwald is proven. [17] At the selected site,a1km long gallery will be excavated, tapping individual caverns,f rom which CHFEs will be conducteda td epths of about4 00 m. Thee xperiments will be continuously monitored from multiple wells that are drilled from the underground laboratory or from the surface.T his will create au nique 4D benchmark datasetofT HMC parameters.T he experimental infrastructure GeoLaB isp lanned as au nique global facilityt os pecifically address the main challengeso fE GSs (Figure 43), which are as follows: 1) High-flow experiments to better understandf luid flow in fractured media;t his will enhance the predictability of geothermal reservoir and lead to better modeling. 2) Development and testing of smart stimulation technologies;t his will lead to higher flow rates with reduced impactfor the public.…”