Hydraulic conductivity of the crystalline crust: Insights from hydraulic stimulation and induced seismicity of an enhanced geothermal system pilot reservoir at 6 km depth, Espoo, southern Finland

Abstract

We analyze hydraulic stimulation data from the St1 Deep Heat Project, which comprised drilling wells to >6 km depth in crystalline rock in southern Finland with an aim to study the conditions for an Enhanced Geothermal System. Combining the stimulation and borehole logging data with induced seismicity results allows for a comprehensive analysis of hydraulic conductivity at rarely tested depths of 5 – 6 km. Hydraulic conductivity was observed to be pressure-dependent, heterogeneously distributed and affected by pre-existing fractures and lithological variation. The pre-stimulation formation fracture frequency is in the range of 0 – 8 m−1 with an average of 1.9 m−1. Only a small fraction of the fractures were open and conductive implying a hydraulically heterogeneous medium. The average natural hydraulic conductivity derived from leak-off tests and well tests before stimulation and from cross-hole pressure data is of the order of 5 0·10−10 - 5 0·10−9 m/s (permeability 1· 10−17 – 1·10−16 m2), which is in agreement with permeability models for the brittle crystalline crust at this depth. Hydraulic stimulation increased conductivity to 10−8 – 10−7 m/s, but it gradually decreased back to the natural level after pressure release. Stimulation generated five micro-earthquake clusters at 4.8 – 6.3 km TVD depth. Hydraulic connections between clusters were apparently not attained. The project generated extensive experience and data sets regarding deep drilling, hydrogeological properties, and seismic response to stimulation of crystalline rock in the upper continental crust. Hydraulic conductivity turned out to be the most challenging issue for the St1 EGS development and the EGS project is not continued at the moment.