Crosswell electromagnetic and magnetotelluric imaging of geothermal reservoirs – evaluation and case studies from Switzerland

Abstract:

Geothermal energy resources are considered important contributors to any future energy mix, as they are renewable and can potentially provide a constant (i.e., baseload) and long-term energy output. However, most regions worldwide are lacking natural, convective hydrothermal resources. As a result, when relatively high subsurface temperatures exist in low-permeability rocks (“hot-dry” rocks), it has been suggested to artificially increase permeability to enable deep fluid circulation and associated advective heat transfer to a production well. One technique to enhance permeability, and thus the ability to extract geothermal heat, is hydraulic stimulation, creating (permeability) enhanced geothermal systems (EGS). In the “Deep Underground Geothermal (DUG)“ laboratory in Switzerland, a meso-scale field experiment is planned, where pre-existing fault systems are hydraulically sheared. The aim of this experiment is to better understand the processes occurring during artificial reservoir creation.

We present our 3D numerical modeling study evaluating the capability of low-frequency crosswell electromagnetic (EM) tomography using magnetic dipole sources to map stimulation-induced changes in electrical conductivity. This geophysical parameter is affected by several subsurface properties including temperature, interconnected porosity, permeability, and the presence of fluids. Electrical conductivity thus provides important information on the effectiveness of geothermal reservoir creation. Besides numerical modeling studies, we report on the current status of instrumentation and realization of crosswell EM measurements at the DUG laboratory.

Furthermore, we present preliminary results of a magnetotelluric (MT) survey conducted at a prominent heat flow anomaly in Northern Switzerland. Here, we test methods to improve data quality of MT measurements in regions that exhibit substantial electromagnetic noise. We also discuss how information on the electrical conductivity distribution may be used to constrain temperature fields at depth.