Imaging of CO2 storage sites, geothermal reservoirs, and gas shales using controlled-source magnetotellurics: Modeling studies

To balance the steady decrease of conventional hydrocarbon resources, increased utilization of unconventional and new energy resources, such as shale gas and geothermal energy, is required. Also, the geological sequestration of carbon dioxide is being considered as a technology that may temporarily mitigate the effects of CO2 emission. Sites suitable for shale gas production, geothermal exploration, or CO2 sequestration are commonly characterized by electrical resistivities distinctly different from those of the surrounding rocks. Therefore, electromagnetic methods can be viable tools to help identify target sites suitable for exploration, and to monitor reservoirs during energy production or CO2 injection. Among the wide variety of electromagnetic methods available, controlled-source magnetotelluric (CSMT) may be particularly suitable because of (i) its ability to resolve both electrically resistive and conductive structures, (ii) controlled sources offering noise control and thus facilitating surveys in populated regions, and (iii) the potential of penetration throughout the depth range accessible by drilling. Nevertheless, CSMT has not yet been widely employed because of logistical challenges of field operations and the requirement of complex and highly computer-intensive data processing. With these difficulties gradually being mitigated by recent technological developments, CSMT may now be reconsidered as an exploration tool. Here, we investigate by 1D and 3D numerical simulations the feasibility of detecting gas shales and identifying sites eligible for geothermal exploration or CO2 sequestration from CSMT data. We consider surface-to-surface, borehole-to-surface, and cross-hole configurations of the sources and receivers. Results and conclusions on the detectability of the targets of interest are presented for various exploration and monitoring scenarios, which are roughly representative of the geological setting of the North German Basin.