Integrated Exploration Strategy ‘ConvEx’ to Detect Hydrothermal Convection in the Subsurface

Abstract

Hydrothermal convection zones can strongly affect the subsurface temperature at reservoir depth and are therefore a major target for geothermal exploitation of low-enthalpy hydrothermal systems. However, their detection is still a challenging task. The standard exploration methods as e.g 2D/3D seismics alone are insufficient to localize convection cells. Furthermore, the mapping of convection cells along major fault zones is complicated by laterally varying fault zone permeability governed by fault activity and orientation with respect to the in-situ stress field. The intensity of hydrothermal convection expresses itself as thermal anomalies and in case of highly saline brines also by a shift in electrical resistivity so that the integration of electromagnetic surveying methods (MT, EM, CSEM) may provide the missing link. The low resolution and non-unique results of EM methods can be overcome by integration of detailed structural models based on high-resolution seismic exploration data, detailed analysis of petrophysical properties and stratigraphic-sedimentary basin-analysis. We will present an integrated modeling approach, which combines a) high-resolution 3D seismic with b) low-resolution EM exploration data (MT, EM and CSEM), c) rock properties derived from lab investigations on cores from reservoir depth as well as from borehole geophysical logs and d) fluid properties. The envisaged thermo-hydraulic-mechanical numerical 3D models will be validated by high-accuracy geothermal gradients determined in medium-deep gradient wells and enable us to map convection cells, where hot brines are ascending and thus control targets for future exploitation. We present our approach for a local case study in the northern Upper Rhine Graben, Germany, where a detailed 3D model based on 3D seismic data as well as various deep exploration wells are available and indicate positive and negative geothermal anomalies and thus hydrothermal convection along a major fault. This data set will be complemented by new exploration data (EM and thermal gradients) to test and validate our exploration strategy and modelling approach, which eventually shall result in a best-practise exploration workflow applicable to similar geothermal play types elsewhere.