3 - Geophysics and resource conceptual models in geothermal exploration and development

Abstract

The application of geophysics to geothermal resource exploration and development is more effective when it is directed at constraining an integrated resource conceptual model, not at identifying anomalies to target or associate with resource capacity. A conceptual model approach promotes the interpretation of geophysics in the context of all available geoscience information. The relevant geophysical applications depend on context: magnetotelluric resistivity surveys are the default exploration method for imaging the impermeable clay cap over volcano-hosted reservoirs and MT is frequently effective for imaging shale caps of sedimentary reservoirs. However, for deep stratigraphic geothermal targets like the Malm carbonate reservoirs of Bavaria, high-quality three-dimensional seismic reflection is essential for targeting wells. For development of high-temperature geothermal reservoirs, exploration geophysical methods are supplemented by precision gravity to monitor boiling and phase change and by earthquake monitoring to track injection fluid movement, characterize the structural compartments of reservoirs, and predict the depth to the base of the permeable reservoir. In undrilled prospects that lack surface manifestations analogous to those emphasized in the exploration risk assessments of previously developed geothermal fields, the value of geophysical methods in reducing the risk of targeting wells is more difficult to assess, but a subjective Bayesian conceptual model approach to the assessment of candidate strategies is recommended.