Resistivity Imaging and Interpretation Strategies to Reduce Uncertainty in Geothermal Resource Capacity Estimation

Abstract:

Best practices in geothermal resource capacity assessment use probabilistic approaches based on conceptual model interpretations that interpolate and extrapolate temperature and permeability from wells using geology, geochemistry, and geophysics data. However, the most significant constraint on resource capacity prior to drilling is often limited to data from surface resistivity surveys. In geothermal exploration of volcanic areas, magnetotelluric (MT) resistivity imaging is commonly used to detect the low resistivity and low permeability smectite clay alteration that caps almost every volcano-hosted geothermal reservoir. Case histories of MT resistivity surveys and drilling results at geothermal fields in New Zealand illustrate the tendency to misestimate capacity if a direct correlation between low resistivity and producible reservoir area is assumed. Although improvements in MT noise mitigation and both 1D and 3D MT inversion have increased the resolution and reliable depth of investigation of resistivity imaging, the main source of uncertainty in applying resistivity imaging to resource capacity assessment is the conceptual ambiguity of the resistivity pattern with respect to reservoir properties. New Zealand case histories indicate that this uncertainty can be mitigated by a conceptually consistent integration of MT resistivity imaging with surface geoscience data, particularly structural indications of abrupt resource margins and geochemical and thermohydrodynamic constraints on upflow and outflow. Further case histories from geothermal fields in Chile illustrate the positive application of this conceptual model approach to geothermal resource capacity assessment.