Integrated MT and Natural State Temperature Interpretation for a Conceptual Model Supporting Reservoir Numerical Modelling and Well Targeting at the Rotokawa Geothermal Field, New Zealand

Abstract

Recent Rotokawa Geothermal Field case histories illustrate the value of combined interpretation of natural state temperature, MT-TEM resistivity, geology and hydrothermal alteration for building conceptual understanding at the development stage of a geothermal reservoir. About 160 MW of generation is currently operated at Rotokawa by Rotokawa Joint Venture, a partnership between Mighty River Power and Tauhara North No. 2 Trust. Important conceptual elements that can be clarified in joint analysis of MT and natural state temperatures, beyond what is typically feasible based on MT exploration resistivity imaging of the geometry of the smectite clay cap, include; details of fluid upflow/outflow, influx and cross-flow paths of cooler groundwater, and permeability variations at the margins of the reservoir. As part of a revision of the Rotokawa conceptual and numerical simulation models, 1D and 3D imaging of over 80 MT stations was combined with natural state temperature interpretations based on numerous pressure-temperature-spinner logs from 32 wells at Rotokawa. Geological information, particularly formation types, Methylene Blue (MeB) smectite clay analyses, and more general alteration mineralogy, was considered in both the MT and natural state temperature interpretation in an iterative process. Interpreted temperatures were initially reconciled between adjacent wells and then interpolated and extrapolated by hand contouring, using the 1D (TE-mode) and 3D resistivity cross-sections and maps as a guide. The product was a general 3D conceptual model of the field that was then reconciled with other geology, geochemistry, geophysics and reservoir engineering data to form a more detailed 3D conceptual model. This conceptual model is being used to inform decisions regarding the numerical model of the reservoir, reservoir management and well targeting.