| Title | A Resource Conceptual Model for the Ngatamariki Geothermal Field Based on Recent Exploration Well Drilling and 3D MT Resistivity Imaging |
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| Authors | Catherine Boseley, William Cumming, Luis Urzua-Monsalve, Tom Powell and Malcolm Grant |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | Ngatamariki, MT, conceptual model, alteration, hydrology, geothermal exploration |
| Abstract | Three successful exploration step-out wells drilled by Rotokawa Joint Venture (RJV) in 2008-2009 at the Ngatamariki Geothermal Field helped demonstrate sufficient resource to support regulatory applications for over 100 MW of generation. This field was discovered in 1984 when four wells were drilled and two wells, about 1 km apart, encountered a permeable 280ºC resource. However, economic and access complications halted further drilling. After initial access was obtained in 2004, RJV conducted MT surveys and completed risk assessments based on a range of conceptual models that supported relatively aggressive well targeting 1.6 to 3 km south of the earlier wells. The resource conceptual models were constrained by integrated interpretations that focused on well results, geochemistry and MT resistivity. The geochemistry from hot springs and well fluids indicated that the hot springs were supplied through a 160ºC intermediate aquifer, not directly from the reservoir. The natural state temperature pattern, flow performance, alteration, and geology data from the wells established the elements of the model that were extrapolated beyond the drilled area using the MT imaging of aquicludes that contained low resistivity smectite clay associated with low permeability. The base-case resource model used in 2008 to target the wells included five main elements. Two smectite-rich aquicludes separated three permeable zones; a 15ºC meteoric aquifer, a 160ºC intermediate aquifer near the initial wells, and a deep 260 to 300ºC geothermal reservoir that extended 3 km south of the initial wells. The basic conceptual scheme was confirmed as new well test data became available and as the resistivity imaging was updated based on revised TDEM corrections to 1D MT inversions and new 3D MT inversions of larger data sets. New details included a reinterpreted deep fractured upflow 1 km north of its initial location and a thin 150ºC permeable zone interpreted within the clay cap. The resource conceptual model presented here was the basis for further reservoir analyses and the numerical simulation of development options such as injection targeting to optimize long term performance. |