Exploring Structure and Stress from Depth to Surface in the Wairakei Geothermal Field, New Zealand

Abstract

The Wairakei Geothermal Field is an important New Zealand energy resource providing electricity to the country since 1958. Knowledge of the structure, stress, and stratigraphy of the field has improved by utilizing geo-scientific data acquired from drilled wells, field studies of the fault systems, geophysical techniques such as magnetotellurics and microseismic monitoring, and borehole image logging. The new insights from this knowledge have been essential in understanding how fluids are transported through the reservoir rocks of this geothermal resource. The permeability of the reservoir rocks in the Wairakei Geothermal Field is controlled by a combination of intrinsic porosity and fluid flow along faults and fractures. At depth (~ more than 3km) heat and fluid flow from the geothermal source is controlled by faulting in the greywacke basement that underlies the Wairakei Geothermal Field. As structure plays such a vital role in the fluid flow character of this resource it is important to characterize the fault and fracture network properties and how structure interacts with the in-situ stress field. Current research is focused on the Te Mihi sector of the Wairakei Geothermal Field, the location of the newly constructed 166 MW Te Mihi Power Station. This research project concentrates on characterizing finer scale structure and stress heterogeneity within the reservoir using borehole image logs, linking this information to detailed mapping of active faults networks at the surface, and larger scale structures at depth identified from detailed analysis of microseismicity. Once this work is completed it will provide information on how to use joint structural characterization methodologies and techniques which can be brought to bear on data from other sectors of the Wairakei Geothermal Field and other geothermal resources elsewhere. This paper synthesizes the current state of knowledge on the structure and stress of the Wairakei Geothermal Field across a range of scales from datasets including borehole image logs, spatial distribution of seismicity, and surface mapping and identification of active faulting. In addition we present the scope of the current Te Mihi sector research project and its preliminary findings.