MEASURING HEAT FLUX DYNAMICS THROUGH THE CLAY CAP IN THE WAIRAKEI-TAUHARA GEOTHERMAL FIELD

Abstract

This paper develops a multidisciplinary analysis of the extent, stratigraphic context, and thermal structure of the hydrothermally altered clay cap in the Wairakei-Tauhara geothermal field of New Zealand. We recovered unprecedented information on the clay cap geometry and heat transfer dynamics that helps in the understanding of these complex hydrothermal systems.
First, using a joint inversion of magnetotelluric (MT) and methylene-blue (MeB) data, we imaged a ~300 m thick conductive clay cap that tracks the primary aquiclude overlying a distinct reservoir unit. Then, by mapping wells temperature and lithology into this structure, we differentiated contemporary hydrothermal clays from relict clays as well as those formed under diagenetic alteration. Also, we confirmed a broad temperature formation range of between 59±15 °C and 199±20 °C for the electrically conductive smectite clay. Finally, by applying a simple heat transfer model that captures vertical conductive and advective heat flow through the inferred clay cap, we estimated a lower bound of 380±21 MW for the system heat output.
Additionally, we tested the incorporation of the clay cap inferred from MT inversions into geothermal reservoir simulations. The inferred clay cap was mapped to a reservoir permeability model to simulate temperatures with the reservoir simulator AUTOUGH2. Modelled temperatures were then compared to observed temperature logs as well as prior models using standard techniques (i.e., without MT information). We applied this scheme to a calibrated permeability model for the Wairakei-Tauhara geothermal field. Results indicated that our inclusion of the low permeability structure led to reductions in the model misfit to temperature logs.
The developed methods allow studying uncertainties when inferring clay cap properties in high-temperature geothermal fields. Imaging the clay cap serves as a guide for developing conceptual models and for defining drilling targets, so the uncertainty inferred for these estimations is of great importance.