Kriging predictions of drill-hole stratigraphy and temperature data from the Wairakei geothermal field, New Zealand: Implications for conceptual modeling

Abstract

Drill-hole temperature and stratigraphic datasets from the Wairakei geothermal field were used for geostatistical predictions using Kriging. In order to adequately constrain Kriging models, anisotropy and trends associated with temperature and stratigraphy were studied using standard variogram analysis, in combination with new regional and local structural data, revised gravity, and available geoscientific and reservoir data. This combined analysis lead to the incorporation of horizontal anisotropy (horizontal to vertical correlation ranging from 8:1 for regional stratigraphic units to 4:1 for local rhyolite bodies) in the case of stratigraphic models and variable anisotropy in the case of temperature models. In the latter, the variable anisotropy was represented by two end members: an isotropic model (horizontal to vertical correlation of 1:1) representative of depths >2000mGL, and an anisotropic model (horizontal to vertical correlation of 3:1) representative of depths <1000mGL. Kriging models of temperature also incorporated a vertical trend which is a combination of two end members at Wairakei: Boiling-Depth-Point Curve (convective) and linear (conductive). The Kriging models succeeded in identifying the primary geological controls on temperature distribution: major upflows largely controlled by structures at depth (>1000m depth) and shallow (<1000m depth) outflows stratigraphically channelled through formation contacts and rhyolite edges. A combination of stratigraphy and faults explain local cold downflows in shallow (750–1000m depth) parts of the field.