New Methods of Determining Rock Properties for Geothermal Reservoir Characterization

The influence of hydrothermal alteration on permeability, thermal conductivity and thermal diffusivity was investigated for more than 300 drill cores from the wells THM18, TH18 and THM19 of the Tauhara Geothermal Field (Wairakei, New Zealand). The measurements were performed with newly developed, portable laboratory devices. The anisotropic, intrinsic permeability was measured with a gas pressure Columnar-Permeameter, while the thermal conductivity and thermal diffusivity were measured with a device based on the optical scanning method. The hydrothermal alteration rank (argillic or propylitic) was determined semi-quantitative by methylene blue dye adsorption tests in combination with thin section analyses. Samples from the Huka Fall Formation and the Waiora Formation, composed of layered mud-, silt- and sandstones as well as pumice-rich tuffs deposited in a limnic environment as well as associated rhyolitic and andesitic intrusive rocks were examined. A prograde alteration with depth is indicated by an increasing amount of illite and the corresponding decrease of smectite. Generally lithologies of higher primary permeabilities are more affected by hydrothermal alteration. With an increase of secondary clay minerals the permeability decreases.