Geochemical Monitoring Before and During Six Years of Power Generation at Ngawha, New Zealand

Abstract

We report the results of an eight-year monitoring programme at Ngawha and examine the data to assess the nature of the reservoir fluids and whether any changes have occurred. The main production reservoir contains liquid at 230?C, 1250 mg/kg chloride, and 1.4 wt% carbon dioxide. Three typical areas of spring discharge are found viz: Spa, Waiariki and Tiger. The highest chloride water (1180 mg/kg) is observed at Jubilee Bath in the Spa area, evidence of the least dilution of the deep water. Waiariki baths (e.g. Universal contains 260 mg/kg) are more dilute and are classified as bicarbonate water. Tiger bath (400 mg/kg Cl and 930 mg/kg SO4) is typical of acid sulphate springs where the deep water is diluted with water containing sulphate formed from oxidation of hydrogen sulphide.
From July 1998 to March 2004, 20 Mt of fluid have been produced by the wells (and reinjected). Linear regression of chemical concentrations are used to determine trends with time. Although some of the data show a large scatter, observed changes are consistent with less deep water and/or more steam and gas reaching the surface features. The production wells, NG9 and NG12 are now discharging some reinjected fluid.