Pressures in the Wairakei Geothermal Field in Its Natural State

Abstract

The near-surface pressure distribution in the Wairakei Geothermal Field prior to exploitation is estimated from early bore hole measurements and drilling logs. Pressure distributions in a bore were rarely measured but can be calculated from the recorded temperature profiles. For a number of bores, a depth and corresponding pressure was estimated, to associate with the undisturbed field conditions,by examining drilling logs for major circulation losses in uncased sections and temperature patterns for internal flows between feeding fissures. In the shallower boiling zone of the field, some points of rapid temperature rise were found in cased sections of bores. These were interpreted as being heated by flow of boiling fluid in fissures outside the casing, and pressures were obtained for these depths from the highest temperatures recorded at them, assuming the fluid to be water at its boiling point.