Broadlands— A gas-dominated geothermal field

Abstract

Broadlands geothermal field is a hot-water system containing a few per cent of non-condensable gas (carbon dioxide). This small fraction of gas makes the field response markedly different from a conventional hot-water system like Wairakei. The gas pressures cause boiling at depths of about 2 km, so that bores exploit only the two-phase zone. The initial pressure response to exploitation is dominated by changes in gas pressures, so that conventional resource assessments are misleading. The fact that bores exploit the two-phase region means that they exploit only a confined area near a bore, and so are very sensitive to local variations in permeability. This contrasts with Wairakei experience. Most presently drilled good bores are in a confined sector of the field. This paper presents a model of the response of this subfield to exploitation, and its recovery.