Operational History of the Ohaaki Geothermal Field, New Zealand

Abstract

The Ohaaki geothermal field is in its year of commercial production. Reservoir change since commissioning has been dominated by enthalpy decline in the West Rank reservoir caused by the ingress of relatively shallow groundwater from the overlying permeable Ohaaki Rhyolite formation. This has been most pronounced in shallow-cased wells that are open to this formation. A major consequence of cold water ingress has been significant increase in calcite scaling which has required the installation of continuous antiscalant dosing systems in some wells. The Bast Bank sector, less to shallow groundwater incursion, saw the development of two-phase reservoir conditions with enthalpy and gas contents peaking after three years and then gradually declining. Separated water injected on the margins of the field has returned to a number of the production wells movement of the fluid in some cases appears to he along narrow structurally controlled routes. Four wells have shown major returns but thermal degradation has occurred in only one. This paper describes the difficulties encountered in managing the field and how they have been mitigated.