Operational History of the Ohaaki Geothermal Field, New Zealand

Abstract

The 116 MW Ohaaki geothermal field has completed about ten years of commercial production. Reservoir changes since commissioning have been dominated by enthalpy decline caused by the ingress of shallow groundwater from the overlying Ohaaki Rhyolite Formation. This has been most pronounced in shallow cased wells that are open to this formation. A consequence of cold water ingress has been an increase in calcite scaling which has required the installation of antiscalant dosing systems. Separated water injected on the margins of the field has returned to a number of the production wells ñ movement of the fluid appears to be along structurally controlled routes. Three wells have shown major returns. A deep exploration programme was undertaken in 1995. Three deep wells were drilled to cross the regional structural trend, seeking to locate permeable fractures and zones within the greywacke below the volcanic formations. The wells were drilled with nominal drilled depths of 2.5km and deviation angles of 45? from vertical. Feed zones in the volcanic formations above the greywacke encountered temperatures up to 300?C. This paper reviews the changes in the reservoir over the period of commercial operation and describes the future production expectations from the Ohaaki geothermal field.