Ngatamariki Project Update

Abstract

In 2008, Rotokawa Joint Venture (RJV) commenced exploration drilling in the Ngatamariki geothermal field to confirm sufficient resource capacity for a new geothermal generation project. This drilling included three ~3000m deep wells and 16 monitoring wells; four deep wells had previously been drilled in the 1980s by the New Zealand government. The recent exploration drilling has confirmed sufficient resource for a ~100MW development. The conceptual model for the resource comprises a 260-285°C geothermal reservoir beneath a deepening clay cap extending to about 3km south of the early wells, overlain by two separate groundwater aquifers. Thermal fluid from the geothermal reservoir upwells into the overlying groundwater aquifer, mixes with groundwater and rises to the surface in three hot spring areas. The most recent exploration well targeted the interpreted upflow to the system. However recent test results have revealed that the upflow was not intersected and therefore its exact location remains uncertain. Numerical modelling of the system indicated a delicate pressure balance between the geothermal reservoir and overlying groundwater aquifer, such that cold groundwater could flow down into the reservoir under certain production scenarios. This has significant implications for future development, and in order to better define the nature of the reservoir-groundwater connection a seven-well monitoring programme was initiated to determine: (1) the permeability of the overlying groundwater aquifer, and (2) to conduct an interference test between the aquifer and the deep reservoir. The results of this testing indicate high overall permeability within the aquifer and that most parts of the aquifer are well connected. No evidence for short term interference was found between the aquifer and the deep reservoir during well flow testing. The numerical reservoir model achieves a good match to the test results, giving confidence to future development planning.