Analyses and Modelling of Reservoir Pressure Changes to Interpret the Rotokawa Geothermal Field Response to Nga Awa Purua Power Station Operation

Abstract

The Rotokawa Geothermal Field has recently increased its fluid extraction by about 400% to provide steam supply to the new 140 MW Nga Awa Purua (NAP) power station. This paper analyzes the reservoir pressure response to the increased take as observed in the continuous downhole pressure monitoring wells and reservoir pressure data from downhole pressure and temperature (PT) logs. The data sets analyzed cover the period from the start-up of NAP in 2010 to the start of a reservoir pressure pseudo-steady state in 2012. Reservoir pressure data showed a strong correlation with the increased production, showing initial high pressure decline followed by a transition to a stable pressure decline rate. Periodic pressure data from downhole PT logs of active wells showed differences in absolute pressure drawdown over the small production area, suggesting heterogeneous permeability and/or strong pressure controls across the field. The difference in pressure drawdown between groups of wells has been interpreted as semi-permeable compartment behaviour. Simple models were used to match the reservoir and compartment pressure responses using Saphir© well test software. The relationship between compartment behaviour and the overall reservoir pressure forecast was also investigated.