Monitoring the injection capacity of the injection wells over time is a key element informing the injection strategy of the Kawerau Geothermal Limited (KGL) plant. Contrary to the expected stimulation behavior of the injection wells due to thermal stimulation, the KGL injection wells have displayed a decrease in injection capacity under brine injection since the start of the KGL plant in 2008. Building on the studies and strategies that have been implemented since 2008 to monitor this decline, three methods have been routinely used. Method 1 and Method 2 only use surface production data, Method 3 uses the information from pressure tubings installed into the wells A and B in 2011 and 2013 respectively. The three methods were compared on well A and B to inform the 2016 injection strategy. The comparison focused on the decline rate that each of these methods provide, the assumptions needed to apply each method, and the advantages of each method for the monitoring of the injection decline. Method 1 relies on mass flow normalisation, method 2 and method 3 rely on the Injectivity Index (II) calculated at the pivot point of the wells. Method 1 became limited for well A and well B when the well head pressure increased steadily in 2015. Method 2 provides a more complete data set than Method 1 but still requires the wellhead pressure to be higher than the saturated pressure to give valid results. Method 3 accounts for whatever depth the liquid level is at but is sensitive to the recharge of Nitrogen into the tubing. The decline rate obtained with Method 2 and Method 3 on well A was almost four times the value obtained with Method 1. One challenge was thus to translate the variation of this downhole parameter to variation of injection capacity at surface. Following this study, the injection monitoring plan was modified with the following key changes: 1) a fourth method was created to link II evolution with forecast injection capacity and 2) the use of pressure tubing was modified to balance the value of information provided with the risks and downsides of downhole instruments continuously present in active wells (i.e. tubing failure, extra cost for tubing pull when other downhole surveys needed). This adaptive field monitoring to the changing behavior of the injection wells is a process that is being continued to ensure the adequacy between the injection capacity of the wells and the injection requirement of the plant.

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