The Rotokawa Geothermal Field has been under development since 1997 with the commissioning of the Rotokawa Power Station and the 2010 commissioning of the Nga Awa Purua Power Station. Since the field has been developed six reservoir tracer tests have been conducted. Recent deep reservoir tracer tests (2013 and 2015) have made use of Iodine-125 (I125) due to the high reservoir temperatures ( more than 320°C) breaking down naphthalene sulfonates, with injection into RK24 (in 2013) and RK22 (in 2015). Results observed from the 2013 tracer test confirmed what wells observed tracer returns, which was informative alongside well geochemistry observations, however a 12-month tracer test was all that was achievable with the injection of 16.1 GBq of I125. Besides practical learnings around project management and injection methodology, the 2013 tracer test results assisted with improvements in the experimental design for the 2015 test which included increasing the activity of tracer injected (to 200 GBq of I125), increasing count times and having the ability to obtain larger samples. All these options were based around increasing the resolution to enable better identification of the tracer peak and to increase the ability to determine the tracer tail shape. The results to-date from the 2015 tracer test indicate that injection fluids move in a similar pathway to those observed in the 2013 tracer test, albeit with a slower first return, with returns being consistent with the presence of compartments within the reservoir. With injection into RK22 the reservoir tracer test results have given confidence that the risk of cooling from rapid injection returns is very low and it has assisted with conceptual understanding of the field which will be discussed in this paper.

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