1 New Zealand Geothermal Workshop 2015 Proceedings 18-20 November 2015 Taupo, New Zealand ROTOKAWA RESERVOIR TRACER TEST HISTORY S.J. Addison 1 , J.A. Winick 2 , B.W. Mountain 3 and F.L. Siega 1 1 Mighty River Power Limited, PO Box 245, Rotorua 3010, New Zealand 2 Schlumberger Water Services, Colorado, USA 3 GNS Science, Wairakei Research Centre, Taupo, New Zealand simon.addison@mightyriver.co.nz Keywords: Rotokawa, Tracer Test, Iodine, NSA, NDSA, Radioisotope, I-125 ABSTRACT 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 five reservoir tracer tests have been conducted. In 1998 Iodine-125 was administered to the main injection line and subsequently down three separate injection wells. In 2006 naphthalene sulfonates were used for the first time on the field with four isomers injected into two wells (1,5-, 1,6-, 2,6- and 2,7-NDSA; two isomers per well). In 2009 1,5-NDSA was injected into one well following re- alignment of injection in the southern part of the field. Following the Nga Awa Purua development a tracer test was conducted in 2011 using four isomers of naphthalene sulfonates (1,5-, 1,6-, 2,6- and 2,7-NDSA) in four separate injection wells. None of these tracers were detected in the field study. Autoclave testing by Mountain and Winick (2012) found these isomers were not thermally stable and samples from the 2011 test were reanalyzed for predicted breakdown products (2-NSA and 1-NSA). 2-NSA was found in a number of samples confirming suspicions around temperature instability. In 2013 Iodine-125 was injected alongside 2-NSA into the main brine injection well in the field to benchmark 2-NSA performance. This paper will discuss each reservoir tracer test, focusing on the field configuration and findings. INTRODUCTION The Rotokawa geothermal field is located within the Taupo Volcanic Zone (TVZ), on the North Island of New Zealand (Figure 1). The first exploration wells in Rotokawa were drilled in the 1960’s with further drilling and testing until 1997 when the field was first developed for power generation with the 24 MWe Rotokawa power station. In 2000, Mighty River Power and Tauhara North No.2 Trust formed the Rotokawa Joint Venture and the Rotokawa power station was expanded to 34 MWe in 2003. In 2010 the 138 MWe Nga Awa Purua power station was commissioned, a triple-flash plant with a dual-flow single-shaft turbine and a direct-contact condenser. As of 2015, the field generates steam from 13 production wells and disposes of waste brines and condensates to 5 deep injection wells. The natural-state chemistry of the reservoir is summarised in Winick et al. (2011a) and Winick (2011b), with insight from Hedenquist et al. (1988). Current conceptual understanding is given in Sewell et al. (2015b), which details fault structures, in particular the Production Field Fault and the Central Field Fault. Geochemical response to production is given in Addison et al. (2015) and early response to production is detailed in Winick (2013). Figure 1: Location of known geothermal fields in the Taupo Volcanic Zone (TVZ) on the North Island of New Zealand as identified by Schlumberger resistivity surveys (Bibby et al., 1995). The Rotokawa field (bold) is approximately 12 km NE of Taupo, 10 km east of Wairakei geothermal field and 10 km south of the Ngatamariki geothermal field. This paper presents the details of five reservoir tracer tests conducted on the Rotokawa geothermal field. These tests have used a range of tracers, including a radioisotope of iodine and various isomers from the naphthalene sulfonate tracer suite. Significant insights into field performance and on the tracers themselves has been obtained over these five tests, with a key learning being that naphthalene sulfonates appear to have limited suitability for the Rotokawa geothermal field due to their apparent temperature instability.