Changes in heat flow associated with exploitation of Wairakei Geothermal Field, New Zealand

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Abstract
From measurements made during 1978-79 the total surface heat discharge from Wairakei hydrothermal system has been determined to have been 600 ± 150 MW. This paper describes and discusses the measurements leading to this result and compares it with the results of earlier surveys. The history of mass and heat flows since exploitation began in 1951 is discussed, and a model describing inferred changes within the 2-phase zone is presented. The analysis supports the conclusion that, while total mass flow appears now to be approaching equilibrium, approximately 800 MW of the total heat discharge continues to be derived from falling reservoir temperatures within the 2-phase zone. The 1978–79 heat flow survey has also revealed a 1 km2 area between the production borefield and Geyser Valley where almost complete cessation of thermal activity has occurred. Faults through this area, which were originally conduits for the ascending hot water that emerged in Geyser Valley, may now be permitting downward infiltration of cold surface water.

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