Early Identification and Management of Calcite Deposition in the Ngatamariki Geothermal Field, New Zealand

Abstract

The 82MW Ngatamariki geothermal power station started operating in 2013. Within a few months, two out of three in-service production wells were observed to decline faster than expected. A decline investigation plan using elements of the reservoir surveillance and monitoring program was carried out to investigate the cause of the decline and to plan the necessary tests. Investigation results identified calcite scaling in the wellbore as a main cause. The occurrence of calcite was not anticipated because pre-development assessment results found the calcite risk to be low. It was also found that a calcite saturation index (CSI) greater than 1 (oversaturated with respect to calcite) is not a reliable indicator to predict calcite deposition in Ngatamariki wells. The uncertainty in calcite deposition resulted in a broad approach to calcite management. It is recommended that additional work be carried out to improve the understanding of calcite deposition mechanisms, improve the design of downhole chemical injection system, and test calcite chemical inhibitors to optimise the calcite management plan.