The official opening of Contact Energy’s $623 million Te Mihi power station last August marked the completion of a significant development that has made the Wairakei field the world’s seventh-largest geothermal generation site.
The seven-year programme involved more than 3.1 million work-hours. The company can now deliver 431 MW from five geothermal power stations – sufficient to power 400,000 homes.
Importantly the development has lowered the average cost of generation from the Wairakei field, improved the flexibility of the operation, and reduced its impact on the environment.
The significant elements of the programme were the 166 MW Te Mihi power station, the award-winning Wairakei Bioreactor, which was commissioned in 2012 to reduce the amount of hydrogen sulphide entering the Waikato River, as well as the drilling a number of new wells and expanding the Wairakei steamfield.
The field now hosts more than 90 wells and 100 kilometres of piping to support its five power stations.
The need to integrate a new power station, wells, steam separators and pipelines into a live and existing steam field was challenging.
Careful planning and flexible design was necessary to ensure Te Mihi’s development didn’t impact on day-to-day production, nor the longer term supply for the company’s existing Wairakei and Poihipi stations.
The steamfield has been designed to provide Contact the flexibility to move its daily take of geothermal fluid between the three stations to maximise the utilisation of available fluid when elements of stations or the steamfield are on maintenance outages.
Should the increased take the company has been allowed prove sustainable longer term, Contact also has the option to prolong the life of the Wairakei plant or add a third unit at Te Mihi.
Given the complexity of the site, Contact deployed a number of health and safety initiatives to minimise risk to staff. It achieved a period of more than two million work hours without a lost time injury.
The company also sought innovations to optimise its use of the resource. Dual flash technology means Te Mihi produces about 25 per cent more electricity than the Wairākei plant for the same volume of geothermal fluid used.
Acid dosing at Te Mihi also prevents precipitation of silica in the low-temperature double flashed and separated geothermal water.
Working with a contractor, Contact was also the first company in New Zealand to utilise broaching, a wireline intervention technique to remove calcite and scale in well casings. After applying this technique to 14 geothermal wells, Contact regained 22 MW of production for less than 5 per cent of the typical cost of completing four well workovers using a drilling rig.
Contact says the construction of Te Mihi contributed about $60 million directly to the New Zealand economy and the development will remain an important source of on-going employment through its operation and maintenance. The project has also contributed to an exponential growth of expertise within the firm.
The Energy Project of the Year award category is sponsored by ITL