The rig left the site of the Pohokura platform late last week and is now off the northeast South Island undergoing repairs and maintenance before heading back to Taranaki waters to drill the Awakino South-1 wildcat well for US independent Discovery Geo in August.

Despite initial problems in early 2006, which included seabed instability at the first wellsite necessitating a shift to a second location, the Ensco 56 drilling campaign proved successful, according to Shell NZ chairman Rob Jager.

"The 56, one of the best rigs Ensco has, performed very well," he told PetroleumNews.net.

Jager said that Pohokura had proved to be an innovative and world-class project and was a showcase development for Shell.

Shell and its partners drilled three onshore-offshore wells and initially planned another six wells offshore, but later found that five would be enough to deliver the reserves currently believed to be in place, he said.

The offshore wells were drilled using "splitter-designed" wellheads that minimised the number of conductors required - a first for New Zealand.

Shell had also used water-based, rather than synthetic, drilling muds and disposed of the muds and drilling cuttings at a local worm farm to minimise environmental impacts.

Now the bulk of New Zealand's gas supply for electricity generation for about the next 20 years would be controlled by a single person in Shell's New Plymouth offices, remotely operating the unmanned platform and unattended onshore production station.

"This too is a first for New Zealand," Jager said.

First gas, from the three deviated onshore-offshore wells, was produced in August 2006.

Field production was limited by the processing capacity of the onshore production station and usually only three of the eight wells were necessary to fulfil take or pay agreements with customers, Jager said.

He declined to specify the capacity of the onshore production station, but former project manager Milan Hendrikse has told PNN that the plant can process up to 200 million cubic feet per day of gas.

Jager also declined to say what annual production is, but Todd Energy managing director Richard Tweedie has told PNN that Pohokura is flowing about 70 petajoules of gas a year, plus associated condensate.

Tweedie and Jager have both said that given continued advances in subsurface technologies the Pohokura partners expect to extract considerably than the official 2P reserves of about 770PJ of gas and 50 million barrels of condensate.

The partners - operator Shell Exploration NZ (48%), Todd Energy (26%) and OMV (26%) - each sell their share of Pohokura gas separately.

Todd is contracted to sell its share to Genesis Energy, OMV to Contact Energy, and Shell to Contact, Genesis, Vector subsidiary NGC and Multi Gas.