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A Borefield is an area that contains the bores or wells through which the geothermal steam and water is extracted. The equipment found in a Borefield is specialised for controlling and transporting steam and water. It includes Bores, Well Heads, Steam Separators, Flash Plants, Silencers and Pipelines.

The western borefield supplies about 50% of Wairakei's steam. It consists of about 30 wells drilled to a depth of about 600m - Image: Heurisko Ltd.

Bores and Borefields

Bores, (also called wells) are the holes drilled into the ground to extract hot water and steam.

Looking East along the Western borefield. The line of bores follows an fault plane that currently provides 50% of Wairakei steam - Image: Heurisko Ltd.

In the Wairakei Borefield the bores reach a depth of up to 2200m. However the deepest bores do not always produce more because the most productive bores are those that intersect a fault plane or are drilled into very porous or fractured rock where steam and water can flow more easily.
Throughout the Borefield the bores are not evenly spaced but instead are grouped above the most productive areas.

The Wairakei Borefield has three main areas of production:

1. Eastern Borefield
   This was the main production area in the 1960's. However its pressure dropped and the reservoir temperature also fell, due to the inflow of cool water.
2. Western Borefield
   About 50% of Wairakei Power Station's steam presently comes from the Western Borefield. It consists of about 30 wells drilled to a depth of about 600m.
3. 

   A modern well head amongst farmland - Image: Heurisko Ltd.
   Te Mihi Borefield
   The remaining 50% of Wairakei Power Station steam presently comes from the Te Mihi Borefield, which produces dry steam from a shallow (330-500m) high pressure (18-25 Bar) steam zone. Below this depth, is a large reservoir of hot water at a temperature of more than 250°C.

Well heads

• support valves and separators that control the flow of water and steam from the well
• support the drill rig while drilling or working down the well

A single Flash Plant that services several bores - Image: Heurisko Ltd.

The design of well heads has also changed over time.

1. Older wells have steam separators built into them, with one separator on each. Modern wells share a common separator.
2. Older well heads were excavated below ground level to help to support the structure. This resulted in a 'cellar' being formed. Modern well heads are level with the ground which helps to prevent the trapping of poisonous gases such as hydrogen sulfide and carbon dioxide.

Steam Separators

Separators separate dry steam and hot water.

Wairakei geothermal field contains mainly pressurised water at a temperature of 230-250°C. However when this water reaches the surface the pressure on the water drops and the water boils as it rises within the well to become a mixture of about 80% water and 20% steam.

Only the steam can be used to drive the turbines so the water must be separated.
The separation technique used was invented by the original Wairakei engineers in the 1950's and uses the principle of centrifugal force.

A single silencer for a single bore in the Eastern borefield - Image: Heurisko Ltd.

It relies on:

1. the steam and water mixture entering the separator at high speed and at an angle.
2. the water having a greater mass than the steam, so that it spins to the outside.
3. the lighter steam molecules collecting in the centre. They are then piped up and out to the Power Station.

The water from the separators may then be discharged or passed to a Flash Plant depending on its temperature and pressure.

Flash Plants

Flash plants produce extra steam (flashed steam) from hot water discharged from separators. Flash plants produce steam by reducing the pressure rather than raising the temperature of the water.

The flashed steam can then be piped to the power station, in separate pipes as it is at a lower pressure, and used for extra generation.

Pipeline leading from the Western Borefield to the Wairakei Power Station - Image: Heurisko Ltd.

Silencers

A silencer reduces the noise of steam escaping into the atmosphere.

Silencers are needed because of the noise created by the pressurised hot water escaping to the atmosphere and producing flashed steam. Without a silencer even a small steam discharge could cause deafness, headaches, and dizziness in people working nearby. With all 54 bores discharging, the noise from the Wairakei Borefield would be heard 10Km away.

Within the silencers there is also separation of steam and water along with the production of massive clouds of water vapour. These clouds are the most obvious feature to visitors.

Pipelines

The pipelines at Wairakei carry both hot water and steam and extend between Borefield, Power Station, Waikato River and Re-injection wells.