Our largest oil and gas field, Maui, was discovered off Taranaki in 1969. Maui commenced production in 1979 and is still producing today.
Background
Offshore drilling has been taking place since the early twentieth century. Offshore drilling in New Zealand commenced in the late 1960’s.
Our largest oil and gas field, Maui, was discovered off Taranaki in 1969. Maui commenced production in 1979 and is still producing today. Subsequently four further fields have been discovered and developed off Taranaki. The majority of NZ’s oil and gas is produced from these five offshore fields.
As of September 2012 a total of 197 offshore wells had been drilled in New Zealand waters, with 176 of these being in the Taranaki region.
Offshore oil exploration and development has the same phases as onshore, however the technologies for operating in the marine environment are different.
Following award of an exploration permit by the government, activity the operators generally takes place as follows:
- Desktop research drawing on existing information sources.
- Seismic surveying over those parts of the permit area where it is considered hydrocarbons might be present.
- Drilling of exploration wells to explore prospects identified from seismic surveys. Further appraisal wells to establish the size of a field would follow a successful discovery.
- Installation of production infrastructure and drilling of production wells to extract oil and gas hydrocarbons and deliver them to markets onshore in New Zealand or export overseas.
Seismic surveying
A marine seismic survey is a method of determining geological features below the sea floor. It works by sending acoustic sound waves into the rock layers beneath the sea floor and then recording the time it takes for each wave to bounce back as well as measuring the strength of each returning wave. It is used to identify geological features that may contain oil or gas deposits.
Seismic surveys generally take place over a few weeks in a given area. Following processing and evaluation of this data, further parts of the area may be surveyed again to provide greater detail. This extra data can assist in mapping potential prospects and the process of deciding where to drill an exploration well. Surveys are also used to reveal changes in the sub-surface rock layers that may present a safety hazard during drilling.
A survey is conducted using purpose-built seismic survey vessels, towing a number of percussion devices at depths of 6-10m below the sea surface. Rapid release of an underwater piston driven by compressed air creates the sound (or seismic) waves, which are directed down into the seabed and reflected back to the surface by the layers of rock under the seabed. The returning sound waves are detected and recorded by microphones (called hydrophones) that are spaced out along a series of cables called streamers. These are towed behind the survey vessel and can be up to 8km in length.
For surveys over larger areas (often referred to as 2-D surveys) the seismic vessel sails up and down grid lines which can be kilometres apart. For detailed surveys of a smaller area (referred to as 3-D surveys) the grid lines are much more closely spaced – as close as 100m apart.
The pattern of recorded signals is then translated into geological cross sections. With 2-D surveys the resultant images provide a general view because the cross sections are relatively far apart. In the case of a 3D survey, however, the cross sections are so close that a three-dimensional picture can be built of the strata under the sea bed. This can be used to identify with greater certainty where oil and gas might be present and to direct the drilling of exploration wells.
Offshore drilling
Drilling exploration or appraisal or wells offshore is undertaken by a variety of mobile rigs suited to operating in different water depths. These include:
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Jack-up rigs are used in water depths of up to approximately 120 metres water depth. The jack-up rig is towed to location with its legs elevated. Once on location, the legs are lowered to the bottom and the platform is "jacked up" above by means of hydraulic jacks.
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Semisubmersibles are the most common type of floating drilling unit and are used in water depths of 100 meters to 3000 metres. The semisubmerible is a column stabilized vessel and with its hull submerged it is very stable. They can be towed into position by a tugboat and anchored, or moved by and kept in position by their own thrusters with dynamic positioning.
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Drillships are vessels fitted with drilling equipment. They are used for exploration drilling for petroleum and for scientific drilling. Modern drillships are able to drill in water depths of up to 3000 metres.
In order to drill, a marine riser is lowered to the seabed with a blowout preventer (BOP) at the bottom that connects to the wellhead.
Production
Production infrastructure to develop an oil or gas discovery can come in various forms depending on the nature, scale and location of the field. New Zealand’s existing offshore fields have been developed using a mixture of fixed platforms such as Maui A and floating installations such as the Tui floating production, storage and offloading (FPSO) facility, the Umuroa.
The oil and gas is extracted from the field then either transferred by pipeline to shore for further processing and distribution or, in the case of FPSO’s, loaded directly on to oil tankers for export.