Report: Tui Operations Will Separate Oil from Water

Oil production from the Tui field in the Taranaki Basin offshore New Zealand is sustainable with water cuts above 90 percent, according to Randy Stewart, Tui project director with New Zealand Crown Minerals.

In a paper on the Tui development presented to the recent 2006 New Zealand Petroleum Conference, Stewart said that a strong formation water aquifer below the thin, high-quality Kapuni F sands reservoir will lead to an early water break and sustained high water cuts.

"Over the expected life of the Tui area accumulations, 10 times more water will be produced than oil," he said.

Stewart described Tui's basic production operation as "oil skimming." Simulation studies indicated that ultimate oil recovery would be proportionate to the length of well bore contact with the reservoir. This will mean long horizontal well bores.

"Early water breakthrough and rapid initial oil rate decline drive the need for high total liquid flow rates and processing capacity over most of the field’s 10 year producing life," Steward noted.

Oil recovery is predicted to increase with increasing oil and water production. Much of the oil production, after initial high flow, would come in a long tail at high water cuts.

First oil from Tui is scheduled for mid-2007. Initial high production rates of 50,000 bopd would decline to about 17,800 bopd within 5 years. Approximately 48 percent of total recovery would occur in years three to 10 at water cuts greater than 91 percent, said Stewart. The economic production limit at prices of US$50 a barrel, would be 2,400 barrels a day.

Gas produced from the field would be used both for the required artificial lift of oil and water (in a closed loop) and to fuel the floating production storage and offloading (FPSO) vessel.

As gas output declined toward the end of the field, nitrogen would be substituted for the gas lift.

The Tui field will be the first in New Zealand to have subsea wellheads. Water depth at the Tui field is 125 meters. Stewart said subsea control modules, which can operate valves and chokes and monitor instrumentation, will be set on each wellhead. A 5-inch by 2-inch, 5,000-psig working pressure Cameron horizontal tree has been selected to equip each of the four producing wells.

Diverless subsea connections were selected over diver-assist connections for installation and intervention at the subsea control modules. Diverless connections had a significant economic advantage when viewed in the context of vessel use, mobilization, and weather downtime.

The subsea wellhead tree will have a multiplexed electro-hydraulic control system to control the tree functions through the umbilical line to the topsides control module on the FPSO. Chokes will be installed to manage flowline pressures--these will have inserts that can be retrievable by a remotely operated vehicle (ROV) operating underwater.

Provisions in the subsea control module trees are made for injecting wax inhibitors, pour point depressants, methanol, and for corrosion and scale inhibitors.

From each well there will be three lines up to the FPSO: the oil production line, the gas lift injection line, and the hydraulic umbilical line. To reduce unnecessary abrasion in the harsh ocean conditions the lines will be partially suspended on mid-water arches.

The lines will connect to the FPSO through a special internal turret mooring system located through the bottom of the ship just behind the bow.

The FPSO will be the chartered Suezmax tanker M/T Ionikos currently being converted by contractor Prosafe Production Services.

Stewart said the internal turret mooring, which allows the FPSO to weathervane 360 degrees around the mooring axis while accommodating sea motion, was chosen to minimize induced loads on the vessel. The vessel is designed to withstand 100-year environmental conditions.

The FPSO, as well as containing the normal processing and storage operations, will also control the subsea system for the field. The vessel will have accommodation for 41 and have a rear helideck for helicopter transport.