Multiphase metering better than test separator for Captain flows

The combination of high viscosity in produced crude and the need to re-inject all produced water back into the formation, led developers of the Captain Extension field in the UK North Sea to opt for multiphase measurement in place of test separation. The flowmeter provides continuous well testing of water, gas, and oil cuts to aid reservoir optimization and oil recovery.

The Captain Extension development requires well testing to aid reservoir optimization and oil recovery. The project partners decided that in this case, multiphase measurements of oil, gas, and water flow providing continuous, real time measurements would enable changes in the well fluids to be detected earlier than using a test separator.

This would enable changing fluid flows that effect individual well performance to be monitored and therefore the optimization of production. According to Coleen Fardley, General Manager of KOP's Flow Measurement Division, the point of water breakthrough is significant. This demands very accurate water cut measurements.

In this regard, the DUET performed exceptionally well during trials of multiphase meters at the National Engineering Laboratory in East Kilbride, Scotland. The DUET provides accuracy to within 2% over 0-65

GVF and 4% over 65-95% for water cut, compared to typical test separator's 10% accuracy range, according to KOP.

Bass Strait trial

In 1998, a prototype version of the meter completed four years in service on the West Kingfish platform in South Australia's Bass Strait, thought to be the longest continuous operational period for a multiphase flowmeter anywhere.

The technology was originally developed for the mining sector by Csiro in Australia. KOP signed an exclusive licence agreement in 1997 to produce and further develop Duet for the offshore industry. Now a subsea version is also available.

DUET is a 1.1 meters long unit, incorporating two gamma-ray densitometers, mounted non-intrusively on a pipe spool carrying well fluids. A single energy density gauge is used to measure mass-per-unit area of the fluids, while the dual-energy gauge determines water cut. Liquid and gas flow rates are derived by combining:

• Mass-per-unit area of fluids from the single energy measurement
• Velocity from cross-correlation of the two gauges' density profile outputs
• Flowline operating pressure and temperature.

Measurements are then combined with software models of the prevailing flow regime, incorporating corrections for slip between liquid and gas phases. (Corrections are made for density variations with temperature and pressure for process fluids).

Mass fraction detection

Mass fractions of oil and water in liquids are determined by detecting the reduction in gamma ray emission intensity from each source, due to the individual absorption coefficients of the oil/water fractions. Water cut is gauged by combining these two measurements with the densities of oil and formation water at the pressure and temperature of the multiphase flow-mix.

A paper by G. Roach and T. Whitaker at last November's North Sea Flow Measurement Workshop outlined the trial results on West Kingfish. The platform is 70 km off the South Australian coast in 80 meters water depth, with 20 wells in production. Prior to the multiphase flowmeter installation, flow from each well had been measured by a test separator twice monthly.

In November 1994, an 18-week trial of a Duet meter was undertaken. In readiness, the test separator was cleaned out to insure good water/gas/oil interface control. However, it soon became apparent that a continuous record during each one hour long well test would be required.

Subsequent comparison revealed that the Duet and separator determination of liquids, water and gas flow rates generally accorded. Accuracies of the multiphase flow meter were then sustained throughout the four-year trial.

Further development of the meter followed a 1996 trial at Texaco's Humble facilities, plus a 10-month laboratory stability test in 1997-98 of an identical system to that on West Kingfish. Key modifications included a switch to lower capacitance cable and development of new algorithms for water cut calculation. These changes improved the accuracy and stability of water cut determination, the authors claim.

Test evaluation

The prototype Duet was removed from West Kingfish in September 1998 for examination. Some of the more severe production characteristics it had to endure included flows containing sand. In its location near a walkway and close to the platform's edge, it had also been largely exposed to the elements. Following almost four years in service, and without maintenance, the meter's surface was coated with salt and other foreign materials.

The meter was dismantled into its three basic components - source pots, electronic hood units, and the outer O-ring. Examination of the inside of the ring revealed no evidence of erosion, scaling, or wax buildup. The seals between the window galleries and electronic head units were unbroken, and there was

little evidence of damage on the front window surfaces, nor of hydrocarbon or production water ingress into the epoxy.

In their paper, the authors attribute the meter's reliability to the fact that it is non-intrusive and does not depend on moving parts. It also reduces operator support and time to determine well flow than test separator systems. Intervention is only necessary to switch the appropriate stream through the pipe on which the Duet is mounted. Esso indicated that the West Kingfish test separator's turbine meters, by contrast, were prone to blockages.

KOP's subsea version of Duet is suitable for water depths to 3,000 meters, and is ROV-installable and retrievable. It can likewise be adapted to pipe diameters of 3-1/2 in.