Well Test Separator Skid

Overview

A well test separator skid is a compact, often portable separator used to measure how much oil, gas and water a single well produces. Operators need this number for each well to manage the reservoir, allocate production among partners, detect problems such as water breakthrough or declining gas, and report to regulators. Rather than build a permanent separator at every well, a facility routes one well at a time to a shared test separator through a test manifold, measures its rates over a test period, and then switches to the next well.

The skid is a self-contained three-phase separator with flow meters on each outlet and a flow computer that totalizes the results. Unlike a Three-Phase Production Separator sized for bulk production, the Test Separator Vessel is sized for accurate measurement and a defined test duration, so it is smaller and built for easy transport and hookup. Many units are mounted on a road-transportable skid with lifting and tie-down points so the same separator can serve several well pads.

How a well test works

The well under test is lined up to the skid through the Inlet Manifold manifold, which isolates that well and routes its flow into the separator while the other wells continue to the production separator. An inlet diverter breaks the momentum of the wellstream, and the fluid separates inside the vessel into gas at the top, oil over the weir, and water at the bottom, the same way a production separator works. The difference is that each outlet now carries a meter.

A test runs for a set period, often several hours to a full day, long enough for the well to stabilize at the test conditions and for the readings to average out short-term slugging. The Control and Test Computer flow computer records gas, oil and water continuously and reports the average daily rates and the cumulative volumes for the test. Sampling points on the inlet and water lines let the operator check fluid properties and confirm the water-cut reading.

Measuring the three phases

Gas is measured by the Gas Metering Section section, usually an orifice plate sized and calculated per AGA 3, with a differential pressure transmitter across the plate and a temperature transmitter for density correction. A back-pressure valve on the gas outlet holds the vessel at the test pressure, which fixes the gas density used in the calculation.

Oil leaves through the Oil Metering Section section, where a turbine or coriolis meter measures the liquid rate. Because the oil may still carry a little water, an inline water-cut meter corrects the reading to a true oil rate. Water leaves through the Water Metering Section section with its own turbine or magnetic flow meter. The flow computer combines all three measurements into the well's gas-oil ratio, water cut and total liquid rate.

Level control and the flow computer

Steady levels are essential for accurate liquid metering, because a swinging level dumps liquid in surges that the meters struggle to follow. The control section uses an interface transmitter and an oil level transmitter to drive the two dump valves, holding both the oil-water interface and the oil level steady so the dumps are smooth. The Control and Test Computer flow computer ties the meters together, applies the AGA and meter-factor corrections, and presents the totals on an operator display. Reflex sight glasses and level switches back up the automatic control and feed the shutdown system.

Relief and shutdown

Even though it is a small vessel, the test separator holds full well pressure and needs the same protection as any production vessel. The Relief and Shutdown section provides an ASME relief valve and a backup rupture disc for overpressure, and an inlet emergency shutdown valve closes on a trip from high or low level, high or low pressure, or fire and gas detection. Pressure transmitters report vessel conditions to the safety system, and the shutdown brings the skid to a safe state without operator action.

Transport and use

The Transportable Skid is built as a single liftable and trailerable unit with the vessel, meters, valves, controls and wiring already plumbed and terminated in a junction box, so field hookup is just the well connection, the outlet lines and power. This makes the skid easy to move between pads and quick to bring into service. Well test separators are used across onshore and offshore production, on new wells during initial flow testing, and on producing fields for routine periodic well tests. By giving an accurate per-well measurement, the skid supports reservoir management and production allocation without dedicating a full separator to each well.