Modified Production Separators
The major advantages of a closed pressurized separators are that it
(1) controls gas from the well and sends it to a flare line under pressure and (2) is serviced by a special crew.
The separator is usually operated under 3 to 5 atmospheres of pressure
(45 to 75 psig). Horizontal units are typically about 9 feet in diameter
and 50 feet long, with a throughput of 5 MMcf or 500 bbl fluid. These are
typical numbers, and sizes and pressure vary according to special jobs.
In areas where H2S is common, where there are strict flare regulations,
or where gas could accumulate in a closed area on an offshore platform, keeping a separator operating under pressure is a critical factor in drilling safety. Further, the separator system is generally instrumented to give a history of annular pressure and measurements of gas, water, and
oil volumes.
Canada began using modified production test separators as closed
drilling separators in the 1990s. Production separators already had the
capability to separate and measure the amount of gas, oil, water, and
BS&W (bottom sediment and water), so they were easily adapted for use
in underbalance drilling projects where H2S was to be expected or where
the drilling fluid was an oil/nitrogen mixture.
Closed separators are gravity separators, often with an involute spiral
feed (a type of centrifugal input). Baffles separate areas inside the
tank and start to isolate cuttings, oil, and water. Gas rises and under
separator pressure is forced to the flare line. The oil rises out of the
drilling fluid and is pumped to an oil tank. Cuttings that start to settle
out in the separator are recycled through the tank, and the drilling fluid
is returned with the cuttings to the shale shaker (Figure 1).
Closed separator systems typically have measurement systems for
water, oil, and gas; fluid height; and tank pressure. There is a separate
upstream trapping system to collect cutting samples. Horizontal closed
separators are the most common, but vertical separators are available
and are typically used on offshore rigs.
Cuttings removal has always been a problem in closed separators.
Most of the cuttings are trapped in the first compartment. In some early designs, a screw, or Moyno, pump automatically pumped the cuttings out. However, generally cuttings are so sticky that they have a high angle of repose and will not fall into the pump suction, and so build up in the front of the tank. Later systems recirculated a part of the mud from the separator back through the cuttings compartment and circulated solids through the system to the rig shale shaker.
