Page 17 - Oil and Gas Production Handbook An Introduction to Oil and Gas Production

allow set up of production "well sets" so that for a given production level, the
best reservoir utilization, well flow composition (gas, oil, waster) etc. can be
selected from the available wells.

For gas gathering systems, it is common to meter the individual gathering
lines into the manifold as shown on the illustration. For multiphase
(combination of gas, oil and water) flows, the high cost of multiphase flow
meters often leads to the use of software flow rate estimators that use well
test data to calculate the actual flow.

Offshore, the dry completion wells on the main field centre feed directly into
production manifolds, while outlying wellhead towers and subsea
installations feed via multiphase pipelines back to the production risers.
Risers are the system that allows a pipeline to "rise" up to the topside
structure. For floating or structures, this involves a way to take up weight and
movement. For heavy crude and in Arctic areas, diluents and heating may
be needed to reduce viscosity and allow flow.

2.2.3 Separation
Some wells have pure gas
production which can be taken
directly to gas treatment and/or
compression. More often, the
well gives a combination of
gas, oil and water and various
contaminants which must be
separated and processed. The
production separators come in
many forms and designs, with
the classical variant being the
gravity separator. Photo: JL Bryan
Oilfield Equipment

In gravity separation, the well flow is fed into a horizontal vessel. The
retention period is typically 5 minutes, allowing the gas to bubble out, water
to settle at the bottom and oil to be taken out in the middle. The pressure is
often reduced in several stages (high pressure separator, low pressure
separator etc.) to allow controlled separation of volatile components. A
sudden pressure reduction might allow flash vaporization leading to
instability and safety hazards.

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