Page 73 - Petroleum and Gas Field Processing

When, on the other hand, salt water is produced with the oil, it is then
essential to use three-phase separators—a case studied in Chapter 4.
Accordingly, oil-field separators can be classified into two types
based on the number of phases to separate:
1. Two-phase separators, which are used to separate gas from oil in
oil fields, or gas from water for gas fields.
2. Three-phase separators, which are used to separate the gas from
the liquid phase, and water from oil.
Oil from each producing well is conveyed from the wellhead to a
gathering center through a flow line. The gathering center, usually located
in some central location within the field, will handle the production from
several wells in order to process the produced oil–gas mixture.
Separation of the oil phase and the gas phase enables the handling,
metering, and processing of each phase independently, hence producing
marketable products.

3.2 THE SEPARATION PROBLEM

High-pressure crude oils containing large amount of free and dissolved gas
flow from the wellhead into the flow line, which routes the mixture to the
GOSP. In the separator, crude oil separates out, settles, and collects in
the lower part of the vessel. The gas, lighter than oil, fills the upper part
of the vessel. Crude oils with a high gas–oil ratio (GOR) must go through
two or more stages of separation.
Gas goes out the top of the separators to a gas collection system, a
vapor recovery unit (VRU), or a gas flow line. Crude oil, on the other
hand, goes out the bottom and is routed to other stages of separation, if
necessary, and then to the stock tank (Fig. 1).
Movement of the crude oil within the GOSP takes place under the
influence of its own pressure. Pumps, however, are used to transfer the oil
in its final trip to the tank farm, or pipeline (Fig. 2). Pressure reduction in
moving the oil from stage to stage is illustrated in Fig. 3.
In order to visualize the changes that occur during the gas–oil
separation process, two illustrations are presented.
First, consider an experiment where a sample of oil–gas mixture is
placed in a cylinder with movable piston to simulate oil reservoir
conditions. The weight exerted on the piston will be proportional to the
pressure found in a reservoir. Initial conditions in the reservoir are as
presented by case 1 in Figure 4, where 1.620 bbl of reservoir liquid with
1200 standard cubic feet (SCF) of dissolved gases are placed in the

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