Page 72 - Petroleum and Gas Field Processing

3

Two-Phase Gas–Oil Separation

3.1 INTRODUCTION

At the high pressure existing at the bottom of the producing well, crude oil
contains great quantities of dissolved gases. When crude oil is brought to
the surface, it is at a much lower pressure. Consequently, the gases that
were dissolved in it at the higher pressure tend to come out from the
liquid. Some means must be provided to separate the gas from oil without
losing too much oil.
In general, well effluents flowing from producing wells come out in
two phases: vapor and liquid under a relatively high pressure. The fluid
emerges as a mixture of crude oil and gas that is partly free and partly in
solution. Fluid pressure should be lowered and its velocity should be
reduced in order to separate the oil and obtain it in a stable form. This is
usually done by admitting the well fluid into a gas–oil separator plant
(GOSP) through which the pressure of the gas–oil mixture is successively
reduced to atmospheric pressure in a few stages.
Upon decreasing the pressure in the GOSP, some of the lighter and
more valuable hydrocarbon components that belong to oil will be
unavoidably lost along with the gas into the vapor phase. This puts the
gas–oil separation step as the initial one in the series of field treatment
operations of crude oil. Here, the primary objective is to allow most of the
gas to free itself from these valuable hydrocarbons, hence increasing the
recovery of crude oil.
Crude oil as produced at the wellhead varies considerably from field
to field due not only to its physical characteristics (as explained in Chapter
2) but also to the amount of gas and salt water it contains. In some fields,
no salt water will flow into the well from the reservoir along with the
produced oil. This is the case we are considering in this chapter, where it is
only necessary to separate the gas from the oil; (i.e., two-phase separation).

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