Page 128 - Petroleum and Gas Field Processing

4

Three-Phase Oil–Water–Gas
Separation

4.1 INTRODUCTION

The concepts, theory, and sizing equations for two-phase gas–liquid
separators have been discussed in Chapter 3. The material presented in
Chapter 3 applies, in general, to the separation of any gas–liquid system
such as gas–oil, gas–water, and gas–condensate systems. In almost all
production operations, however, the produced fluid stream consists of
three phases: oil, water, and gas.
Generally, water produced with the oil exists partly as free water and
partly as water-in-oil emulsion. In some cases, however, when the water–
oil ratio is very high, oil-in-water rather than water-in-oil emulsion will
form. Free water produced with the oil is defined as the water that will
settle and separate from the oil by gravity. To separate the emulsified
water, however, heat treatment, chemical treatment, electrostatic
treatment, or a combination of these treatments would be necessary in
addition to gravity settling. This is discussed in Chapter 5. Therefore, it is
advantageous to first separate the free water from the oil to minimize the
treatment costs of the emulsion.
Along with the water and oil, gas will always be present and,
therefore, must be separated from the liquid. The volume of gas depends
largely on the producing and separation conditions. When the volume of
gas is relatively small compared to the volume of liquid, the method used
to separate free water, oil and gas is called a free-water knockout. In such a
case, the separation of the water from oil will govern the design of the
vessel. When there is a large volume of gas to be separated from the liquid
(oil and water), the vessel is called a three-phase separator and either the
gas capacity requirements or the water–oil separation constraints may
govern the vessel design. Free-water knockout and three-phase separators

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