From Gas Lift Manual

2.5.4 Horizontal and inclined flow Introduction.

In production operations, horizontal and inclined two-phase flow usually occurs in surface flowlines connecting the wellhead to a field separator. The pressure drop in the flowline, if compared to that arising in the well tubing, is of a lesser magnitude. This situation is because flowlines are usually horizontal or near horizontal, therefore no hydrostatic losses are present in the pipe. In summary, horizontal two-phase pressure drop in flowlines is generally of minor importance in the total production system consisting of the formation, the well, and the surface equipment. Exceptions to this rule are offshore wells where the length and especially the profile of the flowline do not meet the previous criteria and considerable pressure drops can be experienced because of inclined two-phase flow. The same way, substantial pressure losses can arise in long-distance pipelines transporting a two-phase mixture over a hilly terrain.

For the purposes of this chapter, i.e. the discussion of production engineering basics, only conventional wells situated on land locations are considered. In such cases, the importance of two-phase pressure drops in surface lines is much less pronounced than those occurring in wells. Accordingly, much less space is devoted to the presentation of pressure drop calculation models.

In order to develop the basic formula for calculating two-phase pressure drops in horizontal or inclined pipes, one solves the General Energy (Bernoulli) Equation for the pressure gradient (see also Section 2.4.2). The resulting formula is, according to Equation 2.90:

where: ?

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