Marching algorithms are the rule rather than the exception in the determination of pressure distribution in long multiphase-flow pipes, both for the case of pipelines and wellbores. This type of computational protocol is the basis for most two-phase-flow software and it is presented by textbooks as the standard technique used in steady state two-phase analysis. Marching algorithms acknowledge the fact that the rate of change of common fluid flow parameters (such as pressure, temperature, and phase velocities) is not constant but varies along the pipe axis while performing the integration of the governing equations by dividing the entire length into small pipe segments. In the marching algorithm, governing equations are solved for small single sections of pipe, one section at a time. Calculated outlet conditions for a particular segment are then propagated to the next segment as its prescribed inlet condition. Calculation continues in a “marching” fashion until the entire length of the pipe has been integrated. In this work, several examples are shown where this procedure might no longer accurately represent the physics of the flow for the case of natural gas flows with retrograde condensation. The implications related to the use of this common technique are studied, highlighting its potential lack of compliance with the actual physics of the flow for selected examples. This paper concludes by suggesting remedies to these problems, supported by results, showing considerable improvement in fulfilling the actual constraints imposed by the set of simultaneous fluid dynamic continuum equations governing the flow.

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