Vapor-liquid multiphase flow has become an increasingly important topic due to the drive for reduced field processing facilities and full wellstream transfer, particularly for offshore fields. In addition, increased computing power makes possible more mechanistic analysis of multiphase flow. Three economically important problems include the calculation of pressure drop-flow rate relations, the determination of flow regime, and the calculation of liquid slug length for separator sizing. Historically, multiphase flow has been analyzed by empirical extension of single-phase flow techniques, but calculations are becoming progressively more analytical. Flow regime determination and slug length calculation remain vexing problems. Subsidiary issues such as multiphase transient simulation, multiphase puntping, and multiphase metering and hydrates complicate the quest for full wellstream transfer. Ultimately, the precision of input data may limit the accuracy of multiphase flow calculations.
Historically, vapor-liquid multiphase flow has had little impact on the gas transmission industry. Gas entering the pipeline network has generally undergone sufficient processing that the dew point was encountered only under unusually cold conditions, or liquid was present only due to lube oil or glycol carry-over. As transmission companies become more closely involved in production operations, particularly offshore, multiphase flow becomes a more pressing problem. Multiphase flow has always been a concern in wellbore flow where large pressure and temperature deviations in a raw gas stream make vapor-liquid flows almost inevitable. The large impact of phase behavior on flow performance makes thermodynamic property prediction much more important and involved than in single phase flows. Increasingly, multiphase flow becomes an issue in field flowlines and gas transmission lines due to the drive for field facilities minimization. In an effort to reduce facilities cost, gas treatment is held to a minimum. This drive is particularly intense offshore where topsides facilities space and weight are at a premium. The limiting case of facilities reduction offshore occurs in subsea production systems where surface facilities disappear altogether, and the full wellstream is piped to remote facilities or to shore. This full wellstream transfer almost always leads to multiphase flow. With this multiphase flow comes a variety of problems including pressure drop-flow rate prediction, flow regime determination, slug length calculation, transient behavior prediction, provision for multiphase pumping and metering, and hydrate suppression. This admittedly superficial review will deal only with the pressure drop-flow rate, flow regime, and slug length questions. Technical Bases for Steady-State Calculations Most steady-state multiphase flow calculations involve more or less empirical extensions of single phase techniques. As with single-phase flows, a momentum equation is used to generate a Bernoulli-like equation in which the pressure drop is broken up into hydrostatic, frictional, and acceleration components. Unlike gas flows, where this momentum equation can be integrated in closed form to produce Weymouth or Panhandle-type algebraic equations (l), a numerical integration of the momentum equation and its associated fluid property and multiphase closure equations must be performed. Multiphase flow analysis is almost exclusively the province of computer programs. The acceleration, frictional, and elevation components are evaluated using single-phase extensions.
