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Abstract

This study pertains to a carefully designed non-intrusive field experiment aimed at generating data for gas condensate pipeline. A compositional two-phase hydrodynamic model is used to analyze the system with a view of validating and tuning the model for field applications. The field test was designed as part of an existing gas distribution system to collect data on gas condensation and pressure drop in gas pipeline. The test section is a 6-inch, 52,000-ft long pipe to which natural gas from three different fields is fed simultaneously. Detailed topographic profile of the pipeline is drawn so as to obtain the various inclination and inclination changes along the test section. 120 inclination changes are present in the test section. The spatial distribution of the pressure and present in the test section. The spatial distribution of the pressure and liquid holdup are not monitored so as to keep the normal operation of the distribution system as intact as possible. However, pressure and gas flow rates are monitored at some key points along the test section. In addition, periodic pigging provides total liquid accumulation over a 4- month period. Gas Chromatographic analysis provides the composition of the transported natural gas.

A compositional two-fluid steady multiphase hydrodynamic model developed from fundamental fluid dynamics is used to describe the formation and dynamic behavior of the gas/condensate system. The results of production runs demonstrate the predictive capability of the model in terms of the basic engineering design variables for this system. A number of interesting findings concerning the behavior of gas condensate flow are made. The model can be used for both design and operational purposes.

Introduction

The increasing role of natural gas as an energy source, especially in the non-transportation sector of an industrial economy, cannot be over-emphasized. It is also certain to play a major role in the transportation sector of the economy as the technology of natural-gas-driven engine improves. This dramatic increase in gas utilization will necessitate the transportation of higher volumes of natural gas than ever before. This implies a critical need for a more efficient utilization of the existing network of pipelines. Optimal design and operation of the gas pipeline is of the essence. A very important complicating factor in this respect is the fact that condensation will in variably take place in these pipelines. It is now a fact generally recognized in the gas industry that adequate design of gas pipelines requires a good knowledge of the impact of condensation on the basic design and operating parameters which in turn would affect the deliverability of the pipeline in question.

Gas pipelines used for distribution and transmission purposes traverse undulating terrain making it imperative to include the effect of varying slopes in their description. Condensation in an otherwise dry gas pipeline is a common occurrence because of the multi-component nature of the gas which is subjected to varying temperature and pressure. Retrograde condensation is thus a rule rather than an exception. In general, the phase behavior of natural gas is quite sensitive to pressure, temperature phase behavior of natural gas is quite sensitive to pressure, temperature and composition. Thus, the amount of condensate formed in the gas-carrying pipeline is dependent upon these variables. What this means is that gas pipeline is dependent upon these variables. What this means is that gas may enter the pipeline as single-phase gas but invariably becomes two contiguous phases somewhere along the pipeline due to condensation. Furthermore, since the slope and orientation of the pipeline vanes, the effect of gravity must also be taken into consideration. It is therefore imperative to utilize a compositional approach in trying to model this system. The most plausible way to do this is to couple the hydrodynamic model with the phase behavior model since the thermodynamic analysis of the fluid is essential to obtain some of the parameters needed in the hydrodynamic model and vice-versa. The model, in order to possess the necessary predictive capability, must be self-sustaining in the sense that it must be capable of predicting both the amount of condensation and where the condensate is formed and its distribution. One of the unique characteristics of the system compared with several other multiphase systems is the fact that mass transfer takes place continuously between the simultaneously flowing phases. Such exchange of mass and the accompanied momentum exchange must be accounted for. This fact has been variously acknowledged in the literature [e.g. Lagiere et al. (1984), Hein and Barua (1986), Furukawa et al. (1986), Cawkwell and Charles (1985), Gm-gory and Aziz (1975)]. More recently, empirical evidence sup-porting the importance of mass/momentum transfer due to gas condensation in pipeline has been evolved through fundamental modeling approach. [e.g. Vincent (1988), Adewumi and Mucharam (1987), Mucharam and Adewumi (1988)].

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