Ingeniero

COMPOSITIONAL HYDRODYNAMIC MODELLING OF GAS/GAS CONDENSATE IN GAS PIPELINE


By


Olaosebikan A. Olafadehana and Gbeminiyi A. Fayanjuolab
aDepartment of Chemical Engineering, University of Lagos, Akoka-Yaba, Lagos, Nigeria
bDepartment of Petroleum and Gas Engineering, University of Lagos, Akoka-Yaba, Lagos, Nigeria
E-mail: olafadehan@yahoo.com; oolafadehan@unilag.edu.ng


ABSTRACTA non-isothermal one-dimensional steady-state compositional two-phase hydrodynamic model is developed to describe the formation and flow dynamics of gas condensate in horizontal natural gas pipelines. The two major constituents of the model, hydrodynamics and phase behaviour, are coupled through the phase-generation/disappearance-related terms in the continuity and momentum equations. Theresulting model equations are numerically solved using semi-implicit Runge-Kutta method combined with step-size adjustment strategy together with application software obtained from www.gpengineeringsoft.com. It is demonstrated that the model is capable of predicting the amount, quantity and distribution of condensate in the pipeline, in addition to the other commonly sought engineering designvariables. Parametric studies show that the model is capable of predicting the phenomenon associated with gas condensation in pipelines.

Keywords: hydrodynamic model, phase behaviour, gas, gas condensate and pipelines.

1.0 INTRODUCTION

Natural gas must be found, produced, gathered, processed and transported for it to be available at the burner, be it in a home or in the factory. Gasgathering, though a standard feature of natural gas production, is a salient component of the exploitation development of any petroleum field. Many gathering lines and long-distance pipelines pass through areas of hilly terrain; this presents no problem in single-phase flow because the potential energy lost going uphill is regained in the downhill section. This is not the case for two-phase flow, becausethe liquid hold-up, and thus the mixture density, is usually much lower in downhill flow. For this reason, pressure recovery in the downhill sections is usually neglected in the design of two-phase pipelines.

Having been processed, natural gas is transported to consumers through a network of pipelines that crisscross the landmass of a nation. Optimal design and operation of such gathering anddistribution system is essential if the ultimate cost to the consumer is to be kept at a reasonable level, especially when the large capital outlay necessary constitutes a sizeable share of the total development costs.

The search for more economic recovery method, particularly for marginal oil fields sometimes in deep water has resulted in decisions to transport mixtures of gas and liquid(two-phase flows) simultaneously in large diameter pipelines over relatively long distances.
Optimization of the design and successful operation of two-phase pipeline systems require knowledge of the behaviour and characteristics of such flows. Various studies have shown that at present no single theory or correlation can satisfactorily predict the characteristics of two-phase gas-liquid in pipeover a wide range of conditions. Gas/liquid flow is a more complex phenomenon than a single flow primarily because the distribution of the two phases is normally unknown and difficult to specify quantitatively. Unfortunately, the present design methods used within the industries are primitive compared to their single phase counterparts. Some of these methods are based on empirical correlationsderived from small scale laboratory systems with small diameter pipes and using simple test fluids such as air and water at low pressure. Extrapolation of these correlations to large diameter, high pressure oil/gas pipelines usually result in unacceptable errors. Another approach to these problems has been the development of very simple steady-state models that cannot reliably predict the wide...