Study of continuity and pressure waves in condensing two-phase flow in a pipeline

The analysis of transient gas condensate flow in pipe is conducted using a one-dimensional compositional two-fluid model. The model is developed using fundamental hydrodynamic equations in which continuity and momentum equations are considered. This model is designed for handling the hydrocarbon fluid flow in pipeline where fluid composition is an important factor in affecting the condensation process. The compositional effect of the fluid is taken into account by coupling a Peng-Robinson equation-of-state-based phase behavior model to the hydrodynamic equations. The resulting system of partial differential equations obtained consists of four dependent variables, namely pressure, gas volume fraction, gas velocity and liquid velocity. The numerical method of lines is utilized to convert the set of partial differential equations to a system of ordinary differential equations, posed as an initial-value problem (IVP). Integration of the IVP is effected using a versatile ODE solver, in which the lannelli-Baker's Implicit Runge-Kutta method for stiff problems is implemented. In the analysis using this model, pressure and continuity wave propagation along the pipeline are predicted for various operational procedures and pipeline geometries. The results show the viability of the model in describing the transient behavior of two-phase gas-condensate flow in pipelines, which is very important for natural gas pipeline design considerations.