The Simulation Theory And Technique For The Natural Gas Liquid(NGL) Pipeline Network

Natural Gas Liquids (NGL) is mainly composed of ethane, propane, butane, condensate oil and their mixtures. It is an important source of fuel and petrochemical raw materials. It is well known that the NGL industry is with high economic values as well as broad application prospects.The resources, such as the condensate gas, shale gas and shale oil are all rich in NGLs. The removal of NGL can not only reduce the hydrocarbon dew point of those gas and oil, but also improve the gas fields’economic efficiency by NGL business. In this business, transportation plays an important role, and pipelines are generally considered as the most economical way for a long-distance and large amount transmission of NGL. However, during the pipeline transportation, the temperature and pressure variations along the pipeline may cause NGL boiling and vaporation. So the gas-liquid two-phase flow may occur in the NGL pipelines. Although the two-phase flow is common in the wet gas pipelines, the flow prediction methods established for the wet gas are not feasible to the NGL pipelines because NGL is much different in its physical parameters, boiling and vaporization characteristics, and two-phase heat and mass transfer characteristics. More importantly, if more than one NGL pipeline are interconnected and forming a complex NGL pipeline network, the prediction of the flow parameters would become even more difficult.nI view of the problems described above, a range of basic laws of NGL are studied based on the fluid mechanics, thermodynamics, and heat and mass transfer theories. These basic laws include the physical parameters variation law, boiling and vaporization law as well as the two-phase flow law of NGL pipelines. With those achievements, the flow prediction model for complex NGL pipeline systems is established and solved. A simulation software named Natural Gas Liquid Pipeline Emulation System (NGLPES) is also developed based on the model and its solution method. The specific research achievements are as follows:(1) Firstly, nine kinds of equation of state (EoS), including SRK equation, PR equation and seven other cubic EoSs, were analyzed based on their prediction accuracy of NGLs’ dew points and densities. However, all these EoS showed limitations when applied to NGL. So improvement of PR equation was accomplished by using the group contribution method (for binary interaction coefficient calculation) as well as the volume translation method. A new EoS named VTPPR78was proposed based on this improvement method, and its accuracy was proved applicable to NGL. On this basis, a set of calculation models were established to estimate the physical parameters of NGL, such as the bubble point, dew point, enthalpy, entropy, etc.. Besides, a viscosity model which was suitable to both the natural gas and liquid hydrocarbon was established based on the Eos VTPRμ. A surface tension prediction model for NGL was also proposed according to the thermodynamic relationship among the chemical potential, fugacity and the surface tension.(2) The NGL vaporation mechanism as well as the bubble dynamics was analyzed based on the Navie-Stokes equation of gas-liquid two-phase flow, the PRI wall boiling model, the VOF (Volume of Fluid) model and the CFD technique. In addition, a numerical simulation model was also established to predict the NGL bubble growth and development process, and the reliability of this model was verified according to the published experimental data. Furthermore, a series of NGL bubble dynamics theoretical models were evaluated and selected by the comparison between the theoretical calculation results and the experimental data/simulation values. Those theoretical models include the bubble departure diameter, bubble takeoff diameter, the bubble departure frequency, bubble activation core density models.(3) Based on the assumption of discrete gas cavity, a mathematical model which was feasible to the low gas-ratio NGL pipelines was proposed according to the Navie-Stokes equation. The method of characteristic and the finite difference method were utilized to solve the model. More importantly, for the first time, the NGL evaporation rate in the low gas-ratio NGL pipelines was successfully calculated by using the bubble takeoff diameter, bubble departure frequency and bubble activation core density models.(4) Based on the Navie-Stokes equation and the simplification of one-dimensional flow in the pipelines, a gas-liquid two-fluid flow model which was applicable to the NGL pipelines in the non-equilibrium thermodynamics condition was established with the consideration of the temperature difference between the gas and liquid phases on the same section. On the basis of eigenvalue theory, the mathematics feature of this two-fluid flow model was studied, and three conclusions were generated:①when the velocities of the gas and liquids are equal to each other in the pipelines, the type of this two-phase flow model is hyperbolic;②when the velocities of the gas and liquids are unequal to each other in the pipelines, the type of this two-phase flow model is parabolic;③when the two-phase flow degenerates into single-phase flow in the pipelines, the two-fluid flow model degenerates into single-phase control equations, and the mathmatical type is hyperbolic. The Barnea model was adopted to identify the two-phase flow pattern in the pipelines. The discrete method of the two-phase flow model was also studied based on the finite volume method.(5) The NGL pipeline network is always composed of multiple pipeline sections, inlet sources, outlet sources as well as a lot of non-pipe elements. As a result, structure of the network is usually random and complex. For this reason, the "node-element associated matrix" method was adopted and successfully to describe the pipeline network with any physical structures. On this basis, for the first time, the NGL gas-liquid two-phase flow simulation model which can be applied to the network with any structures was proposed by gathering all the mathematical models of the nodes, elements and non-pipe elements. This simulation model is of highly nonlinear characteristics. So the model linearization method is studied, and its solving method is also obtained based on the LU decomposition method as well as the damped Newton-Raphson method.(6) Based on the "Pipieline Emulation System PES" platform which was proposed by Southwest Petroluem University, the "Natural Gas Liquids Pipeline Emulation System NGLPES" was developed according to above theoretical achievements. This software has a completely independent intellectual property rights, and can be applied to the two-phase flow pipeline system with any structures. The accuracy of the software was also tested in reference to the OLGA results and the measured values form the NGL pipelines of Tarim oil field in China. The comparison results showed that the average absolute deviation between the NGLPES simulation values and measured values is0.0854t/h in flow rate and1.29℃in temperature, and the relative deviation of flow rate is0.956%. In the dynamic conditions, also based on the parameters of Tarim NGL pipelines, the average absolute deviation between the NGLPES simulation values and OLGA results is58.5kPa in pressure and0.03in holdup. These results suggest that NGLPES software is accurate and reliable.The software NGLPES has gained national software copyright (copyright No.2014SR043986) in China, and it has been practically applied to the NGL pipelines belongs to the Ministry of Oil and Gas distribution of PetroChina Tarim Oilfield Branch.