Investigation On Gas-Liquid Two-Phase Flow Characteristics And Transient Slug Flow Model On The Drastic Undulating Ground

The scheme of mixed transport of oil and gas is widely used in the process of oil and gas extraction because of its low investment and operating cost.Due to the influence of topographic relief,the complex flow state of gas-liquid two-phase pipeline in the complex terrain area(deep-sea oil field,desert oil field,Changqing oil field,etc.)is easy to induce the slug flow,which brings great hidden trouble to the field safety production.At present,most of the relevant studies focus on horizontal,small fluctuation or vertical pipelines,and relatively few studies focus on gas-liquid two-phase flow in drastic undulating ground.Therefore,a series of experimental and theoretical studies have been carried out on the flow characteristics and transient model of slug flow in large undulating gas-liquid mixed transport pipeline.Firstly,a detailed experimental study was conducted on the flow pattern characteristics of gas-liquid two-phase flow in undulating gas-liquid mixed transport pipeline.Seven different flow patterns were observed in the experiment.The periodicity of severe slug flow Ⅰ,Ⅱ and Ⅲ was relatively obvious and the main frequency was obvious in the power spectral density(PSD)diagram.Slug flow,bubble flow,emulsion flow and annular flow have no periodic rule.The amplitude of annular flow PSD is minimum.When the angle of the up-dipping pipe increases and the angle of the down-dipping pipe changes from -15° to -30°,the amount of liquid accumulates in the lower concave area at the bottom of the pipe,the area of serious slug flow Ⅰ increases,and the area of annular flow decreases.The calculation results of the improved Taitel-Dukler flow pattern discrimination criterion are basically consistent with the experimental data at different inclinations.The observed bubble flow area is smaller than the calculation result of the criterion,and the circular flow area calculated by the criterion is larger than the experimental observation result.The change of forward and backward angle of Ⅴ-type pipeline has a significant influence on the range of severe slug flow.The correction coefficient C of B?e criterion under the condition of large fluctuation angle in this experiment is 3.The probability density function(PDF)feature was combined with support vector machine(SVM)to realize flow pattern recognition and the recognition accuracy of severe slug flow reached 94%.The variation law of slug flow characteristic parameters(slug translational velocity,slug length,slug distribution,slug frequency and pressure drop)under different pipe inclinations and different gas-liquid velocity was analyzed.Based on the experimental data,the existing slug characteristic calculation formulas are compared and theirs applicability in drastic undulating ground is obtained.Based on the Renault model and the slug capture theory,a transient slug flow model was established to simulate the formation and development process of gas-liquid two-phase flow.The liquid phase equation of the model is solved by traveling wave method.In the solution process,the sparse wave is approximated by shock wave,the exact solution between cells is replaced by an approximate solution,and the dry zone is replaced by a thin liquid film.So that the traveling wave method is applicable to all computing units and the model can be solved quickly.The proposed adaptive large time step(ALTS)is used to simulate the transition from the layered flow to the slug flow in the initial stage,which further improves the computational efficiency under the condition of ensuring the accuracy.The transient slug flow model can spontaneously simulate the dynamic process of slug formation and development.At the initial moment,the gas-liquid interface generates a small disturbance,and the flow pattern in the tube changes from layered flow to wavy flow.With the enhancement of disturbance,the flow pattern develops into a stable slug flow.By comparing the computing efficiency of this model with the Renault model under different grid numbers,the computing time of this model was reduced by 30.5% on average compared with the Renault model,indicating that the simplified method proposed in this paper greatly improved the computational efficiency.By comparing the experimental results with the model calculation results,it is found that the characteristics of slug are basically consistent with the experimental results,and the calculation results of transient pressure drop of the model under different pipe inclination angles are basically consistent with the experimental values,and the model has a good adaptability to topographic relief.In this paper,by studying the gas-liquid two-phase flow characteristics of pipelines in drastic undulating ground,the variation law of slug flow characteristic parameters and the applicability of the calculation formula are clarified.Based on the Renault model,a transient slug flow model for gas-liquid two-phase flow hydraulic computation was established by using the travelling wave method and the adaptive large time step format.The conclusion can be used to predict and evaluate the slug flow in drastic undulating ground and is of great significance to the production safety of oil and gas mixed transport pipelines in complex terrain.