Comparative Study On Three-Phase Stability Analysis And Flash Algorithms Of Hydrocarbon System

The stability analysis and flash calculation of hydrocarbon system is an important part of compositional reservoir simulation,which is computationally intensive and timeconsuming.During reservoir simulation processes,if the number of phases from phase stability analysis or the results from flash calculation was incorrect,which is not only time-consuming,but also leads to wrong decision of the project.Therefore,the accurate,stable and efficient implementation of the stability analysis and flash calculation of the system is very important.This paper makes a lot of theoretical research on the literature of phase equilibrium thermodynamic models,and it is found that the method of reducing variables is attractive to solve the flash calculation.There are two ways to achieve variable-reduction for a flash problem: formula fitting binary interaction parameters or spectral decomposition binary interaction parameters.However,the above two reduced methods have received less attention of the calculation effect(accuracy and reliability)than the traditional flash algorithms.Therefore,this study implemented the following algorithms: 1)Successive Substitution Method,2)Newton-Raphson Method,3)Minimum Variable NewtonRaphson Method,4)L&J Reduced Variable Method,5)Spectral Decomposition Reduced Variable Method,and 6)New Variable Spectral Decomposition Reduced Variable Method.The above methods were programmed with MATLAB to perform the gas-liquid two-phase equilibrium stability analysis,gas-liquid two-phase flash calculation,gasliquid-liquid three-phase equilibrium stability analysis and gas-liquid-liquid three-phase flash algorithm of hydrocarbon system.The flash methods are applied to solve the minimum miscibility pressure,and the Multiple Mixing Cell Method is used to solve it,and the differences between the above reduced variable method and the traditional flash method were compared and tested in terms of calculation speed and reliability.Three algorithms are used: 1)Newton-Raphson Method,2)Minimization Method and 3)Trust Region Method to solve the complex Richford-Rice equation in three-phase flash calculation.Each algorithm is tested with a variety of hydrocarbon fluids.The accuracy,reliability,running speed and convergence trajectory of each algorithm were comprehensively compared and analyzed,and the recommended algorithms were given for different application scenarios(two-phase flash calculation,three-phase flash calculation,minimum miscible pressure solution,etc.).The results show that,in general,the speed of the accelerated algorithm is greatly improved compared with the Successive Substitution Method,the initial value accuracy of the Newton Method is higher than that of other accelerated algorithms,and there is a convergence problem under some conditions of temperature,pressure and composition.The Li-Johns Reduced Variable Method has the fastest computational speed,but the accuracy of the calculation results is related to the fitting of binary interaction parameters,especially when it comes to three-phase flash calculation and three-phase equilibrium stability analysis.The Spectral Decomposition Reduced Variable Method has fast speed and good robustness,and can be used as a versatile algorithm.