Calculation Of Minimum Miscibility Pressure For CO₂-Hydrocarbon System

Gas flooding is one of the most popular enhance oil recovery technique.Minimum miscibility pressure(MMP)is a key parameter during the gas injection.Hence,it is essential for academia and industry to determine the MMP correctly and quickly.There are two ways to determine the MMP: experimental method and numerical method.Slim tube experiment and rising bubble experiment are two widely-used experimental methods to test the MMP.However,both of them are costly and timeconsuming.1D compositional simulation,method of characteristic(MOC)and multiple mixing-cell method(MMC)are the numerical methods which can correctly calculate the MMP.1D compositional simulation is the safest numerical method the calculate MMP.However,it can be affected by dispersion and it is tedious to eliminate the effect of dispersion.Moreover,it is time-costly compared to MOC method and MMC method.MOC method is the fastest method to calculate the MMP correctly at present,but it is more complicated and could be limited to some complicated system.MMC method is simple and robust and can be a good alternative for MOC method.The basic work during MMP calculation is flash calculation.In this study,four typical flash methods(SSM,MVNR,Michelsen and Li-Johns)are compared through scope of application,number of iteration and convergence time.The L-X method combined SSM method is proposed as the flash scheme during MMP calculation.The LX method is applied first during flash calculation and will switch to SSM method if the L-X method is broken or the iteration number exceeds 20.In view of the limitation of the MMC method(AJ-MMC)proposed by Ahmadi and Johns(2011),in this study,we proposed an acceleration algorithm towards the AJ-MMC method to directly locate the minimum key tie-line through the control of the gas-oil mixing ratio during calculation process.The algorithm contains the following key components:(1)a strategy for determining an appropriate mixing ratio for a given reservoir fluid system to move the cell number corresponding to the minimum tie-line length to the middle of the diagram(we named as TL profile)that represents the tie-line length versus the cell number after a certain number of contacts;(2)the AJ-MMC model is used to generate a certain amount of base cells on which the proposed accelerat ion algorithm will carry to develop the minimum key tie-lines;(3)the cells which have little contribution to the development of the minimum key tie-lines are cut off from the expanding cell-triangle to avoid the severe increasement of the flash calculations in the following contacts.The algorithm is shown to be effective in developing the minimum key tie-line with three case studies.