Prediction Of Minimum Miscibility Pressure On CO₂ Drive Based On Multiple Mixing Cell Model

At present,most of the oil field has been developed in the mid-late period,deliverability decline rapidly,composite water cut is high,development effect is not predominance,it is difficult to improve the recovery of low permeability oil field.Improve oil recovery is the urgency topic that is needed to be resolved.The advantages of carbon dioxide injection are not only its widely application,lower cost.but also remarkable enhanced effect of oil recovery.Therefore carbon dioxide injection has become an important green way to improve oil recovery in our country and abroad.According to the mechanism of CO2 drive,it can be divided into the miscible displacement and immiscible displacement,oil recovery efficiency of miscible displacement is higher than immiscible displacement,miscible drive efficiency tend to 100%,average reservoir recovery rate tend to 90%.To achieve CO2 miscible displacement,formation pressure must be higher than the minimal miscibility pressure.So the minimal miscibility pressure is an important parameter,it is the key factor to the enhanced oil recovery,rational forecast of minimal miscibility pressure has an important meaning for the design of miscible displacement.Experiment and theoretical calculation are the mainly methods used to forecast the minimal miscibility pressure presently.The result obtained from experiment is accurate and reliable,but it spent much time,the cost and the demand of accuracy to instrument is high,and it is easily affected by human factors.The theoretical calculation method is simple and direct,but the results are rough,adaptability is not strong.So,it is very necessary to search a method to predict minimal miscibility pressure.In this paper,the results of multiple mixing cell model used to predict minimal miscibility pressure are reliable,and calculation time is short.First of all,thermodynamic problem of contact of CO2 and crude oil multiple is studied.Material balance condition equations which describe balance gas liquid composition and material balance of molar are established.Equilibrium constant and effusion degree thermodynamic material balance equations and state equation that used to calculate phase balance are depicted.It provides theoretical basis for establishment of multiple mixing cell model.Secondly,filling slime tube used in the experiment of slime tube test is scattered to a series of cell cube which has same volume,continuous injection gas is scattered to a series of the same volume batch,and setting the temperature and pressure are constant in every flow unit,there are no physical diffusion and capillary pressure between flow units,every flow unit is calculated by thermodynamic P/T flash vaporization multiple,and mixing cell model is established.Thirdly,for a series of specific oil and gas as an example,to study the gas oil ratio and diffluent function of multi-level mixed element model have an effect on the path component and the development process of mixed-phase.To analysis the feasibility of using the multi-level mixed element model to forecast minimum miscibility pressure.Analysis the reliability of multi-level mixed element model,the results show that:the average relative error of multi-level mixed element model is only 2.78%,with good prediction accuracy.Additional,multiple mixing cell model is used to calculate ternary system,quadruple system and multivariate system.The key nodal line obtained is used to predict minimal miscibility pressure.The effects of CO2 gas purity,reservoir temperature and crude oil composition to minimal miscibility pressure are analyzed.The result shows that:?The critical temperature of impure gas is higher than CO2.As the CO2 gas concentration is increasing,the minimal miscibility pressure is bigger.Conversely,it decreased;?ln the condition that other reservoir characteristics are the same,as the temperature is increasing,the minimal miscibility pressure is bigger;?As the light and medium components of crude oil is increasing,the minimal miscibility pressure is smaller.At last,according to oil block of Fang 48 and Huan26,based on the minimal miscibility pressure that is determined by tubule experiment as the standard.All sorts of theoretical methods that can determine the minimal miscibility pressure are compared,the sphere of application and superior-inferior of every method are obtained.The error analysis shows that the method to forecast minimal miscibility pressure by the multiple mixing cell mode is most accurate.