Numerical Well Test Theory And Interpretation Method Of Carbon Dioxide Flooding

As one of the most important methods to enhace oil recovery（EOR）,the CO₂ flooding can not only improve the ultimate oil recovery,but also mitigate the greenhouse effect,which makes it more and more attractive.However,the big difference between the viscosities of CO₂ and in-situ oil can trigger viscous fingerings close to displacement front,and the large difference between the gas and oil densities provokes the gravity override possible.One of the efficient ways to overcome those negative factors is to reduce the viscosity and density differences by developing the miscibility between the in-situ oil and the injected gas.Therefore,it is important to monitor the injection process to know the in-situ distributions of phases,based on which we can adjust the injection and production strategies to guarantee the miscibility underground.We will focus on the well test approach to monitor the process of CO₂ flooding.First,use the equation of state（EoS）,phase equilibrium equation and flash calculation to describe the complex displacement mechanisms,then establish the numerical well test model of CO₂ flooding based on compositional model.There are three inner boundary models include standard well model,wellbore storage coefficient model and multi-segment well model.We can pick up the proper well model according to the real situation to reproduce the well test curve accurately.Next,a corresponding analysis method is proposed to analyze the effect of rock/fluid properties at any position on the well test curve in detail;based on that,we analyze the well test curve of injection well at immiscible,multi-contact miscible and first-contact miscible conditions,then we have a better understanding of the features and influence factors of the well test curve;a character line method is proposed to determine the point on the well test curve corresponding to the CO₂front;the well test curve of production well is also analyzed.Third,to deal with realistic models,we design a special meshing strategy for the 3D unstructured grids;a flow based grid resolution analysis is proposed to obtain the optimal gridding strategy,which can reduce the grid number significantly with high accuracy simulation results;to improve computational efficiency further,we apply the dynamic mesh technique to solve the the well test model.Fourth,we establish the well test model of CO₂ flooding in low permeable reservoir,the well test model of CO₂ flooding in fractured reservoir based on the discrete fracture network model,the well test model of CO₂ flooding of fractured well based on the discrete fracture model;based on these models,we discuss the features of well test curve separately.Fifth,an optimization method of the injection/production stratey is proposed to develop the miscibility underground,and then enhance the oil recovery and the storage of CO₂.Sixth,the SPSA method is applied to perform well test curve matching to reduce the uncertainties underground.We establish the numerical well test theory and interpretation method of CO₂ flooding,the optimization method of the injection/production strategy based on well testing technique,which provide a theoretical basis and technical support for the efficient application of CO₂flooding technique.