Research And Application Of The Method To Improve Miscibility And Sweep Efficiency In The Gas Flooding Reservoir

A large number of practices and studies show that gas injection technology can greatly improve oil recovery,which has a wide range of applications broad prospect.However,in the middle and late stage of gas flooding,the formation energy deficit is serious,which makes it difficult to form miscibility between oil and gas,and it is often accompanied by severe gas channeling in the gas flooding reservoirs.All of these lead to a rapid decline in oil production and poor performance during the gas flooding.Establishing a rational evaluation system during the gas flooding and formulating reasonable and effective adjustment measures are crucial for improving the performance of gas flooding and popularizing the gas injection technology.Firstly,based on the phase equilibrium theory and reservoir numerical simulation method,this paper analyzes the characteristics of composition distribution and interfacial tension distribution of the reservoir fluids during gas flooding,and puts forward the idea of subdivision of miscible state and sweep efficiency.Then two parameters,immiscible and miscible critical interfacial tension,are defined and miscible states are subdivided into four states,namely complete miscible,miscible,near miscible and immiscible.Further,two parameters,the volume coefficient and degree coefficient of miscibility,are defined to represent the distribution range and the contribution rate to oil recovery of different miscible states.Considering the strong diffusion capacity of gases,the concepts of component sweep efficiency,effective component sweep efficiency and phase sweep efficiency are put forward to characterize the sweep range of the injected gas.Based on the above,the numerical simulation models are established to analyze the influence of reservoir and development parameters on the degree of miscibility and sweep efficiency,and the main control factors are determined through orthogonal experiments.Aiming at the identification of gas channeling,the paper uses equivalent thought and flow tube bundle model to equate fracture and matrix into a series of flow tube bundles,which converts the two-dimensional seepage problem into a one-dimensional tube bundle flow.Through the superposition method,the gas channeling models of main-fracturetype and differential-fracture-typ are solved and the concentration equation of produced gas of different gas channeling can be obtained.Then the influence of different parameters on the component concentration and its derivative are discussed and the identification methods of gas channeling and fracture parameter are proposed by fitting the actual data.Based on the above two parts,the zoning control and step-by-step control technology is proposed to improve the degree of miscibility and sweep efficiency during the gas flooding.Using the characterization parameters of sweeping and miscibility of gas flooding,the reservoir is divided into five parts: unsweeping zone,low potential zone,near-miscible zone,high-pressure immiscible zone and low-pressure immiscible zone.Moreover,the zoning control method is proposed.Aiming at the multistage fracture channeling in the process of gas channeling,the type of gas channeling and fracture parameters are determined by using the gas channeling identification method,and then the step-by-step control method of injection of polymer-water-gas displacement system is proposed.For gas flooding reservoirs,an evaluation system of nine evaluation indexes including development performance,sweep state,pressure state and miscibility state are selected.A comprehensive evaluation method for gas floodingeservoirs is proposed with the fuzzy mathematics method and then the performance of the zoning control and stepby-step control technology is evaluated with this method.Finally,the established zoning control and step-by-step control technology is applied to a large carbonate reservoir and a set of development and adjustment schemes are proposed.The adjusted schemes are compared and evaluated,which provides an effective means to improve the performance of gas flooding in this reservoir.