Study On Action Mechanism And Mass Transfer Law Between Shale Oil And Injected Carbon Dioxide Huff-n-Puff

The application of carbon dioxide huff-n-puff in shale reservoir can effectively improve the properties of shale oil,increase reservoir energy and further improve its recovery.However,for shale reservoirs with tight pores,the injected carbon dioxide and shale oil involve complex mass transfer process,and are affected by many factors,thus the final development effect behaves differently.In this thesis,based on NMR and imaging technology,the experiment of carbon dioxide huff-n-puff of shale oil is carried out to obtain the macro mass transfer characteristics and laws of carbon dioxide and shale oil in matrix and pores under different injection and drainage modes.Then,molecular dynamics simulation is used to study the interaction law and mass transfer characteristics of crude oil and carbon dioxide under different conditions.Through the understanding obtained from micro simulation,the results of macro experiment are analyzed.Finally,the influence of different pressure disturbance modes in injection and drainage production stages on development effect in the process of carbon dioxide huff-n-puff is obtained.The results show that the increase of pressure will have two effects on crude oil.On the one hand,pressure will inhibit the change of volume strain and viscosity of crude oil system and restrict the movement of crude oil.When the pressure increases by 10MPa,the average volume strain of crude oil decreases by about 1.106%;On the other hand,pressure helps carbon dioxide enter the crude oil system and promotes the volume expansion and viscosity change of the crude oil system.Under various pressure conditions,carbon dioxide can promote the crude oil system to produce about 3.107%volume strain.By setting different gas injection and drainage methods,under the same pressure impulse,the average crude oil production degree of linear pressurization mode is 26.12%,which is better than the pulse pressurization mode with 19.95%average crude oil production degree.Gradual pressurization has a good effect on crude oil production in macropores,at the same time,after gradually accumulating more two-phase molecules on the two-phase contact surface in small and medium pores,the two systems achieve a better contact,forming a better convective mass transfer effect,and play a better extraction effect in the process of carbon dioxide movement.In small pores,the solubility parameter difference of two-phase fluid in linear pressurized injection is 2.169（J/cm³）^1/2,which is lower than 2.531（J/cm³）^1/2 in pulse pressurized injection.The two-phase under linear pressurized injection shows a better dissolution state.Compared with the linear drainage mode,under the pulse drainage mode,the carbon dioxide system is greatly affected by the quick pressure changes from the critical state to the gaseous state,and the convection mass transfer environment is damaged,resulting in a worse crude oil stripping effect.