Study On The Mechanism Of Air Injection For Enhanced Oil Recovery In Typical Shale Reservoirs

As a typical tight reservoir,shale oil is widely distributed in China.However,due to the poor continental sedimentary environment of shale reservoirs in China,shale oil development is facing the difficulties of low single well production and rapid production decline.It is urgent to find an economic and effective method to enhanced oil recovery.The studies have shown that air flooding has attracted much attention due to its wide gas source,expansion and viscosity reduction.However,the physical and chemical reactions and dynamic oil displacement mechanisms involved in the process of air flooding in shale reservoirs are extremely complex,and there are still gaps and deficiencies in domestic basic theoretical research.Therefore,it is necessary to study the mechanism of air flooding in shale reservoirs and the law of fluid migration in porous media.In this thesis,the analyses of cast thin sections,mercury injection,and nuclear magnetic resonance were carried out first.Combining with medical CT,micro-CT scanning technology,MAPS technology and advanced mathematical algorithms of Avizo visualization software,which reconstructed 3D digital of shale.The comprehensively quantitative study of pore structure characteristics was studied.Moreover,combined with PVT experiments,the phase physical properties of shale oil and the characteristics of air flooding seepage were studied.The online NMR and online CT system were used to carry out air injectability and visual dynamic oil displacement experiments,and the air displacement mechanism and fluid production laws under different development conditions were studied.Finally,the enhanced oil recovery mechanism and matrix fracture material exchange law under different injection timings were studied.The results show that:（1）The pore-throat scale of shale reservoirs is widely distributed,and the pore-throat distribution is bimodal,which can be divided into three orders of magnitude.The pore throat distribution has the characteristics of"large pore and fine throat".The contribution rate of 0.12-1.75μm throat is the highest,the content of nano-scale pore throats is high,and the submicron pore throats have a greater contribution to seepage.The low angle inclined bedding has low contribution to shale oil drainage and production.（2）The oxygen-containing air helps to dissolve the gas phase into the oil phase,reduce the saturation pressure of the system and improve the mobility ratio of crude oil,but（Oxygen-reducing）air has a poorer effect on improving the physical properties of crude oil than CO₂.The permeability loss ratio of overburden gas measurement can reach 92%,while the liquid measurement can reach 42%,and the liquid flow is extremely difficult due to boundary layer effects during gas flooding.（3）Increasing the injection pressure difference and fracturing can effectively improve the injection effect and shorten the energy replenishment period.Depletion production mainly produces crude oil with pores of 0.2～2μm,and air flooding mainly produces pores of 0.02～5μm.During the air flooding process,the macropores and submicron pores produce oil first,with the increase of the displacement pressure difference,the macropores produce less oil,medium pores become the main source of oil production,and the oil production from the nano-scale pores shows an increasing trend.（4）Under different oxygen content（5%,8%,21%）,the oil displacement efficiency gradually increases（41.62%,43.57%,45.71%）.The higher the oxygen content,the higher the gas injection pressure,the higher the permeability,the more obvious the thermal effect and viscosity reduction effect,and the higher the oil recovery rate.（5）The shale pore network model based on digital core reconstruction has high precision and accuracy.The total recovery factors of air flooding under different depletion pressures are 43.28%,41.39%and 39.27%respectively,and the lower the depletion pressure,the lower the recovery rate of the air flooding.The proper fracturing before air injection can help the matrix supply oil to the fractures,which can improve the oil displacement effect of air injection.