Experimental Study On Mechanism Of Foam Assisted Gas Flooding In Typical Fracture-vuggy Medium Of Carbonate Reservoirs

The storage spaces of fractured-vuggy carbonate reservoirs are mainly composed of structural fractures,dissolved pores and vugs,the distributions of which are extremely uneven.Besides,the number,location and size of fractures connected with vugs vary greatly.So the heterogeneity of fractured-vuggy carbonate reservoir is extremely severe.In the early stage,the exploitation of carbonate reservoirs depended on natural energy in formation.Then the water or gas was injected to supply the formation energy after natural energy depletion.In the process of water flooding,it occured the principle problem of rapid rise of water cut,which caused low ultimate oil recovery.In the process of gas flooding,gas channeling easily formed due to the formational heterogeneity of fractures and vugs.Foam fluid,a profile control agent,could control gas mobility,adjust gas suction profile,and finnaly delay gas channeling time.Foam assisted gas driving(FAGD),which is the alternated injection of foam slugs and nitrogen slugs,could reduce the use of chemical agents.However,the EOR mechanisms of FAGD in fractured-vuggy reservoirs are still not clear.Therefore,it is necessary to carry out the laboratory physical experiments to study the mechanisms of FAGD.In this paper,Four typical visualized fractured-vuggy models including the 2D fractured model,the 2D vuggy model,the 2D fractured-vuggy combination model and the 3D fracture-vug model,were designed and fabricated based on the analysis of the fracture-vug scale,the spatial distribution and the connectivity relation,etc.FAGD experiments were subsequently conducted based on the physical models.The EOR effects and mechanisms of FAGD in different models were simultaneously evaluated and analyzed.The experiments in the fractured model showed that the gas migration status in fractures was related to the fracture aperture.Gas escaped faster in the expanded fractures,while the inertial force played a decisive role in the variable aperture fractures because of the gas buffer effect of foam slugs.The study in the vuggy model showed that it easily formed gas channeling in the higher part of vugs.But in the process of foam flooding,it could stabilize the oil/gas interface,effectively control gas mobility and expand the swept volume.The research in the fractured-vuggy combination model showed that gas fractional flow rate in paralleled combination model differed widely in gas flooding process.The foam slug could adjust the gas suction profile and reduced the difference of the fractional flow rate,thereby expanding the swept volume.In the tandem combination model,the foam could expand the swept volume of the proximal vugs and delay the gas cap expansion rate of the remoted vugs.Finnaly,the mechanisms of FAGD was verified through the physical experiments in the 3D fractured-vuggy model.The results showed that it would form the foam slug in fractures and high parts of vugs due to the longitudinal and horizontal propagation and accumulation,which could change the path of gas driving,reduce the difference of gas flow distribution in the fractured-vuggy joints,and expand the spread range of gas flooding.Meanwhile,the injected foam could stabilize the gas cap interface and create a synergistic effect with the bottom water.After defoaming,the free gas could also enhance the gas cap energy,and the foam stabilizer could increse the viscosity of the water phase.Furthermore,the foaming agent could reduce the oil/water interfacial tension that a higher degree of the remaining oil was activated and swept.