Study On EOR Mechanism In Fractured-Vuggy Carbonate Reservoir

The Ordovician of Tarim Basin is a typical fractured-vuggy carbonate reservoir with strong heterogeneity and various storage space,which leads to an unsatisfied development effect of fractured-vuggy carbonate reservoir in China.Hence,it is necessary to study the EOR mechanism in fractured-vuggy carbonate reservoir.In this paper,based on Ordovician carbonate reservoir in Tarim Basin,both the numerical simulation and experiment method have been carried out to study the EOR mechanism in fractured-vuggy carbonate reservoir.Firstly,the reservoir development characteristics have been analyzed,and the typical fractured-vuggy combination pattern has been built with energy situation and reservoir development characteristics.Then,four types of models have been introduced including single fracture models with different vug positions,single fracture models with different vug density,multi-fracture models with different vug porosity and models with different fracture-vug networks.Secondly,in terms of water injection development,the well selection criteria of water injection displacing oil is proposed systematically by analyzing the influence factors of the water injection displacing oil in the actual reservoir.Based on the typical fractured-vuggy combination pattern,the infuence mechanism on water flooding recovery efficiency has been determined.The oil recovery factor of waterfooding is mainly influenced by gravity,vug density and vug porosity,fracture-vug connectivity,vug distribution and density of fracture along the water flooding direction.The main reason of water breakthrough is the high density of fracture along the water flooding direction,and the existence of vug between the fractures can delay the water breakthrough.The higher the density of fracture along the water flooding direction is,the greater the influence of well spacing on the oil recovery factor will be.And then the well selection criteria of water flooding is built.Thirdly,as for water injection development,the oil physical property experiment,phase analysis and gas flooding numerical simulation have been carried out on the oil samples in Tahe oil field,which implies the EOR mechanism with gas injection displacing oil.The EOR mechanism with gas injection displacing oil is further discussed in the aspect of miscible and immiscible phases,which shows that the selection of dispacement method depends on the residual oil occurrence state,and the immiscible displacement is more suitable for the single well fractured-vuggy unit model with constant volume,its EOR mechanism is to reduce oil viscosity,increase formation energy by volume expansion of gas,gas displace oil in small pores,and form the artificial gas cap with gravitational differentiation to displace remaining oil at the top of reservoir.Based on the typical fractured-vuggy combination pattern,the influence mechanism on immiscible gas flooding recovery efficiency has been studied,finding that the main factors which influence the oil recovery of immiscible gas fooding and the main reason of the gas breakthrough are similar to those of waterflooding.Compared with waterflooding,the gasflooding process uses gravitational differentiation more fully,and is more sensitive to fracture.It is better to carry out water flooding when the density of fracture along the flooding direction is highe.Then the well selection criteria of gas flooding is built.Fourthly,the indexes are optimized from two stages including natural energy development period and supplement energy development period.Analytical and numerical simulation methods are introduced to solve the reasonable productivity and production pressure difference with different energy and reservoir types.Based on the actual reservoir areas,the single well fractured-vuggy unit with constant volume model,multiple well fracture-vug unit with high fractured conductivity model,and multiple well fracture-vug unit with non-fractured conductivity model is built to optimize the develop way and development indicators.The results provide a solid foundation for the development decisions in fractured-vuggy carbonate reservoir and have a certain application value.