The Percolation Mechanism And Parameter Design Of Water Alternating Gas Development In Low Permeability Tight Reservoirs

Waterflood in low permeability tight reservoir is characterized by shorter water free production period and faster watercut growth rate due to extremely low permeability and strong heterogeneity.Water and gas alternate injection(WAG)can effectively suppress water and gas breakthrough.However,complex oil-gas-water multiphase flow exists in porous media during water and gas alternate injection,and there are few documented studies on the distribution and transportation of three-phase system in complex lowpermeability tight reservoirs,which directly affects the development and application of this technology.First,3D digital core reconstruction of low permeability tight reservoir is carried out,water film dynamics in circular and angular pores are analyzed and evaluated,and then the characteristics of oil/water two-phase flow in low permeability tight reservoir are studied with consideration of the water film effect.Secondly,a pore network model considering the micro-nano scale effect of gas is established in order to investigate the characteristics of gas flow during WAG.On this basis,the influence of pore structure on gas flow through low permeability and tight formation is studied.Thirdly,the influence of water film on oil-gas-water three-phase flow is studied,and the relative permeability characteristics during WAG are analyzed by a combination of pore network simulation and empirical hysteresis model.Finally,based on the theoretical study,a numerical model was established with incorporation of start-up pressure gradient,permeability hysteresis effect and gas slippage effect.Taking SJG block as field example,optimization is implemented on the basis of the numerical model.The main work and conclusions of the dissertation are given as follows:(1)The pore network structure of typical cores in SJG block is reconstructed.The pore size distribution of 4 cores from SJG block in eastern Ordos was obtained by highpressure mercury injection method,and the digital core reconstruction based on nano-CT experiment laid a foundation for the subsequent pore network simulation.The results show that the porosity and permeability of the reconstructed cores are 7.64% and 0.14 m D,respectively.The results are close to laboratory results 8.98% and 0.18 m D,which can well reflect the pore structure characteristics of the core.(2)The traditional oil-water two-phase pore network model is improved to consider the influence of thin water film.The water film thickness in micro-nano pores was quantified,and the fluid distribution,capillary entry pressure and conductivity of each phase in pores of different shapes during drainage and imbibition were revised,forming an oil-water two-phase pore network model suitable for low permeability tight reservoir.The results show that when the average pore throat radius of the reservoir is less than1000 nm,the influence of water film on the relative permeability cannot be ignored.Water film mainly affects the relative permeability by changing water saturation,fluid connectivity and dynamic thickening and thinning in the pore throat system.(3)A 3D gas-water two-phase pore network model was established to investigate the characteristics of gas flow under different water-gas distribution modes.The model incorporates gas-water distribution characteristics,real gas properties and multi-scale gas transport mechanism,and then studies the effects of different injection processes and pore structure characteristics on gas flow.It is found that the gas slippage factor increases monotonically in the process of gas injection,while it increases first and then decreases during water injection,and there exists a critical water saturation,which is affected by the pore structure characteristics.(4)An oil-gas-water three-phase pore network model considering thin water film was constructed,and then a design method of WAG was formed,which takes start-up pressure gradient,permeability hysteresis and gas slippage effect into account.Previous two-phase pore network model is extended to three phase.Based on combination of pore network and empirical hysteresis method,relative permeability characteristics during different cycles are revealed.Finally,a platform which can interact with CMG is established to incorporate the variation of relative permeability with time.The result showed that regardless of the thin water film will overestimate the maximum gas saturation during gas injection process,and it affects the prediction and optimization process of WAG development.