Study On Injectivity Of CO₂ Water-alternating-gas Flooding In Low Permeability Reservoirs

CO₂ water-alternating-gas flooding in low permeability reservoirs combines the advantages of CO₂ flooding and water flooding,and can greatly enhance oil recovery,which has broad prospect in the future.However,there appears the problem of reduction of injectivity during the process of CO₂ water-alternating-gas flooding,which seriously influence the development of water alternating gas injection.Currently,in our country,there is lack of recognition and research on the reason and mechanism that cause the reduction of injectivity when CO₂ water-alternating-gas flooding in low permeability reservoirs is implemented.Therefore,it is necessary to study the variation law of injectivity from different scales.In micro-scale,considering the effect of transferring and diffusing between CO₂ and oil,according to the difference in fluid distribution patterns,the process of water alternating gas can be divided into different seepage stages.Introducing the concept of equivalent resistance,the unequal radius capillary model used for CO₂ water-alternating-gas is presented,which is solve by the iteration method.As there is a well in the center of closed boundary reservoir,considering the dissolution of CO₂ in oil and injected water and the effect of CO₂ on the vaporization of light components in oil,after modifying the B-L equations and combing the theory of multi composite reservoir,the semi-analytical model of CO₂ water-alternating-gas in low permeability reservoirs is developed.The model is solve by applying the method of Laplace conversion and stehfest numerical inversion,which is validated by comparing the numerical simulation results.In order to reveal the injectivity changing ruler in large scale,such as the scale of reservoir,based on the characteristics of geology and fluid in practical field,the numerical simulation models used to simulate CO₂ water-alternating-gas in low permeability reservoirs are developed.The factors that have influence on injectivity are analyzed and optimized.The results demonstrated that water-alternating gas injectivity decreases with the increase of oil viscosity and the interfacial tension between CO₂ and oil.As the length of artificial fractures and CO₂ slug increase,the injectivity will become larger.In the five-spot well pattern,the best development effect can be achieved.In terms of the reservoir parameters,permeability has the largest effect on the injectivity.The injectivity in oil-wet formations is larger than water-wet formations.The permeability anisotropy,the sedimentary rhythm in vertical direction and plane heterogeneity have different permeability distribution modes,hence lead to the formation of difference in injectivity.The achievements of this research can provide theoretical instructions and technical supports for the scheme design of CO₂ water-alternating-gas in low permeability reservoirs.