The Macroscopic Description Study Of Seepage Flow Laws On CO₂ Flooding In Low Permeability Reservoirs

At present, most of Chinese continental oil fields use water-flooding development, but the recovery of most reservoirs are very low because of the impact of reservoir heterogeneity and other reasons, and the development effects are far from satisfactory. As a kind of greenhouse gas, CO₂ is also a non-polluting displacing agent. Many countries have already focused on the study of CO₂ in the oilfield development. It is noteworthy that the low-permeability oil fields which has been proved that CO₂ flooding can obtain good results for the low permeability and small fault block oil reservoir in which the water flooding has proven ineffective, but in our country the study of the Application of CO₂ is still at the initial stage.therefore, description and research of seepage laws of CO₂ flooding in low-permeability reservoirs are of great significance.In this article fluid flow parameters at the process of CO₂ flooding are modificated according to the geological and fluid datas of field oilwells and combining with the characteristics of CO₂ flooding, and the capillary force and actuating pressure gradient are taken into account, on this basis a one-dimensional non-miscible mathematical model of CO₂ flooding is built, then numerical solution is obtained, and also matches the experimental data to verify the reliability of the mathematical model. The results show that: the higher the viscosity of crude oil, the more obvious the effects of viscosity reduction by CO₂, and CO₂ solubility in crude oil drops with the drop of formational pressure; under the action of capillary force, the FMP is smaller; the actuating pressure gradient has little effect on the CO₂ drive in low-permeability reservoirs, therefore, CO₂ can be applied as a displacement agent for the development of low-permeability reservoirs.Considering the characteristics of CO₂ flooding, a math equation is set up and the characterizations of viscous fingering and influencing factors are studied. The laws of the impact of the injection-production pressure drawdown and reservoir temperature on the viscous fingering are regressed, and they can guide the actual production in the oil field. The results show that: With the injection-production pressure drawdown increases, the extent of viscous fingering appears in the trend of first increased then decreased; when injection-production pressure drawdown is maintained as a constant, with the increase of reservoir temperature, the extent of viscous fingering in CO₂ flooding is in growth trends. Actuating pressure gradient exists in oil region will lead to severe viscous fingering.Math equations of CO₂ miscible flooding are built considering the diffusion mechanism of mass transfer, and a numerical solution is carried out, and then the analysis of the effects of diffusion mechanism of mass transfer on CO₂ flooding is focused on. The differences between concentration-weighted correction method and empirical formula correction method are compared in view of correction problem of the fluid viscosity. The results show that: when diffusion of mass transfer is taken into account, CO₂ concentration distribution is superior to that diffusion of mass transfer is not taken into account; the value of CO₂ concentration derived from concentration-weighted correction method is greater than that derived from the empirical formul correction method , but because the empirical formul correction method takes full account of CO₂ dissolution characteristics and flooding characteristics, so it's more suitable for the amendment of the fluid parameters in CO₂ flooding.