Study And Application Of CO₂ Leakage Migration Under Oil Flooding And Storage

With the improvement of petroleum exploration and development and the urgent need to reduce carbon emissions,CO₂ flooding oil seal technology is becoming a new crossover technology.However,geologically sealed CO₂ may migrate upward from deep reservoir to shallow reservoir through potential leakage channels.The study of CO₂migration law under complex geological conditions is an important part of monitoring design work.In this study,a geological model of shallow surface and reservoir was established based on the actual CO₂ flooding and sequestration test area.TOUGH2 software was used to simulate and discuss the effects of CO₂ source rate,formation parameters and permeability heterogeneity on CO₂ migration and diffusion process in shallow surface under static and dynamic groundwater conditions.At the same time,the hypothetical CO₂ leakage from the cap layer and from the cap layer to the surface was simulated on the scale of the storage body.Finally,a set of CO₂ leakage risk assessment method in the storage body was formed based on the result rule combined with the comprehensive index analysis method and the analytic hierarchy process.The results show that the CO₂ migration is controlled by pressure difference,density difference and concentration difference in the saturated zone to unsaturated zone of shallow surface,resulting in dissolution,convection and diffusion effects.The overall migration trend of CO₂ is upward,and after passing through the aquifer,it spreads laterally in the soil aeration zone to form the“mushroom top”,and forms the maximum diffusion range on the surface.High CO₂ source rate,high formation porosity and permeability,and high water saturation will increase the size of the leak zone.Compared with the loess ridge and mound area,the high permeability thin layer in the terrace area of the valley has a worse tolerance to CO₂ leakage,and the water level of the submerged layer has an obvious rising trend.Surface rainfall and Wells jointly affect and form complex shallow groundwater environment,which indirectly affects CO₂ migration.Rainfall and transverse groundwater inhibit vertical CO₂ diffusion.The well extraction effect directly changes the leak zone,depending on the relative location of the well to the source and the size of the bottom hole differential pressure.The downstream well extraction effect is more significant.The CO₂ source rate is the decisive factor of the diffusion height.Under the leakage rate of 1×10^-5 kg/s,CO₂ does not migrate upward to the vadecapsules,but all migrate along the flow.Simulation at the sequestration scale shows that CO₂ does not break through the cap layer in 500 years.The pointing phenomenon caused by the heterogeneity in the strata above the cap will accelerate the CO₂ migration,and the high anisotropy will produce a larger scale of the leakage zone.Under the influence of pressure superposition,the CO₂ transport rate of multi-source leakage is less than that of single-source leakage.Finally,the shallow and deep factors affecting the CO₂ migration and the risk threshold were determined.On this basis,the safety assessment method of CO₂ leakage risk was established to ensure that the prediction or monitoring results could scientifically and objectively identify the risks,which is of great significance to the environmental safety monitoring and early warning work of CO₂flooding and storage.