Mechanism Study Of Interactions Of CO₂-crude Oil-micropore Systems During CO₂ Flooding In Tight Sandstone Reservoirs

CO₂ flooding has been widely applied to enhanced oil recovery from conventional reservoirs.Nanoscale pore throats are more developed in unconventional tight sandstone reservoirs.However,the performance of CO₂ flooding in tight oil reservoirs could be overshadowed by not only the fact that the injectivity of CO₂ is low in the tight sandstones,but also by the fact that the asphaltene precipitation due to CO₂-oil interaction can cause formation damage issues.Simultaneously,abundant micro/nano pores in such reservoir are readily blocked due to the complex interactions among CO₂,brine,and rock during the CO₂ flooding.It is very important to do the reveal the mechanism of interaction between CO₂,oil and microscopic pore-throat system during the CO₂ flooding in tight sandstone reservoir.The tight sandstone reservoirs have characteristics of small pore throats,complex structures,and strong heterogeneity.Before the CO₂ flooding test,the cores samples were analyzed by cast thin sections,scanning electron microscopy,high-pressure mercury injection,constant-rate mercury injection,X-ray diffraction,field emission scanning and nano-CT.Core samples were selected for physical simulation tests.After the experiment,the asphalt content and the ion content of the outlet fluid were analyzed to determine the amount of asphaltene precipitation and the degree of mineral dissolution.For each core-flooding experiment,the T₂ responses of hydrogen nucleus in the core samples are measured before and after CO₂ flooding,and then compared to quantitatively determine the blackage degree of pore throats during the CO₂ flooding.The following conclusions are obtained:?1?It is found that in the immiscible flooding stage,the degree of asphaltene precipitation increases with an increase in the CO₂ injection pressure.After entering the miscible flooding stage,asphaltene can be still precipitated,but with a much less magnitude.The core permeability tends to be reduced after CO₂ flooding,and the permeability reduction is positively correlated with the amount of asphaltene precipitated.The amount of asphaltene deposition is positively correlated with the CO₂ injection pressure.?2?At a low injection pressure,a small portion of the nanoscale pores?10nm?1000nm? and microscale pores?1?m?1000?m? is filled up with the asphaltene precipitated.When the injection pressure increases above the MMP,more than 55.13% of the nanoscale pores are blocked,while only 19.58% of the microscale pores is affected by the asphaltene.As the pressure further increases,72.32% of the nanoscale pores are blocked.?3?After the CO₂-pore-throat interaction,the pH value of the brine after the experiment is lower than that of before the experiment,the change of the ion concentration before and after CO₂ flooding reflects the dissolution of feldspar and carbonate minerals.When the interaction time is 60h,5.58%-7.74% of the nanoscale pores?100nm?1000nm? was blocked;when reaction time reach at 120h,the blockage degree is more than 30.98%.But,the microscale pores?1?m?100?m? has not been significantly blocked?3.05%?.?4?CO₂-oil-pore-throat has the high degree of blockage in cores with low permeability and poor pore structure The damage due to the CO₂-pore-throat interaction on the core permeability was significantly higher than that of the porosity.The damage degree was positively correlated with the reaction time,and had no significant correlation with the CO₂ injection pressure.Because of the small pore size and low permeability of tight sandstone cores,the particles produced by the interaction of CO₂-pore-throat will have a serious plugging effect on the pore-throat structure,which in turn will result in a decrease in pore-throat seepage capacity and core permeability.Finally,the CO₂ flooding efficiency and oil recovery will be reduced.