Study On CO2-N2 Flooding Characteristics In The Ultra-low Permeability Homogeneous Cores

In targeting the common problems found during devleopment of ultra-low permeability such as injection difficulty,fast water breakthrough,low production,etc.and recognizing the advantage of better injectivity of gas in ultralow permeability reservoir and its synergistic characterisitic in waterflood,perform experimental oil displacement with CO₂-N₂ micisible flood,CO₂-N₂ slug flood in ultra-low permeability reservoir to assess the applicability of these methods.In order to produce qualified experimental low-permeability/dense cores,the optimization of process parameters for artificial core production was carried out.Through the experiments,the influence of cement dosage and reaction time on the cement core strength were obtained;the regular pattern of pressing pressure and pressure holding time on core permeability were obtained.Accordingly,the parameters such as dosage of cementing agent,reaction time,pressing pressure and pressure holding time were optimized,the minimum permeability of produced cores can reach10^-2×10^-3?m².N₂ percolation experiments were conducted using artificial dense cores and natural cores in high pressure and atmospheric pressure conditions.The results show that the high-pressure N₂ flow deviates significantly from Klinkenberg's first-order slip theory;the consistency of the N₂ percolation test results between the artificial cores and the natural cores shows that the artificial dense core can satisfactorily simulate the main properties of the actual reservoirs.Using long artificial ultra-low permeability core?30cm?,five oil displacement simulation experiments separately using CO₂,CO₂-N₂ mixture gas,CO₂-N₂ slug,intermittent injection CO₂ and CO₂-water alternate injection were conducted in the high pressure conditons.Comparing the five oil flooding methods,the author concluded that the CO₂-water alternate injection has the highest oil displacement efficiency?75.56%?;the efficiency of CO₂-N₂ mixture gas oil flooding?55.87%?is higher than that of the CO₂ flooding.Considering the injection pressure,the injection pressure gradient of CO₂-water flooding in the ultra-low permeability cores is as high as 33.42 MPa/m,while the injection pressure of CO₂-N₂ gas mixture?6.31 MPa/m?is significantly lower than that of CO₂-water alternate injection flooding.Comprehensively evaluating from aspects of flooding efficiency and injection capacity,CO₂-N₂ mixture gas and CO₂-N₂slug drive are a kind of highly prospective technology for enhanced oil recovery in ultra-low permeability reservoirs.