Influence And Mechanism Research Of Porosity And Permeability Parameters On MMP Of CO₂-Oil

CO₂ miscible flooding is the most promising EOR technology that can be widely used in low permeability sandstone reservoirs.The minimum miscibility pressure(MMP)of CO₂-oil is the key parameter to determine the miscible flooding.At present,the generally accepted method of measuring MMP is a slim tube experiment,but the slim tube test has been questioned to be quite different from the actual reservoir,and it didn't saturated with formation water,so the measured minimum miscibility pressure can't represent the actual reservoir.The research on the influencing factors of MMP focuses on the purity of CO₂,the influence of O₂,the composition of crude oil and the temperature of reservoir.However,few have studied the effects of reservoir permeability,porosity and formation water salinity on MMP.In view of the above problems,firstly,this paper used three groups artificial cores with different permeability and three groups artificial cores with different porosity to measured CO₂-oil MMP by coreflood test,and compared with the result of traditional slim tube test,obtained the influence of porosity and permeability parameters on MMP.Secondly,three groups of Berea cores with similar porosity and permeability were used to measure the MMP of CO₂-oil by coreflood test after saturating different salinity formation water.Combined with the CT scan results of core slice,the influence mechanism of formation water salinity on MMP was obtained.In the experiment of the influence of permeability and porosity on MMP,the cores with permeability of 45×10^-3?m²,104×10^-3?m²,342×10^-3?m²,the measured MMP were 20.80MPa,21.26MPa,21.32MPa,and the cores with porosity of 23.56%?28.31%?31.74%,the measured MMP were 20.80MPa?21.36MPa?21.70MPa,respectively.The higher the porosity and permeability,the higher the measured MMP.The analysis shows that:in cores with low porosity and permeability,CO₂ flooding has a long breakthrough time,increase the contact time between CO₂ and crude oil,intense the molecular diffusion,mass transfer and extraction,so it is easy to reach the miscible phase.In the experiment of the influence of formation water salinity on MMP,the cores with salinity of 0mg/L,6778mg/L,15000mg/L,the measured MMP were20.90MPa,19.93MPa and 19.13MPa.With the increase of salinity,the MMP of CO₂-oil decreases,because of that with the increase of formation water salinity,the solubility of CO₂ in formation water decreased and more CO₂ dissolved in crude oil.During the CO₂ flooding process,the porosity and permeability of the high salinity core decreased,so the measured MMP was smaller.In the coreflood test,core porosity is the most important factor affecting the determination value of CO₂-oil MMP,followed by formation water salinity,and core permeability is the least important factor.