Experimental Study On Enhanced Oil Recovery By Supercritical CO₂ Huff And Puff In Tight Reservoir

With the development of oil and gas field development technology,unconventional oil and gas resources such as tight oil have gradually formed an industrial scale all over the world,especially the successful development of tight oil in the United States,which has greatly promoted the development of the global oil and gas industry and brought significantly promote exploration and development of global oil and gas.Tight reservoir has some problems such as formation natural energy shortage,water drive energy supplement difficulty and strong heterogeneity,CO₂ has a strong ability of diffusion,and can be diffused into the deep oil reservoir more quickly,resulting in swelling of crude oil volume,reduction in the viscosity of crude oil and enhanced flow capacity of crude oil.Thus,the crude oil production can be improved and the residual oil saturation can be reduced,therefore,the study of CO₂ enhanced tight oil recovery technology is of great significance for the effective development of tight and shale reservoirs.Through deep research on the mechanisms of supercritical CO₂ huff and puff to improve oil recovery,the minimum miscible pressure of CO₂ and crude oil was determined,and the laboratory experiment of tight oil supercritical CO₂huff and puff to improve oil recovery was set up,and the influences of huff and puff parameters on oil recovery were studied.Firstly,basic parameters were obtained by conducting experimental studies on physical parameters of tight cores and physical parameters of crude oil.Then,by designing the dead crude oil expansion experiment in the high-pressure visual kettle,the fitting parameters of the fluid were obtained,and the influences of pressure,temperature and molecular weight on the swelling and extraction mechanism were analyzed.In order to make the experiment repeatable and easy to operate,the effects of lithology and porosity,CO₂ injection amount,huff and puff cycles,injection time and attenuation rate on enhanced recovery by CO₂ huff and puff were studied with n-dodecane as the simulated oil.The research results show that the reservoir formations are compact with porosity less than10%and permeability extremely low.Micro-fractures exist in tight sandstone.Dead crude oil is a light oil with a viscosity of 3-5 m Pa·s.its saturated hydrocarbon content is as high as 68.52%and its asphaltene content is relatively low,which is only 0.79%.Near miscible pressure,the interaction between CO₂ and crude oil is the best,the swelling factor is the largest,and the extraction effect is the most obvious.When the molar composition of CO₂ is low,temperature has no influence on the swelling of the interaction between CO₂ and crude oil,and the variation trend of temperature on the expansion coefficient is different in different pressure ranges.High temperature is conducive to the interaction between CO₂ and crude oil,and the smaller the molecular weight of crude oil,the more obvious the swelling and extraction.The law of enhanced recovery by supercritical CO₂ huff and puff is as follows:the higher the porosity,the higher the total recovery,the influence on matrix recovery is greater than fracture recovery.Due to the adsorption and dissolution of kerogen,shale recovery is lower than tight sandstone.When CO₂ injection pressure is at the miscible pressure and close to the miscible pressure,the CO₂injection rate rises sharply,and the effect on fracture recovery is lower than that on matrix recovery.Recovery rate increases with the increase of injection time,increases with the increase of huff and puff cycles,but decreases gradually,and slowly increases with the decrease of attenuation rate.