Experimental Research Of CO₂-Assisted Steam Flooding On Heavy Oil Reservoir In Qigu Group Of Nine-six Block

Due to the large reserves and highly developing difficulties of heavy oil resource in China,traditional methods such as cyclic steam injection(CCS)and steam flooding not meet the production demands,especially in the middle or later developing stage.Applying CO2 assisted steam flooding technique not only can strengthen steam driving oil effect,highly enhanced oil recovery,but reducing CO2 emission and mitigating greenhouse influence.After more than 20 years steam injection developed,Qigu group of Nine-six block has entered high water cutting and low oil and gas ratios developing tail,so CO2 assisted steam flooding as the precursive test is carried out in this field.However,the mechanisms of this technology not be well understood,and injection parameters at reservoir condition should be further optimized.In this paper,we firstly researched physical and chemical properties of crude oil,and PVT experiments were conducted to study the effect of CO2 on high-pressure properties of crude oil.Then,oil emulsification and CO2 demulsification experiments were carried out to investigate emulsion rheology and CO2 demulsification efficiency.Finally,injection parameters were optimized by carried out sand-packed tube physical simulation experiments.The results showed as follows:(1)The relative density of crude oil in target reservoir is 0.9435,and the oleic viscosity is 23670 mPa s at normal temperature and pressure(20?,101.325KPa).The oils is extra-heavy oil,which's viscosity is sensitive to temperature.Oil carbon numbers are mainly distributed in C19 to C30,which account for 61.986%of the total carbon content.The light constituents(saturate and aromatic)of SARA account for 70.89%,and 29.11%is composed by heavy fractions(resin and asphaltene).The crude oil is highly acidic oil and the oleic acid value is between 3.2 to 5.1 mgKOH/g.Carbon numbers of oleic acid components are mainly distributed in C16 to C44,multi-constituents of acid components including O1,O2,N1,N1O2 etc..(2)The experimental results of the influence of CO2 on oleic high-pressure properties are shown that with the increment of temperature and dissolved CO2 gas content,the volume factors of crude oil increase,the relative densities and API degrees decrease,and the oleic viscosities decrease significantly.The oleic volume increased by 12.2%,the volume factor increased to 1.122,and the relative density decreased to 0.854 at 120? and dissolved 25%of CO2.Viscosity reducing rate achieved 54.7%at 80? and dissolved 25%of CO2.(3)Heavy oil of Nine-six block is easily emulsified to form water-in-oil emulsions,which are characterized by higher viscosity than crude oil.Emulsion viscosity is insensitive to temperature when water cut of emulsion is 58.11%.With the increment of water cut of emulsion,emulsion rheology is characterized by shear thinning behaviors,the fluid is transferred from Newonian fluid to non-Newonian fluid,and the linear viscoelasticity region(LVER)is narrowed.In high frequency circumstance,the decreased difference between elastic modulus(G')to viscous modulus(G")of emulsion is achieved,and mobile properties of emulsion are transferred from fluid to colloid colloform.(4)CO2 has demulsification function,and the demulsified efficiency increased with demulsified pressure dropped.Viscosity reducing rate of emulsion reached 98.21%when the demulsified efficiency is 63.96%,indicating remarkable viscosity reducing effection.(5)Comparing steam flooding,improvements of 14.85%to 23.25%are achieved by using CO2 assisted steam flooding method,and recovery factor of alternating injection is higher than mixed injection.Alternating injection of CO2 and steam is characterized by water alternative gas(WAG),which can achieves maintance of pressure,improving viscosity reduction effection of CO2 dissolved and demulsification,enhancing mobility of crude oil,improving water/oil mobility ratio,reducing water cutting,increasing sweep efficiency,and prolonging production cycle.The higher steam temperature,the stable thermal front is formed,and the significant viscosity reduction effection is achieved by heat and thermal dilatation effection,and then reducing residual oil saturation and enhancing oil recovery.(6)In the process of steam and CO2 alternative injected,CO2 orc steam channeling is caused by injected CO2 too early or too later,as well as injected slug volume ratio of steam and CO2 too large or too small,which leads to fast pressure dropping,and low sweep efficiency and low displacement efficiency,and low oil recovery.When injected 0.6 PV steam and then injected CO2 and the slug volume ratio of steam and CO2 is 10:1,the favourable water and gas slug is formed,the maintance of pressure and improvement of sweep efficiency and displacement efficiency are achieved,and the cumulative recovery factor is 84.02%.So it is the optimalizing CO2 injection time and the optimalizing slug volume ratio of steam and CO2.In this paper,the achievements of CO2 assisted steam flooding,are further enriched and improved the researches of this technique.At the same time,it has practical guided significance to promote this enhanced oil recovery method conducted in the heavy oil reservoir at Qigu group of Nine-six block.