Study On The Effect Of Supercritical CO₂ Treatment On The Rheology Of Changqing Dissolved/Degassed Crude Oil And The Stability Of Emulsions

Supercritical CO₂ enhanced oil recovery（Sc CO₂-EOR）is a new and efficient oil field EOR technology,especially for heavy oil reservoirs and low permeability reservoirs.This technology can greatly improve the oil recovery,so it has a broad application prospect.When CO₂ is injected into the reservoir,CO₂gradually changes from liquid state to supercritical state with the increase of temperature and pressure.The oil recovery mechanisms of sc CO₂-EOR include the extraction of light compone nts,swelling crude oil,improvement of oil-water fluidity ratio,miscibility with crude oil and acid unblocking technique.C urrent researches on Sc CO₂ flooding mainly focus on reducing the minimum miscibility pressure（MMP）of CO₂,reducing the corrosion caused by CO₂,and improving the efficiency of CO₂flooding,while researches on the laws and mechanisms of flowability and stability of crude oil and its emulsion after Sc CO₂ flooding in the gathering and transportation process are still lacking.Changqing crude oil and its emulsion were used as the research objects.CH₄ and CO₂were represented the associated gas composition before/after CO₂ flooding,respectively.A self-made Sc CO₂ treating equipment was used to prepare the dissolved/degassed crude oil and its emulsion before/after Sc CO₂ treatment.The effect of Sc CO₂ treatment on rheology of dissolved/degassed crude oil and the stability of dissolved/degassed crude oil emulsion was studied.The solubility and coefficient of volume expansion of dissolved CO₂/CH₄ crude oil/emulsion were analyzed,while the composition and asphaltene properties of degassed crude oil before/after Sc CO₂ treatment were compared.The sensitivity of degassed crude oil to thermal treatment and pour point depressant before/after Sc CO₂ treatment was investigated,while the sensitivity of degassed crude oil emulsion to demulsifier before/after Sc CO₂treatment was investigated.The main research contents and conclusions are as follows:⑴The effect of Sc CO₂ treatment on the solubility,volume expansion coefficient,pour point,viscosity-temperature relationship and yield stress of C hangqing crude oil was analyzed by changing the pressure,temperature and molar ratio of CO ₂/CH₄.It was found that the solubility and volume expansion coefficient of CO₂ in crude oil are higher than that of C H₄,and the rheology of crude oil dissolved CO₂ such as pour point,viscosity and yield stress are significantly lower than that of crude oil dissolved CH₄.Although the solubility of CO₂/CH₄mixture in crude oil is between that of CO₂ and that of CH₄,the presence of of CH₄（around20 mol%）promotes the swelling effect of CO₂ on crude oil,making the volume expansion coefficient higher than that of the crude oil dissolved CO₂,and further improving the rheology of dissolved crude oil.Molecular dynamics simulation results show that CH₄ can significantly reduce the interaction energy between hydrocarbon molecules（from 409.05 kcal/mol to370.13 kcal/mol）,which is advantage to CO₂ molecules forcing hydrocarbon molecules away from each other.Therefore,the CO₂ miscible with small amount of CH₄ can further improve the swel ing effect of CO₂ and flowabilty of Changqing dissolved crude oil.⑵The mechanism of Sc CO₂ treatment on crude oil rheology（effect of thermal treatment and EVA pour point depressant）was revealed by comparing the changes of crude oil composition,asphaltene property and wax crystal characteristics before/after Sc CO ₂treatment.It was found that Sc CO₂ can peel off the solvation layer of asphaltenes,enhancing the asphaltene polarity,which weakens the dispersing of asphaltenes in the crude oil and the eutectic effect of asphaltene with paraffin molecules.The high carbon number hydrocarbon extracted from asphaltene solvation layer is present in crude oil in the form of microcrystalline wax,providing crystal nucleus for paraffin wax crystal.More nucleation points and worse eutectic effect of asphaltene result in more,sma ller wax crystal aggregate with higher spatial complexity,which worsens the low temperature flowability of Changqing degassed crude oil.The dispersion and gelation effect of thermal treatment on asphaltenes in the degassed crude oil worsen after Sc CO₂ treatment,weakening the eutectic effect of asphaltene with paraffin molecules.Therefore,Sc CO₂ treatment weakens the effect of thermal treatment on improving the flowability of Changqing degassed crude oil.Sc CO₂ treatment can further improve the densifica tion degree of wax crystal aggregate by promoting the EVA molecules to adsorb on the surface of asphaltene,which is advantage to forming asphaltene-EVA-wax crystal ternary crystal.Therefore,Sc CO₂ treatment improves the effect of EVA on improving the flowability of Changqing degassed crude oil.⑶The influence of the oil-water interface film on the solubility of CO₂/CH₄ was analyzed by comparing the change of solubility of CO₂/CH₄ in oil phase,water phase and emulsion phase.And the changes of flocculation sedimentation stability（oil separatio n rate）and coalescence demulsification stability（water separation rate）of dissolved crude oil emulsion during constant and decreasing pressure were investigated from the interfacial characteristics（interfacial tension and interfacisl modulus）and interfacial pressure.It was found that the interfacial asphaltene film formed by emulsification reduces the polarity of oil phase and promotes the dissolution of CO₂/CH₄ in the oil phase,which improves the solubility of CO₂/CH₄ in the dissolved crude oil emulsion.Under constant pressure,Sc CO₂treatment can reduce the interfacial tension,increase the interfacial elastic modulus,and reduce the interfacial expansion loss angle,making the droplets more stable and difficult to coalesce,which reduces the water separation rate of the dissolved crude oil emulsion.Moreover,the elasticity of the interfacial film weakens with the dissolved gas pressure increase,which worsens the emulsion stability.In the process of degassing,the CO₂ bubbles grow in the inner phase droplets,which forces the droplets to expand,decreasing the concentration of active substances at the oil-water interface.Thus,the oil-water interfacial pressure decreases,which worses the droplet coalescence stability,promoting the dissolved crude oil emulsion demulsifing.Moreover,the interface pressure decreases more rapidly and the emulsion stability becomes further worse with the increase of the pressure decreasing rate.⑷The asphaltenes adsorbed at the oil-water interface and dispersed in the oil phase were isolated,and analyzed from the polarity and dispersity before/after Sc CO ₂ treatment.The mechanism of Sc CO₂ treatment on the stability（effect of AP125 demulsifier）of crude oil emulsion was revealed from the interfacial characteristics.It was found that Sc CO₂ treatment can increase the polarity of asphaltenes absorbed at interface,and reduce the dispersing ability of asphaltenes absorbed at interface into the oil phase,making the asphaltenes more easily adsorb at the oil-water interface,which increases the ad sorption amount of asphaltenes at the interface.Thus,the droplets are difficult to coalesce,and the stability of degassed crude oil emulsion is improved after Sc CO₂ treatment.Sc CO₂ treatment reduces the adsorption of demulsifier molecules on the oil-water interface by improving the densification degree of asphaltene film.Moreover,the asphaltenes are difficult to be driven into the oil phase by demulsifier molecules,which reduces the effect of demulsifier molecules on the interfacial strength.Therefore,Sc CO₂ treatment weakens the effect of AP125 on promoting the demulsification of degassed crude oil emulsion.