Study On The Patterns Of Volume Change Of CO₂-petroleum Model Compound Systems Using Fused Silica Capillary Cell With In Situ Raman Spectroscopy

Under the background of global warming,more and more attentions were paid to the sequestration of CO₂.The proposed methods for CO₂ sequestration include geological sequestration,ocean sequestration,mineral sequestration,and so on.Among them,geological sequestration in oil-bearing reservoirs has the best economic benefit,and is the most promising method.This technique can effectively reduce greenhouse gas by permanent storage of CO₂ in geological formations.Moreover,the dissolution of CO₂ in oil reduces both the viscosity and interfacial tension of oil while increasing the mobility and swelling of the oil volume in oil-bearing reservoirs,and therefore dramatically improves oil recovery.As a fundamental information required for efficient CO₂ capture and storage-enhanced oil recovery（CO₂ CCS-EOR）,the volume expansion of CO₂-petroleum model compound（e.g.,alkanes,cycloalkanes and aromatics）systems has been studied over the past decades using fixed volume or variable volume pressure-volume-temperature（PVT）methods.However,the shortcomings of these methods are their low efficiency and their limited applicable P-T range.Therefore,development of a new experimental method to obtain accurate expansion data is not only immediately valuable to the petroleum industry,but also important for theoretical research.In this study,we developed a new experimental system,which combined an optically transparent fused silica capillary cell（FSCC）,heating-cooling stage,pressure generator,and a confocal Raman spectrometer,to investigate the volume expansion of CO₂-petroleum model compound systems under applicable geological conditions.The FSCC was constructed at the micro scale to reduce the temperature gradient and reagent consumption.A section of water was loaded between the petroleum model compound and the CO₂ in the fused capillary tube to seal the petroleum model compound and,at the same time,allow diffusion of CO₂（gaseous or supercritical）into the petroleum model compound after diffusion through the section of water.Raman spectroscopy was applied during our experiments to verify that the system reached phase equilibrium.The volume changes of the CO₂-petroleum model compound systems in the FSCC were observed under a microscope;the images were recorded continuously with a digital camera and the volumes were calculated from the measured inner diameter of the fused silica tube and measured length of the liquid column by using a micrometer.This experimental method was found to be a viable technique for assessing volume expansion of the CO₂-petroleum model compound under applicable geological conditions.Over an extended temperature and pressure range,our new method was thus shown to be more efficient and accurate than the traditional PVT methods.Using this new method,the thermal expansion factor of petroleum model compound（hexane,octane,decane,dodecane,tetradecane and hexadecane）were determined from 30 to 80°C.Moreover,the volume expansion factor of the CO₂-petroleum model compound systems were determined from 1 to 21 MPa and from 30 to 80°C,the fundamental and useful information needed for efficient CO₂CCS-EOR.The thermal expansion factor of petroleum model compound increased linearly with the increase of temperature from 30 to 80°C,and decreased with the increase of the carbon number of alkanes under the same temperature.The relative volume of petroleum model compound（the volume at T,1 atm/the volume at 30 ^oC 1 atm）obtained can be expressed by the equation RV_{Cn H2n+2}=(–7.464×10^-7 n³+3.148×10^-5 n²–4.606×10^-4 n+3.280×10^-3)（T–303.15）+1,6≤n≤16,303.15 K≤T≤353.15 K.The volume expansion factor of CO₂-hexane/octane/decane/dodecane systems increased with pressure from 1 to 14 MPa,but decreased with the increase of temperature from 30 to 80°C.According to principle of“like dissolves like”,as a nonpolar molecule,CO₂ is easy to dissolve in weak polar solvens such as alkanes.And,the solubility of CO₂ in petroleum model compound increased with pressure,but decreased with the increase of temperature,which made the patterns of volume change of CO₂-petroleum model compound systems showing a similar tendency.The patterns of volume change under different temperature and pressure conditions are quite different when tetradecane and hexadecane come into contact with CO₂.The volume expansion factor of CO₂-tetradecane/hexadecane systems increased with pressure from 3 to 21 MPa.A turning point has been observed on the volume expansion factor curves of CO₂-tetradecane/hexadecane systems at 30 and40^oC,which was found for the first time in CO₂-alkane systems.It is plausible that this phenomenon is related to an abrupt change in the solubility of CO₂ in polar molecule（e.g.,long chain alkane）near the critical region of CO₂（the critical point of CO₂ is 31.1^oC and 7.38 MPa）.The volume expansion factor of CO₂-tetradecane system decreased with the increase of temperature from 30 to 80°C.And,it seems that the physical characteristics of hexadecane caused the volume change pattern of CO₂-hexadecane system with temperature more complex than those for tetradecane.Under the same temperature and pressure conditions,the volume expansion factor of the CO₂-petroleum model compound systems decreased with the increase of the carbon number of alkanes.This is related to the molecular polarity of petroleum model compound increased with their carbon number,which made the solubility of CO₂ in petroleum model compound decreased with the increase of carbon number.The phase behavior of the CO₂-petroleum model compound systems in the range of experimental temperature and pressure regions showed that the attractive forces between CO₂ and hexane/octane/decane/dodecane were stronger than their respective repulsive forces;the attractive forces between CO₂ and tetradecane/hexadecane decreased with the increase of volume expansion of CO₂-tetradecane/hexadecane systems,and finally lower than their respective repulsive forces.Moreover,the unexpected discovery of a good quadratic relationship between the Raman peak intensity ratio and volume expansion factor,which indicates that this new method can be used not only just to obtain more accurate experimental data,but also to obtain additional volume expansion data from measured Raman peak intensity ratio data.This relationship can also be used for predicting the volume change patterns of complex CO₂-petroleum model compound systems and protracting their volume expansion factor curves or curved surfaces accurately.