Raman Spectroscopic Study Of Liquid Saturated Hydrocarbons Under High Temperature And High Pressure

At present the temperature of petroleum stability and pressure effect are still uncertain. Whenprevious scholar investigated cracking of oil or alkanes by thermal simulation experiment, fewpeople ever considered the role of pressure. Those people who thought pressure effect existed onlyincreased temperature at a constant pressure which was basically low because of limitedexperimental technologies. It is far from real geological condition. In addition, the issue is thepressure gauge extensively used in geology science such as the origin of petroleum, hydrocarboninclusion and experimental study under high temperature and high pressure, and at present,organic matter as a pressure gauge have not been nearly investigated. Mainly aiming at the abovethree scientific questions, we have studied the laser Raman scattering spectra of some liquidsaturated hydrocarbons, which can be commonly found in the petroleum, such as n-hexane、n-heptane、n-pentadecane、cyclohexane，and their mixture such as cyclohexane－n-heptaneblends、cyclohexane－n-pentadecane blends, and water－n-heptane blends、water－cyclohexaneblends、water－n-pentadecane blends by the experimental technique of diamond anvil cell. Wehave got some achievements as following.Whether at room temperature or at high temperature, and whether the liquid saturatedhydrocarbons became crystallized or not, the Raman bands of CH3and CH2stretchings and thering breathing mode of the liquid saturated hydrocarbons in ternary system (pure system、twoblends system of liquid hydrocarbon samples and water system) shifted to higher wavenumberswith increasing pressure, in general, not only the asymmetric stretching shifted to higherwavenumbers more quickly than corresponding to the symmetric stretching, but also at ambienttemperature above all these Raman bands shifted to higher wavenumbers more quickly thancorresponding to the vibrational mode at high temperature.In the process of increasing temperature, there were two opposite effects which weretemperatures and pressures, but pressure effects exert larger influence than temperature effects inliquid saturated hydrocarbons. Increasing pressure can retard thermal destruction of liquidsaturated hydrocarbons, so it is important for liquid saturated hydrocarbons to keep stable at hightemperature.Any visible changes didn’t occur in the liquid saturated hydrocarbons where were in ternarysystem and they kept their own usual state under their experimental temperatures and pressures.Their maximum stable temperature range from280℃to315℃below300MPa which imply theminimum stable temperature of petroleum, and it is higher than the results of previous studies.Although Raman shift and equation of pressure gauge of cyclohexane changed in two blendssystem of liquid hydrocarbons, the stability of liquid hydrocarbon and the role of pressure weren’taffected. The Raman shift of a mean C-H stretching vibrational mode was affected by mixture liquid hydrocarbons, but the equation of P-p-T wasn’t varied.The Raman spectrum of a mean C-H stretching vibrational mode can be used as a barometerof fluid inclusions, especially hydrocarbon inclusion, and its equation is as following:P=75.56+0.3508T+60.7（23≤T≤405、P＜2160MPa）Water retard the decomposition of liquid saturated hydrocarbons in water system, so liquidsaturated hydrocarbons can be kept stable at higher temperature. Moreover, water may play acertain role during primary migration of petroleum.The liquid/solid equilibrium line and isometric line of liquid saturated hydrocarbons can beused as not only estimating the minimum captured pressure of hydrocarbons, but also roughlydetermining the hydrocarbon component of fluid inclusions, especially hydrocarbon inclusion.Cyclohexane can be used as a pressure gauge, and it has different equations in three systemsas following:Pure cyclohexane system: P=60.337(Δ p)2933+0.818T+4.999（20℃≤T≤315℃、P≤1100MPa）and P=79.488(Δ p)802+1.910T-44.910（20℃≤T≤315℃、P≤1100MPa）.Cyclohexane－n-heptane blends system: P=144.865(Δ p)801+1.954T-75.355（20℃≤T≤315℃、P≤2100MPa）.Cyclohexane－n-pentadecane blends system: P=135.484(Δ p)803+1.721T+64.045（20℃≤T≤405℃、P≤2100MPa）.Water system: P=154.55(Δ p)2855+338.35（ambient temperature，0＜(Δ p)2855≤2.3cm-1）,and P=75.425(Δ p)2933+0.462T+20.285（20℃≤T≤300℃、P≤1550MPa）, and P=98.424(Δ p)802+1.99T-67.58（20℃≤T≤300℃、P≤1550MPa）.n-Heptane can be used as a pressure gauge, and it has different equations in two systems asfollowing:Pure n-heptane system: P=159.73(Δ p)2880-55.457（ambient temperature，0.35≤(Δ p)2880≤6.9cm-1）, P=157.81(Δ p)2935+36.798（ambient temperature，-0.23≤(Δ p)2935≤6.4cm-1）and P=70.476(Δ p)2965+0.0628T+29.036（20℃≤T≤315℃、P≤2000MPa）.Water system: P=122.13(Δ p)2880+1.3328（ambient temperature，-0.01＜(Δ p)2880≤9.5cm-1）, P=148.09(Δ p)2857-11.301（ambient temperature，-0.8≤(Δ p)2857≤7.8cm-1）, P=126.72(Δ p)2935+171.66（ambient temperature，-1.35≤(Δ p)2935≤7.7cm-1）, and P=81.264(Δ p)2965+0.397T+96.691（20℃≤T≤315℃、P≤2200MPa）.Note: P－Pressure (MPa)；T－Temperature (℃). Δ p、 are wavenumber shifts relative tothe line position at room temperature and pressure（in cm-1）. i.e. Δ pis p-0and is p-0. pand are Raman shifit at a certain temperature and pressure,0and are Ramanshifit at room temperature and pressure.