The Influence of Pressure on the Thermal Cracking of Oil

Abstract

The influence of pressure on gas, liquid, and solid products of thermal cracking of a C9+ fraction of a saturate-rich Devonian oil from the Western Canada Basin has been investigated. Confined pyrolysis was performed in sealed gold tubes at 350, 380, and 400 °C and pressures ranging from 90 to 2000 bar for 72 h. At the temperatures investigated, the effect of pressure on oil cracking and product generation is small. Rates of early hydrocarbon gas generation (350 and 380 °C, 72 h) decrease with increasing pressure by 9−15% in the 90−210 bar range and by 7% for gas generation (400 °C, 72 h) in the 90−345 bar range. Gas generation rates then steadily increase 10−15% to a maximum at 690 bar for all temperatures. From 690 to 2000 bar, the rates of gas generation steadily decrease by 5−17%. Activation volume values were estimated to be ΔV ⧧ = 47 cm3/mol in the 90−210 bar range, ΔV ⧧ = −14 cm3/mol in the 345−690 bar range and ΔV ⧧ = 5 cm3/mol in the 690−2000 bar range. Extrapolation of results to geologic conditions shows that the pressure effects on oil cracking are larger under geologic conditions than laboratory pyrolysis conditions but still secondary to temperature. The effect of pressure on gas generation rates is also reflected in methane carbon isotopes, which show nearly 2‰ fractionation with increasing pressure to 1380 bar. Ethane and propane showed almost no detectable fractionation with pressure. At 350 and 380 °C, C8+ n-alkane yields generally increase as pressure increases from 90 to 690 bar and decrease as pressure increases from 690 to 2000 bar, parallel to that observed for the gases. At 400 °C, however, the n-alkane yields are highest at 345 and 2000 bar, where gas yields are lowest. This suggests that n-alkanes are generated, in part, from heavier molecules at 350 and 380 °C, and at 400 °C, n-alkanes are cracked more rapidly than they are formed to produce gas.

Retardation of oil cracking to gas and pressure induced combination reactions to account for viscous oil in deep petroleum basins: Evidence from oil and n-hexadecane pyrolysis at water pressures up to 900 bar

“…The magnitude of the retardation effects of pressure on the initial stages of oil cracking is much greater than found in previous studies using confined gold bags pyrolysis method (Hill et al, 1996;Al Darouich et al, 2006) that generally show less retardation effect compared to the unconfined water pyrolysis method used here. The decomposition of oil to gas occurs via beta scission of hydrocarbons to generate free radicals.…”

“…At lower pressures, Fabuss et al (1964) concluded that pressure accelerates cracking between 1 and 400 bar while a decrease was observed at higher pressure (800 bar). This trend of an initial increase followed by a decrease at higher pressure has been confirmed for oil cracking (Behar and Vandenbroucke, 1996;Hill et al, 1996). On the contrary, Jackson et al (1995) observed a continuous retarding effect for n-hexadecane pyrolysis between 120 and 600 bar.…”

“…Most of the work on the effect of pressure has been undertaken on model compounds, e.g. saturates and aromatics, although Hill et al (1996) pyrolysed the C 9 + fraction of a saturate-rich, West Canadian Devonian oil in sealed gold tubes at temperatures between 350 and 400 °C for 72 h and at pressures between 90 and 2000 bar, and found that the effect of pressure on the rate of oil 6 cracking and product generation was small. In a number of studies, a retardation effect was observed during the pyrolysis of saturated hydrocarbons at pressures higher than 400 bar (Fabuss et al, 1964;Dominé, 1991;Behar and Vandenbroucke, 1996).…”

Thermal Cracking of Oil under Water Pressure up to 900 bar at High Thermal Maturities. 1. Gas Compositions and Carbon Isotopes

“…41 The model of activation volume is included to account for such effects, which has been elaborated in the previous works. 31,42 Thus, the gas yields at 470-900 bars are lower than at 200…”

“…On the other hand, concentrations and collision rates of reactants would be enhanced with increasing pressure, resulting in increased overall reaction rates. 31 Although cage/diffusional effects generally retard reaction rates with increasing pressure, collision rates among reactants will increase with pressure and reach a maximum at a given pressure threshold, resulting in increased reaction rates and gas yields. 31 This could be the case in this study, which shows higher yields at 750 bar than at 470 bar when oil cracks at the elevated stage in the wet gas window.…”