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.