The Effects of High Pressure on Oil-to-Gas Cracking during LaboratorySimulation Experiments

Chen Zhonghong,* Ma Zunjin, Zha Min, Sha Xiaoy, Zhang Shouchun and BaoYoushu.

Journal of Petroleum Geology, Vol. 37(2), April 2014, pp 143-162.

The effects of high pressures on the yield and kinetics of gas generatedby the cracking of crude oil were investigated in laboratory simulationexperiments. Samples of a low maturity nonmarine oil were recovered from thePaleogene Shahejie Formation in the Dongying depression, Bohai Bay Basin,eastern China. The oils were cracked to gas under different pressure and temperatureconditions in an autoclave. Initial temperatures of 300 °C were increased to650 °C at rates of either 30 or 100 °C/h. Reaction products were analysed atthe end of each 50 °C temperature increase. Pressure conditions were either 0.1MPa (i.e. atmospheric) or 20 MPa. Results show that high pressures inhibit ordelay oil to gas cracking and retard the initiation of the cracking process.The temperature at which oil was cracked and the activation energy of theformation of C1–5 hydrocarbons increased under high pressure conditions,demonstrating the effects of pressure on the kinetics of the oil to gascracking process. High pressures and high temperatures inhibited the conversion of C2-55 hydrocarbons to methane during secondary cracking. In addition, highpressures retarded the generation of N2, H2 and CO during cracking of oil. Thepresence of water increased the yields of total cracked gas, C2–5 hydrocarbonsand CO2 in high-pressure conditions. The simulation results show that CO2 and C2–5 hydrocarbons have similar yields during oil-to-gas cracking.

Using the kinetic parameters determined from the laboratory experiments,the yield and production rate of gas generated during the cracking of oil fromMember 4 of the Paleogene Shahejie Formation in the Minfeng-Lijin sag (Dongyingdepression) were calculated. The results indicate that only limited volumes of natural gas in this area werederived from the cracking of oil, and that most of the gas was derived from thethermal decomposition of kerogen.