THE EFFECTS OF HIGH PRESSURE ON OIL‐TO‐GAS CRACKING DURING LABORATORY SIMULATION EXPERIMENTS

The effects of high pressures on the yield and kinetics of gas generated by the cracking of crude oil were investigated in laboratory simulation experiments. Samples of a low‐maturity non‐marine oil were recovered from the Paleogene Shahejie Formation in the Dongying depression, Bohai Bay Basin, eastern China. The oils were cracked to gas under different pressure and temperature conditions in an autoclave. Initial temperatures of 300 °C were increased to 650 °C at rates of either 30 or 100 °C/h. Reaction products were analysed at the end of each 50 °C temperature increase. Pressure conditions were either 0.1 MPa (i.e. atmospheric) or 20 MPa. Results show that high pressures inhibit or delay oil‐to‐gas cracking and retard the initiation of the cracking process. The temperature at which oil was cracked and the activation energy of the formation of C_1–5 hydrocarbons increased under high pressure conditions, demonstrating the effects of pressure on the kinetics of the oil‐to‐gas cracking process. High pressures and high temperatures inhibited the conversion of C_2–5 hydrocarbons to methane during secondary cracking. In addition, high pressures retarded the generation of N₂, H₂ and CO during cracking of oil. The presence of water increased the yields of total cracked gas, C_2–5 hydrocarbons and CO₂ in high‐pressure conditions. The simulation results show that CO₂ and C_2–5 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 from Member 4 of the Paleogene Shahejie Formation in the Minfeng‐Lijin sag (Dongying depression) were calculated. The results indicate that only limited volumes of natural gas in this area were derived from the cracking of oil, and that most of the gas was derived from the thermal decomposition of kerogen.