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

Abstract

In this study, a C 9 + fraction of saturate-rich Tertiary source rock-derived oil from the South China Sea basin was pyrolyzed in normal and supercritical water using a 25 mL vessel at a range of temperature from 350 to 425 °C for 24 h, to probe pressure effects up to 900 bar on gas yields and their stable carbon isotopic compositions during thermal cracking. Pressure generally retards oil cracking, as evidenced by reduced gas yields, but the trends depend upon the level of thermal evolution. In the early stages of cracking (350 and 373 °C, equivalent vitrinite reflectance of < ∼1.1% R 0 ), the suppression effect increases with pressure from 200 to 900 bar, but it is most marked between 200 and 470 bar. At the later stages in the wet gas window (390, 405, and 425 °C, equivalent vitrinite reflectance of >1.3% R 0 ), pressure still has a strong suppression effect from 200 to 470 bar, which then levels off or is reversed as the pressure is increased further to 750 and 900 bar. Interestingly, the stable carbon isotopic composition of the generated methane becomes enriched in 13 C as the pressure increases from 200 to 900 bar. A maximum fractionation effect of ∼3‰ is observed over this pressure range. Due to pressure retardation, the isotopically heaviest methane signature does not coincide with the maximum gas yield, contrary to what might be expected. In contrast, pressure has little effect on ethane, propane, and butane carbon isotope ratios, which show a maximum variation of ∼1‰. The results suggest that the rates of methane-forming reactions affected by pressure control methane carbon isotope fractionation. Based on distinctive carbon isotope patterns of methane and wet gases from pressurized oil cracking, a conceptual model using "natural gas plot" is constructed to identify pressure effect on in situ oil cracking providing other factors excluded. The transition in going from dry conditions to normal and supercritical water does not have a significant effect on oil-cracking reactions as evidenced by gold bag hydrous and anhydrous pyrolysis results at the same temperatures as used in the pressure vessel.

Coupled mineral alteration and oil degradation in thermal oil-water-feldspar systems and implications for organic-inorganic interactions in hydrocarbon reservoirs

“…9) in the hydrous system suggest that water promoted the oil (eicosane) degradation reactions in the present study. This result is in agreement with some previous studies (Shuai et al, 2012;Wang et al, 2015), although some other researchers claimed that water retards oil degradation (Hesp and Rigby, 1973;Lewan, 1997;Price, 1993;Uguna et al, 2016) or has little impact on oil degradation (Xie et al, 2016). The oil-NaCl water systems generate even more gases, liquid hydrocarbons, and organic acids than the oil-D water systems (Fig.…”

Constrain on Oil Recovery Stage during Oil Shale Subcritical Water Extraction Process Based on Carbon Isotope Fractionation Character

“…Specially, in this study, we did not consider the effect of pressure, temperature, and rock characteristics on the carbon isotope fraction process. According to other research, the pressure has a suppression effect of gas generation in the early stage from 20 to 90 MPa, with a maximum carbon isotope fractionation effect of ≈3‰, and it increases with temperature from 350 to 373 • C [51]. However, the effect on the δ 13 C CO 2 − CH 4 characteristics may be less, as it is a relative change.…”

Thermal Cracking of Oil under Water Pressure up to 900 Bar at High Thermal Maturities: 2. Insight from Light Hydrocarbon Generation and Carbon Isotope Fractionation

“…At this highly mature stage, cage and possibly diffusional effects play important roles to account for the lower yields from 470 to 900 bar, which generally suppress reaction rates with increasing pressure due to the activation volume dynamics. ,,, On the other hand, although the collision rate among reactants is mainly temperature-dependent, it is also pressure sensitive and has a pressure threshold for its maximum . Therefore, the overall reaction rates depend on competition between collision rates and cage/diffusional effects …”

“…To the best of our knowledge, no attempt has been made to probe this issue and the mechanisms involved. In the early work of this study, using a C 9− free of saturate-rich oil derived from Tertiary source rock in South China Sea, fixed-volume pressure vessel pyrolysis experiments at a range of temperature from 350 to 425 °C were conducted under up to 900 bar water-pressure condition, C 1 –C 5 gas compositions and carbon isotopes were reported . The present study further addresses the yields and stable carbon isotopic compositions of different compound classes ( n -, iso -, and cyclo -alkanes and aromatics) in the LH range for the same pressurized oil cracking experiments.…”

“…If in-reservoir oil cracking occurs in deep-buried basins, the LHs must be the intermediate products carrying important structural information. Previous studies mainly focused on analyzing C 12+ hydrocarbons or C 1 –C 5 gas hydrocarbons when considering oil cracking during pressurized pyrolysis experiments, ,,,, and little is known about the evolution regularities of LHs with respect to pressure. Although there are clear descriptions on how temperature influences the molecular and stable carbon isotopic compositions of LHs during oil cracking, , pressure can theoretically complicate oil cracking and result in different pathways affecting molecular compositions and stable carbon isotopes of LHs.…”