Carbon and hydrogen isotope fractionation for methane from non-isothermal pyrolysis of oil in anhydrous and hydrothermal conditions

Kun He,Shuichang Zhang,Jingkui Mi,Yu Fang,Wenlong Zhang

doi:10.1177/0144598719857189

Abstract

In this study, to ascertain the carbon and hydrogen isotope fractionation of oil cracking gas (secondary gas) in hydrothermal conditions, non-isothermal pyrolysis of oil with and without water was carried out by a gold-tube system. By determination of the yields of individual gas products, it is found that the presence of water enhanced the yields of hydrocarbon gases. However, kinetic calculations indicate that Ea for the generation of methane and C2–5 in pyrolysis in hydrothermal conditions are essentially identical with those in anhydrous pyrolysis. The yields of carbon dioxide (CO2) and alkene gases in pyrolysis in hydrothermal conditions are evidently higher than those in anhydrous pyrolysis. It is reasonable that water–hydrocarbon reactions occurred and contributed to the generation of secondary gas in hydrothermal conditions. Meanwhile, the presence of water resulted in a slight depletion of 13C for methane and an evident depletion of 13C for CO2. Thermodynamic calculations suggest that water–hydrocarbon reactions in non-isothermal pyrolysis are dominated by free radical mechanism rather than ionic mechanism. Moreover, δ2H values of methane are apparently different in pyrolysis involving water with different δ2H. This result demonstrates that water provided hydrogen for hydrocarbon gas generation. Finally, we established mathematical models based on isotope fractionation to quantitatively determine the contribution of water–hydrocarbon reactions for gas generation in both experimental and geological conditions.

Highlights

Water was suggested to affect and even participate in the petroleum generation from organic matters (Helgeson et al, 1993; Lewan et al, 1979; Lewan, 1997; Seewald, 2003, Seewald et al, 2000)
This result may be attributed to the relative low amounts of water introduced and the fast heating rate applied in our experiments, where water–hydrocarbon reactions did not play a dominant role for secondary gas generation
Isothermal pyrolysis of oil/hydrocarbons with higher contents of aqueous solutions at 330–420C for 48–936 h has observed 0.5–3.0 times increase in hydrocarbon gas yields compared with anhydrous pyrolysis (He et al, 2018a; Shuai et al, 2012)

Summary

Introduction

Water was suggested to affect and even participate in the petroleum generation from organic matters (Helgeson et al, 1993; Lewan et al, 1979; Lewan, 1997; Seewald, 2003, Seewald et al, 2000). Numerous hydrous or hydrothermal experiments have been conducted to understand the effects of water on oil and gas generation (Behar et al, 2003; Gao et al, 2014; He et al, 2018a; Lewan et al, 1979; Lewan, 1997; Lewan and Roy, 2011; Pan et al, 2009; Wang et al, 2017) These works observed that water can provide hydrogen and oxygen for hydrocarbon generation, and alter the features of individual products as well as acting as a medium for petroleum expulsion from source rocks (Hoering, 1984; Lewan, 1997; Schimmelmann et al, 2001). Water–hydrocarbon reactions were observed at elevated temperature (Helgeson et al, 1993; Leif and Simoneit, 2000; Seewald, 2001), the effect of water on the generation of oil cracking gas (secondary gas) remains controversial (He et al, 2011, 2018a; Hesp and Rigby, 1973; Shuai et al, 2012)

Results

Discussion

Conclusion