Influence of water on yields and isotopic fractionations of gas hydrocarbons generated from oil cracking

Abstract

Few investigations have been made to discuss the influence of water on the yields and carbon and hydrogen isotopic ratios of gas hydrocarbons generated from oil cracking although water occurs in oil reservoirs. In this study, a pyrolysis experiment for a crude oil under hydrous and anhydrous conditions in a gold tube confined system was performed at T = 370°C, P = 21 MPa and times from 1 to 7 days. It is found that water has an obvious inhibition on the gas yields generated from oil cracking and a pronounced influence on the carbon and hydrogen isotopic fractionations of the cracking gas. Compared with the anhydrous pyrolysis, the hydrous pyrolysis has a smaller C 1-5 alkane yields and lighter carbon and hydrogen isotopic ratios of the C 1-3 hydrocarbons within maturity levels investigated. The differences in their C 1-5 alkane yields reach 3-12 ml/g, in their δ 13 CH 4 , δ 13 C 2 H 6 and δ 13 C 3 H 8 values reach 1.0-2.7‰, 0.2-0.8‰ and 0.2-0.3‰, respectively, and in their δD CH4 , δD C2H6 , δD C3H8 values reach 5-16‰, 22-80‰ and 4-13‰, respectively. Moreover, the hydrous pyrolysis experiment formed a greater amount of H 2 and alkenes and a smaller amount of CO 2 than the anhydrous pyrolysis experiment did. The differences of H 2 , ethene, propene and CO 2 between the two experiment conditions are 0.44-2.12 ml/g, 0.03-0.05 ml/g, 0.20-0.34 ml/g and 0.08-0.98 ml/g, respectively. These results indicate that these differences in carbon and hydrogen isotopic ratios of generated gases between the hydrous and anhydrous experiments is possibly because water affects the free radical reactions of oil cracking and inhibits the combination between alkyl and hydrogen to form alkane to some extent, rather than due to the isotopic exchange reactions between water and some pyrolysates as usually believed for kerogen hydrous pyrolysis.

Experimental Simulation Of Hydrocarbon Expulsion

“…For bulk parameters (elementary analysis, Rock-Eval) no difference between hydrous and anhydrous pyrolysis was noticed by Monthioux et al (1985). A large impact on gas generation and carbon and hydrogen isotopic fractionation was observed by Jin et al (2009). In this study, oils were matured artificially with and without addition of water.…”

“…The authors traced it to selectively oxidization of specific functional groups within the lignite by water, as described earlier by Lewan (1997). In contrast, hydrous oil cracking experiments showed a higher production of hydrogen, but a lower formation of CO2 in comparison to anhydrous pyrolysis (Jin et al, 2009). This indicates that main reactions forming CO2 are linked to kerogen-to-bitumen decomposition and not to further oil cracking.…”

“…In addition, hydrogen-balance calculations of hydrous and anhydrous pyrolysis show that water does not contribute to the net amount of initially bonded hydrogen in the products of hydrous pyrolysis (Lewan et al, 1979). Moreover, experiments which were carried out with pure compounds (Weres et al, 1988) or with oils (Jin et al, 2009) show high yields of alkenes, which should be prevent by hydrogen donors like water. Larsen (1999) see in this the proof that the role of radical reactions is secondary in formation of oil and gas from kerogen.…”

“…In contrast to this results, the authors of the second study notice only small influences to generation by pressure, secondary to the effects caused by temperature. The possibility to add water to the sample and to perform hydrous pyrolysis led to comparative studies, which examine the influence of water to the products of pyrolysis (Jin et al, 2009;Monthioux et al, 1985;Pan et al, 2010). Regarding to expulsion, the same limits exist which were discussed for MSSV and CSVP.…”

“…The method of gold tube pyrolysis is preferably used for anhydrous pyrolysis. The hydrous pendant was used by some studies for investigation of the influence of water on pyrolysis (Jin et al, 2009;Monthioux et al, 1985;Pan et al, 2010). For bulk parameters (elementary analysis, Rock-Eval) no difference between hydrous and anhydrous pyrolysis was noticed by Monthioux et al (1985).…”