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_2H_6 and δ^(13)C_3H_8 values reach 1.0-2.7‰, 0.2-0.8‰ and 0.2-0.3‰, respectively, and in their δDCH_4, δDC_2H_6, δDC_3H_8 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.