Kinetics of gaseous hydrocarbon generation with constraints of natural gas composition from the Barnett Shale

Abstract

A kinetic compositional model covering the generation of methane through pentanes is calibrated with both experimental and field data. A continuous distribution of activation energy (E=E0+yEs) is applied to describe the heterogeneity of hydrocarbon generation kinetics, with E0 (activation energy E at conversion y=0) as the location parameter and Es as the scale parameter. The best fit gives E0=53.0kcal/mol and Es=4.0kcal/mol for methane generation with frequency factor A=3×1011s−1. The values of E0 and Es for the generation of ethane, propane, n-butane and n-pentane decrease with carbon chain length, with a constant A value. The generation of branched alkanes (isobutane and isopentane) has higher E0 and narrower Es values compared with the generation of their n-alkane isomers. The model adequately describes natural gas compositional properties with thermal maturation, including: (1) decreased wetness, (2) a log-log relation between the C2H6 through C5H12 concentration, and (3) increased i-C4H10/n-C4H10 and i-C5H12/n-C5H12 with thermal maturity. The contribution of secondary-cracking gas at higher maturity, which may change the above trends, was also quantified.