Ethane CC Clumping in Natural Gas: a Proxy for Cracking Processes?

Clog, M.D. , Ferreira, A.A., Santos Neto, E.V. and Eiler, J.M. (2014) Ethane CC Clumping in Natural Gas: a Proxy for Cracking Processes? AGU Fall Meeting, San Francisco, CA USA, 15-19 Dec 2014.

Abstract

Ethane (C2H6) is the second-most abundant alkane in most natural gas reservoirs, and is used to produce ethylene for petrochemical industries. It is arguably the simplest molecule that can manifest multiple 13C substitutions. There are several plausible controls on Δ13C2H6in natural gas: thermodynamically controlled homogeneous isotope exchange reactions analogous to those behind carbonate clumped isotope thermometry; inheritance from larger biomolecules that undergo thermal degradation to produce natural gas; mixing of natural gases that differ markedly in bulk isotopic composition; diffusive fractionation; or combinations of these and/or other, less expected fractionations. There is little basis for predicting which of these will control isotopic variations among natural ethanes, but we think it likely that addition of this new isotopic proxy will reveal new insights into the natural chemistry of ethane. We have developed a method to measure the abundance of 13C2H6 in natural samples, using high-resolution mass spectrometry. We define Δ13C2H6 as 1000 . ((13C2H6/12C2H6)measured/(13C2H6/12C2H6)stochastic -1). We studied several suites of natural gas samples and experimentally produced or modified ethane. Natural ethanes, including closely related samples from a single natural gas field, exhibit surprisingly large ranges in Δ13C2H6 (4 ‰ overall; up to 3 ‰ in one gas field). Such ranges cannot be explained by thermodynamic equilibrium at a range of different temperatures, or by diffusive fractionation. Kinetic isotope effects associated with 'cracking' reactions, and/or inheritance of non-equilibrium carbon isotope structures from source organics are more likely causes. We observe a correlation between Δ13C2H6 and the concentration of alkanes other than methane in several suites of natural gases, suggesting the causes of clumped isotope variations are tied to the controls on gas wetness. An experiment examining ethane residual to high-temperature pyrolysis confirms this trend could be an isotopic fingerprint for ethane destruction.

• Item Type: Conference or Workshop Item
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Clog, Dr Matthieu
• Authors: Clog, M.D., Ferreira, A.A., Santos Neto, E.V., and Eiler, J.M.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre