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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
New Insight into Gas Source-Tracking: A Predictive Model for Kinetics and δ13C of CH4 Generated from Methylated Monoaromatics during Thermal Cracking of Oil in High-Temperature Reservoirs
(1) Total, Pau, France.
(2) IFP, Rueil-Malmaison, France.
(3) Curtin University, Perth, WA, Australia.
(4) CNRS, Paris, France.
(5) CNRS, Nancy, France.
The scope of the present study was to validate an approach that could be used in order to elaborate an integrated model predicting the kinetics and δ13C of gases generated during thermal cracking of oil in high-temperature (HT) reservoirs. For this feasibility study we have focused on methylated monoaromatic hydrocarbons (MMH) present in oil using the model compound 1,2,4-trimethylbenzene (TMB) and we have proceeded in 4 steps.
- Pyrolysis experiments at 395, 425, 450, and 475°C and 100 bar were performed in order to study the whole range of TMB conversions. All pyrolysis fractions were recovered and quantified. Products up to C18 were quantified individually.
- A mechanistic kinetic model was achieved for thermal cracking of TMB until 70% conversion. It involved 122 reversible free-radical reactions and 47 species up to C18. It allowed the characterization of CH4 generation processes involving components up to C18 at all temperatures.
- A lumped kinetic model was achieved for thermal cracking of TMB on the whole range of conversions using the mechanistic model in order to constrain its reduced reaction scheme. This scheme was composed of 4 pathways for CH4 generation: (Pa) the dimerization of TMB, (Pb) its demethylation into xylenes, (Pc) the condensation reactions of dimers and C18+ compounds, and (Pd) the dimerization of xylenes and their demethylation into toluene. Associated activation energies were in the range 52-61 kcal/mol and frequency factors all close to 10^12 s^-1. Below 5% conversion, Pb and Pc governed CH4 generation, followed by Pa. Above 5% conversion, Pc became the main source of CH4, followed Pb and Pa, respectively. Pd showed negligible CH4 yields up to 95% conversion. Simulations under conditions met in HT reservoirs revealed that the thermal stability increased in the series methylated polyaromatics < MMH < saturates. They also demonstrated the CH4 generation potential of MMH and the risk for heavy components generation when conversion increased.
- Pa, Pb, and Pc were selected as relevant contributions to δ13C(CH4) until 100% TMB conversion. Kinetics for the generation of 12CH4 and 13CH4 were expressed separately. Associated ratio of frequency factors Ω = 1.028 and variations of activation energy ΔEi ranged from 36 to 79 cal/mol. Simulations under conditions met in HT reservoirs were performed and illustrated the importance to determine the magnitude of the isotopic precursor effect for natural compounds.