oa 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

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