Change in hydrocarbon composition in rock samples as a function of time: A thermodynamic evaporation model

Abstract Recent pyrolysis analytical methods such as Shale Play (Romero-Sarmiento et al. 2016), HAWK-PAM (Maende et al., 2017) and Incremental S1 (Beti et al., 2018) enable improved understanding of the quantity and quality of free petroleum present in rock samples. However, it is well established that the evaporation losses restrict the ability of an experiment to accurately quantify and qualify free petroleum in rock sample (Michael et al., 2013; Jiang et al., 2016; Li et al. 2020). This article presents a thermodynamic model to explain the change in hydrocarbons composition of rock samples residing in a laboratory as a function of time. The model involves multicomponent residue curves and vapor-liquid equilibrium calculations. The changes in hydrocarbon composition predicted using models were compared with experimental data obtained from thermal desorption-gas chromatography (TD-GC) analysis of rock samples. High correlation coefficients are observed between predicted compositions and the experimental data inferring a good model fidelity. Relatively higher deviations were also observed between experimental data and predicted composition with time. The proposed model is useful in complementing the experimental results and enhancing the fundamental understanding of hydrocarbon loss from rock samples. This model also has the potential to eliminate multiple experiments to estimate the compositions after hours of sample collections.