The Influence of Individual Solvents as Hydrogen Donors on the Hydrothermal Conversion of Organic Matter in Shale Rocks of Domanik Deposits

Abstract

Abstract The influence of individual solvents (ethyl acetate and tetralin) as hydrogen donors on the nature of the conversion of organic matter of low-permeable shale Domanik rock from the Mendym deposits of the Upper Devonian of the Tavel deposit (Tatarstan, Russia) during hydrothermal processes was studied. The experiments were carried out at a temperature of 300°C in a CO2 environment with a water content in the reaction system of a 1:1 to sample of rock and an exposure time of 24 hours. The degree of conversion of kerogen into extractable hydrocarbons was determined by Rock-Eval and thermal analysis methods. The changes in the composition of the formed hydrocarbons were evaluated using different technic such as gas chromatography (GC) analysis of evolved gasses, SARA analysis, GC-MS of saturated and aromatic hydrocarbons) and elemental analysis before and after hydrothermal experiments. The results showed that, ethyl acetate does not exhibit the properties of a hydrogen proton donor but intensifies the destruction of kerogen. The results of GC-MS showed that, the ethyl acetate almost completely decomposes to acetic acid, which, in turn, due to its effect on the carbonate component of the rock, promotes the extraction of high-molecular components associated with the rock and the destruction of kerogen along the least stable C-S bonds. This is demonstrated by a higher presence of H2S in the gas composition compared to the control hydrothermal experiment. Additionally, there's an increase in the yield of extract from the rock, along with a higher content of high molecular weight components, consequently raising the sulfur content and the abundance of benzothiophenes in the aromatic fraction. Under these experimental conditions, tetralin not only enhances the conversion of organic matter from the Domanik rock into extractable hydrocarbons but also facilitates the removal of heteroatomic compounds by transferring hydrogen donors, which inhibit radical reactions. This leads to the elimination of alkyl substituents and the formation of lower molecular weight hydrocarbons. The results of GC-MS measurements of the aromatic hydrocarbons obtained confirm these processes. Specifically, a comparison with the control experiment reveals the formation of 1,4-dihydronaphthalene, naphthalene, and 1-, 2-methylnaphthalene in the composition. In addition, tetralin is not completely converted under these conditions and partially remains in the extract, also acting as a diluent, which can facilitate easier transportation of the extracted oil. The utilization of tetralin during hydrothermal treatment of kerogen-containing Domanik rock leads to several notable outcomes. Firstly, there is an increase in the yield of hydrocarbon gases. Additionally, the yield of oil extract rises, marked by more than a two-fold increase in the content of saturated hydrocarbons. Furthermore, there is a notable reduction in the content of resins by 1.5 times and a three-fold decrease in benzothiophenes within the aromatic fraction. Moreover, the presence of dibenzothiophenes is not detected, and the extract exhibits the lowest total sulfur content compared to all conducted experiments.