Evaluation the Effect of Temperature and Reaction Time on Hydrothermal Conversion of Oil Shale: Insights into Synthetic Oil Generation and Micro-Scale Pore Structure Change

Abstract This research presents a systematic study on the hydrothermal conversion (artificial thermal maturity) performance of oil shale from Domanik Formation (Russia). The experiments were conducted at different conditions including temperatures of 300°C, 350°C, and 400°C, and reaction times of 1, 3, 6, 12, and 24 hours. For this purpose, the quantity and quality of produced synthetic oils, the composition of released gases, as well as changes in pore size and structure of shale consolidated core samples by X-ray computed tomography were evaluated. In addition, SARA group composition, carbon number distribution including light fractions (C10 – C20, with a boiling point of 216–342°C, representing petroleum, kerosene and diesel cuts) and heavier fractions (C21 – C32, with a boiling point of 356 – 458°C), elemental analysis, and the compositions of obtained synthetic oils (including aliphatic and aromatic hydrocarbons) and the conversion degree of kerogen were analyzed using multiple techniques including, Low Field - NMR, GC, GC–MS, and Rock-eval pyrolysis. The obtained results confirmed that the high quality of synthetic oils is characterized by a higher amount of light fractions (C10 – C20) compared to heavy fractions (C21 – C32), with a higher amount of saturate and aromatics than resins and asphaltenes. The degree of OM conversion varies depending on the temperature and reaction time. Among 300, 350, and 400 °C, the highest conversion rates were 25, 70, and 99% for reaction times of 24, 24, and 6 h, respectively. Hydrothermal conversion at 300°C for 24 hours, 350°C for 24 hours and 400°C for 3 hours showed the highest quality of synthetic oils. Notably, hydrothermal treatment caused a substantial change in pore size, leading to an improvement in porosity and the formation of connected pore channels. The porosity increased approximately 4 times (from 2.6 to 11.9%). In general, the promising obtained results technically demonstrate the feasibility of developing Domanik oil shale by in-situ hydrothermal retorting.