In-situ pyrolysis of oil shale in pressured semi-closed system: Insights into products characteristics and pyrolysis mechanism

The in-situ conversion of oil shale has become an inevitable trend for exploitation due to the environmental friendliness and adaptability of deeper reservoirs. In-situ pyrolysis process actually occurred in a semi-closed system, in which kerogen cracking and hydrocarbon migration are inevitably affected by pressure. Additionally, pyrolysis zones far from the injection well remain below 400 °C, leading to prolonged heating stage. The atmospheric pressure rapid pyrolysis of oil shale is dissimilar from in-situ mining conditions. In light of this, this study comprehensively investigated the effects of pressure, temperature, and heating time on the pyrolysis behavior, yield and composition of pyrolysis products. For the effect of pressure, the shale oil yield of 8 MPa at 500 °C declined by 65.7 %, while the gas yield increased by 93.7 % when compared to atmospheric pressure. The pressure promoted the generation of light components and accelerated the conversion of alkenes to alkanes and aromatics in shale oil. The release temperature of gases increased under pressure, encouraging the production of alkane gas while reducing the hydrogen yield. Finally, the pyrolysis mechanism for oil shale coupling of temperature and pressure was proposed. Besides, with the extension of holding time, the maximum shale oil yield reached 65.90 %, 80.81 %, and 83.03 % of the Fischer oil yield at 350 °C, 380 °C, and 400 °C, respectively. Temperature and time exhibited a compensatory effect, with a distinct boundary observed between 350 °C and 380 °C for shale oil production. GC-MS analysis revealed that the proportion of medium- and short-chain alkanes in shale oil exceeded 75 %, while longer time enhanced the release of long-chain alkanes and exacerbated aromatization. Additionally, the accumulation of bitumen could enhance the heat capacity of semi-coke with an appropriate insulation at 350-380 °C, although it did not reverse the deterioration of combustion performance. An alternate pyrolysis pathway of organic matter based on bitumen transformation was proposed during the medium-temperature, long-duration in-situ pyrolysis process.