The behavior of Irati oil shale before and after the pyrolysis process

Oil shale is a sedimentary rock that contains organic complexes of kerogen throughout its mineral matrix. Kerogen can be converted into hydrocarbons through various chemical processes, such as pyrolysis, which is used by Petrobras for oil production in Brazil. To process oil shale, the shale is placed into a vertical cylindrical reactor and subjected to a gas stream that induces pyrolysis at a temperature of 500 °C. The purpose of this study was to examine possible inorganic and petrofabric transformations in oil shales as a result of the pyrolysis process. Samples of oil shale and retorted material (residue from pyrolysis) were analyzed to compare possible chemical, mineralogical and petrofabric variation between the two materials. X-ray fluorescence (XRF), X-ray diffraction (XRD), petrographic analysis and X-ray computed microtomography (μCT) were used for this purpose. Additionally, the effects of temperature and heating rate on the thermal degradation of the samples were determined using thermogravimetric analysis combined with differential thermal analysis (TGA/DTA). The samples of retorted material were obtained with a pyrolysis simulator called BSTU. The chemical and mineralogical results for the oil shale and retorted material showed that the pyrolysis process did not cause significant changes in the rock. The chemical analysis showed that the chemical composition was dominated by SiO2 and Al2O3, while the mineralogical analysis detected mainly quartz, feldspar and clay minerals, as well as pyrite. The thermogravimetric curves showed that organic matter and pyrite decomposed between 400 °C and 550 °C. The main changes occurred in the petrofabric, as the pyrolysis process caused intense fracturing parallel to the original bedding structure of the shale. The process was more pronounced in shales with higher levels of organic matter and led to pore expansion and higher pore connectivity, thus allowing the escape of fluids and gases.