Investigation on pyrolytic, fracture evolution and seepage of oil shale under high temperature

Oil shale resources are rich in China, and kerogen-dominated organic matter in the oil shale is transformed into shale oil after heating, which is expected to make up the “short board” of China’s energy structure. It is of great scientific value to exploring the evolution of pores, fractures and the law of fluid migration in oil shale under the influence of pyrolysis final temperature and high temperature to promote the commercial development of it in-situ exploitation in China. In this paper, the influence of multiple pyrolysis final temperature and heating duration was considered, the physical experiment method and the introduction of electron microscope observation technology (SEM) are used, the pyrolysis of oil shale samples and gas seepage tests were carried out, and the pyrolysis property, pore and fissure evolution and permeability were investigated. The results show that, with the increase of pyrolysis temperature, the pyrolysis rate of organic matter mainly kerogen increases exponentially, the remolding of the internal structure of oil shale heated include the pores produced by organic pyrolysis and cracks formed by non-uniform deformation of inorganic materials. The SEM results showed that the pyrolytic areas originally distributed in discrete spots are connected with each other, and the dark area keeps expanding. The uneven change of the materials in the pyrolytic and non-pyrolytic areas leads to cracks at the interface between them. The pyrolysis process had a development process of “small-scale discontinuous fissure -- penetrating distributed fissure -- penetrating main fissure”, which improves the fracture rate in oil shale reservoir. Under the influence of Klinkenberg effect, the permeability of oil shale after pyrolysis decreases with the increase of gas injection pressure in an exponential function law, and it increases with the heating temperature in an approximate “S” shape, which is almost opposite to the TG curve of Balikun oil shale pyrolysis. Organic matter in oil shale has the highest pyrolysis efficiency and enhanced effective permeability in the range of 450−500 ℃. The research results provide a certain experimental basis for improving the theory of oil shale pyrolysis mining.