Pore evolution of oil shale during sub-critical water extraction

Abstract

The porous structure of oil shale plays a vital role in heat transfer and mass transport. In this study, the pore evolution of oil shale samples during sub-critical water extraction was investigated by scanning electron microscope (SEM), N2 adsorption/desorption, and low field nuclear magnetic resonance (NMR). The following results were obtained: (1) With increased extraction time and extraction temperature, the yield of bitumen increased, pores in spent samples obviously developed and extended to the inner of the shale matrix, and their pore size gradually increased from the nano to micron size; (2) Pore volume and surface area of mesopores increased with increasing yield, indicating that the extraction of organic matter improves the development of organic matter pores distributed in mesopores; (3) The formation of secondary organic matter pores primarily contributes to the increment of pore volume in oil shale samples. The diameter of kerogen may range from 100 to 1600 nm; (4) Fractures probably propagated parallel to the bedding direction, and their evolution led to an initial increase in the total pore volume followed by a decrease. This is likely because fractures will be strongly compacted by pressure due to the weakening of inner support after more organic matter is extracted.