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.
Highlights
Oil shale is a natural, fine-grained, laminated black or brown combustible material, and it consists of complex organic material of high molecular weight called kerogen, which is finely distributed in an inorganic matrix [1,2,3,4,5,6]
The pore evolution of oil shale during sub-critical water extraction remains a topic of interest to be explored, and findings related to it will serve as a guide towards the development of in-situ conversion [13,14]
The yields of bitumen extracted by sub-critical water increased with increasing extraction time and temperature
Summary
Oil shale is a natural, fine-grained, laminated black or brown combustible material, and it consists of complex organic material of high molecular weight called kerogen, which is finely distributed in an inorganic matrix [1,2,3,4,5,6]. Oil shale is structurally heterogeneous, porous, and amorphous [7,8]. Sub-critical water extraction technique is a promising method; its operating cost is low and environmental pollution caused by organic solvents and catalysts can be avoided. Sub-critical water as a new medium and good solvent can recover organic matter from oil shale owing to its solvent properties similar to polar organic solvents [9,10,11]. During sub-critical water extraction, pore evolution of oil shale influences the processes of conversion and hydrocarbon expulsion [12], especially for organic matter. The pore evolution of oil shale during sub-critical water extraction remains a topic of interest to be explored, and findings related to it will serve as a guide towards the development of in-situ conversion [13,14]
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