(185f) Analysis and Discussion of Specific Surface Area in Nanoporous Kerogen: From Molecular Simulation to Applicability of BET Theory with Different Criteria

The specific surface area (SSA) of shale media plays an important role in the gas-in-place estimation. Shale contains a large number of pores in nanometer range, especially micropores (<2 nm) and mesopores (2~50 nm). The Brunauer, Emmett and Teller (BET) theory has been widely used to characterize SSA of nanoporous media by converting N₂ adsorption isotherms at 77 K. However, the applicability of BET theory to calculate SSA in nanoporous media is still under debate. Specifically, the relative pressure range for BET fitting applied in shale studies follows the standard method (P/P₀ between 0.05 and 0.3), while previous works on other microporous materials (such as MOFs) emphasize the importance of the consistency criteria to determine the validity of BET theory. As a main constituent of organic matters, kerogen contains large amount of micropores, but the relations between BET SSA and geometrical surface area in kerogen nanopores have not been addressed yet.

In this work, molecular simulation is performed for kerogen nanopores to analyze the applicability of BET theory in the description of SSA. The simulation data of N₂ adsorption at 77 K are used to obtain the BET SSA for kerogen nanopores with different pore sizes. The BET SSAs for organic matters are obtained by applying conventional relative pressure range as well as with consistency criteria. The calculated BET SSAs are compared with geometrical surface areas which are well defined from kerogen atomic structures. We find that the discrepancies between BET SSA and geometrically surface area is sensitive to pore size, adsorbate-adsorbent interactions and surface roughness. In particular, the BET areas for kerogens are lower than the geometrical surface areas with standard relative pressure range, while an appropriate relative pressure range on the basis of consistency criteria provides a better representation of SSA. In addition, the surface roughness imposes negative effects on the SSA prediction from BET theory. Our work should provide important insights into the accurate characterization of SSA in kerogen nanopores.