(645a) Effect of Energetical and Geometrical Heterogeneity of Kerogen on BET Surface Area Characterization and Methane Adsorption

As a transition fuel, natural gas plays an ever-increasingly important role to meet the global energy demand, while reaching net-zero carbon emission by 2050. On the other hand, due to the continuous depletion of conventional natural gas reservoirs, shale gas has become an important natural gas source. For example, in the United States, ~80% of total dry natural gas production is from shale formations in 2020. Unlike conventional reservoirs, surface adsorption plays a dominant role in shale gas due to the presence of a significant amount of nanosized pores in shale media. Kerogen is the main constituent of organic matter, which generates hydrocarbons via chemical decomposition. It is also the main methane storage site as methane adsorption capacity in shale rocks has shown a positive correlation with the total organic carbon (TOC) content. Therefore, the accurate characterization of surface area in kerogen nanoporous media becomes utterly important in the prediction of shale gas-in-place (GIP).

The Brunauer–Emmett–Teller (BET) method has been widely used to characterize 77 K N₂ adsorption to obtain the surface area (the so-called BET surface area, S_BET) in various porous media, such as activated carbons, metal organic frameworks (MOFs), silica, and zeolite. It is also one of the standard methods to obtain the surface area in isolated kerogen. BET theory assumes that the multilayer adsorption of ideal gas takes place on a perfectly-smooth ideal homogeneous surface. On the other hand, in contrast to the basic assumptions in BET theory, kerogen surface may not be perfectly smooth (i.e., geometrical heterogeneity), while it carries energetical heterogeneity with a number of heteroatoms such as N, S, and O. While there have been a number of studies to investigate the effect of energetical and geometrical heterogeneity on S_BET, the careful analysis of their effect on kerogen S_BET is still lacking. In addition, whether S_BET can be a good indicator for methane adsorption capacity in kerogen nanopores remains unanswered.

In this work, we conduct 77 K N₂ adsorption in 13 kerogen slit mesopores to study the effect of kerogen geometrical and energetical heterogeneity on the S_BET characterization by using grand canonical Monte Carlo (GCMC) simulations. We find that within the BET pressure range, N₂ adsorption sites are mainly within concave ("basin") and valley regions on kerogen surfaces, while in convex regions its adsorption rarely takes place. In addition, while S_BET agrees well with the geometric surface area (S_GEO) in graphene mesopores, in kerogen mesopores, S_BET is generally lower than the S_GEO. The deviation becomes more significant as kerogen surface roughness becomes more significant and the fraction of convex ("ridge") surface increases. On the other hand, surface chemistry shows non-negligible impacts on N₂ adsorption when the presence of heteroatoms affects surface topology. Interestingly, S_BET correlates well with methane excess adsorption in kerogen mesopores, while it outperforms S_GEO. It is probably because both nitrogen-kerogen and methane-kerogen interactions are mainly van der Waals type, while their adsorption sites largely overlap.

This work provides some crucially important fundamental understanding about S_BET characterization in kerogen mesopores which can guide methane adsorption capacity prediction in kerogen nanoporous media and shale gas-in-place estimation.