Assessment of Various Approaches in the Prediction of Methane Absolute Adsorption in Kerogen Nanoporous Media

Abstract

It is crucial to accurately characterize methane absolute adsorption in kerogen nanoporous media for gas-in-place evaluation and well productivity prediction. Assuming that methane forms a single-layer adsorption in kerogen nanopores, a large number of approaches have been reported to convert the experimentally measured excess adsorption to the absolute adsorption. Recently, we have shown that methane adsorption behavior depends on the pore size and may be very different from the single-layer adsorption model, such as the Langmuir model. Thus, it is necessary to explicitly consider the pore size distribution (PSD). While these conversion methods have been extensively used, their validity in the characterization of methane absolute adsorption in nanoporous materials, such as kerogen, has not been systematically assessed. As in our previous work, we used model kerogen with varying PSDs and grand canonical Monte Carlo simulations to model methane adsorption up to 500 bar to assess various commonly used methods converting excess adsorption to absolute adsorption. We find that the predetermined density methods using 373 or 424 kg/m3 may show unphysical phenomena and Langmuir as well as SDR models can largely overestimate the absolute adsorption. On the other hand, the Ono–Kondo (OK) lattice model with PSD can accurately characterize the absolute adsorption in nanoporous media. Interestingly, Langmuir and SDR models coupled with PSD can provide comparable predictions to OK with PSD. In addition, we also suggest to use the high-pressure excess adsorption data (up to 500 bar), instead of the commonly used low-pressure excess adsorption measurements (up to 150 bar). Our work also calls for the accurate characterization of PSD in nanoporous materials to obtain their absolute adsorption capacity.

Comprehensive Review about Methane Adsorption in Shale Nanoporous Media

“…279 On the other hand, m abs monotonically increases with pressures. 88 It should be noted that in eqs 23 and 24, neither V a nor ρ a can be directly measured from experiments. 125,164,279 One needs to make certain assumptions to convert the experimentally measured m ex to m abs .…”

“…As a result, a high-pressure and hightemperature gas adsorption measurement is necessary to mimic the in situ condition and describe the accurate adsorption behaviors. 87,88 However, most shale gas adsorption measurements are limited to relatively moderate pressure conditions. 57 Moreover, the current gas adsorption measurements can obtain the excess adsorption from a macroscopic perspective, while the absolute adsorption amount which represents the actual adsorption amount needs to be converted.…”

Methane Absolute Adsorption in Kerogen Nanoporous Media with Realistic Continuous Pore Size Distributions

“…Based on the characterization shown in Figure , the OK models are divided into six types. In contrast to the GCMC simulations, a constant adsorption layer width W a = 0.38 nm is applied in the OK model . From Type I to Type V in the OK model, the density in each layer is higher than ρ b , while in Type VI, the density in the 3rd layer in the mesopore is very close to ρ b and the region beyond the 3rd layer is regarded as free gas.…”

“…In contrast to the GCMC simulations, a constant adsorption layer width W a = 0.38 nm is applied in the OK model. 40 From Type I to Type V in the OK model, the density in each layer is higher than ρ b , while in Type VI, the density in the 3rd layer in the mesopore is very close to ρ b and the region beyond the 3rd layer is regarded as free gas. It should be noted that the occupation in each layer k in different adsorption types can be different, which is given as x k, j = ρ a, k, j /ρ am , where j is the specific adsorption type.…”

“…AED is calculated by using the Steele 10-4 potential for each specific pore, while the adsorbed phase is regarded as liquid with a constant density coupled with a thermal expansion effect over all studied pore size ranges. Our recent works , applied the OK model associated with varying adsorption models in different pores and PSD to reliably predict m abs in kerogen nanoporous media. Four adsorption types are included in the OK model based on the typical CH 4 density distributions in micropores and mesopores from the grand canonical Monte Carlo (GCMC) simulation.…”