Shale, which consists of various of both inorganic and organic materials with pores ranging from a few nanometers to several micropores. Accurate characterization of methane absolute adsorption plays a key role in gas-in-place (GIP) estimation and economic viability (Thomas et al., Energy and Fuel, 2014, 28 (5), 2886-2901). While experiments can measure excess adsorption, the absolute adsorption is converted from the excess adsorption based on single-layer adsorption assumption coupled with constant adsorbed phase density or adsorption models such as Langmuir and SDR, ignoring rock compositional heterogeneity and pore size heterogeneity. We have shown, using grand canonical Monte Carlo (GCMC) simulations, that the presence of a transition zone beyond the first adsorption layer can adversely affect the accuracy of commonly used conversion methods. Explicitly characterizing the transition zone using a modified multilayer adsorption model coupled with the Ono-Kondo (OK) lattice theory yields an accurately estimation of the methane absolute adsorption in kerogen mesopores. Such multilayer adsorption model also extends to the heavier hydrocarbon adsorption in kerogen mesopores. By using artificially generated PSD and GCMC simulations, we effectively model methane adsorption in nanoporous kerogen (micropores&lt;2 nm) where methane can exhibit layering. Based on the varying density profiles in different nanopores from GCMC, we propose a corresponding adsorption model in each nanopore and apply an OK model. We find that our proposed OK with PSD (OK-PSD) agrees with GCMC simulation. We also systematically assessed other conversion methods for methane absolute adsorption in kerogen nanoporous media and found it necessary to explicitly take into account the PSD. OK-PSD offers the best performance. We also use GCMC simulations to characterize methane adsorption behavior in inorganic nanopores of various sizes. Based on methane density distributions in nanopores of various types and sizes, we propose corresponding adsorption models, which can be coupled with the OK model. By regressing the excess adsorption from GCMC in shale nanoporous media (consisting of organic/inorganic and varying PSD), OK-PSD can readily obtain the absolute adsorption in an excellent agreement with GCMC simulations.

Methane absolute adsorption in shale reservoirs: Central challenges arising from dual heterogeneity

2019 Fall National Meeting

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