(206g) Absolute Adsorption of Methane in Shale Nanoporous Media: Challenges Arising from Dual-Heterogeneity

Accurate estimation of methane adsorption amount is essential for reliable gas-in-place (GIP) evaluation and well productivity prediction in shale media. While only excess adsorption isotherm can be obtained by experimental measurements, the absolute adsorption which represents the actual adsorption amount needs to be converted from the excess adsorption. In previous works, it has been shown that methane adsorbed phase density is dependent on pressure and temperature, indicating that the constant pre-determined adsorbed phase density may not be applicable for absolute adsorption calculation. The widespread pore size distribution (PSD) in shale reservoirs leads to varying adsorption mechanisms in micropores and mesopores. Our recent studies have shown that it is necessary to explicitly consider PSD in the estimation of absolute adsorption in kerogen from excess adsorption. On the other hand, shale consists of organic (kerogen) and inorganic matters (clay and quartz, etc), while both can contain an extensive number of nano-scale pores with drastically different pore surface properties. The pore size heterogeneity and rock heterogeneity, the so-called dual-heterogeneity, imposes a grand challenge in methane absolute adsorption estimation in shale media.

In this work, we use grand canonical Monte Carlo (GCMC) simulations to describe methane adsorption behavior in various sizes of organic and inorganic nanopores. For simplicity, we use carbon and illite slit nanopores to represent organic and inorganic nanopores. The methane adsorption in nanopores is divided into six and five adsorption types for carbon and illite slit pores, respectively, based on density profiles from GCMC simulation. The characterized adsorption models with lumping in terms of the corresponding pore size are applied in the Ono-Kondo (OK) model. The validity of our proposed OK model is examined by 1000 sets of artificially generated PSDs ranging from 0.7 to 30 nm. We find that by fitting the excess adsorption isotherm, OK model coupled with new adsorption models shows a good agreement with GCMC simulation in terms of absolute adsorption. The proposed model can faithfully capture the methane adsorption in organic-rich, clay-rich and organic-clay mixed (each contribution is comparable) shale nanoporous media. On the other hand, we find that without considering the dual-heterogeneity, the popularly used models, such as Langmuir and SDR, show noticeable deficiencies. Our work shows the applicability of OK model for accurate calculation of methane absolute adsorption in shale nanoporous media and the necessity of explicitly considering dual heterogeneity in shale gas-in-place estimation.