--> Nano-Scale Pore Structure Characteristics of Organic-Rich Longmaxi Shale Using Argon Gas Adsorption

AAPG ACE 2018

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Nano-Scale Pore Structure Characteristics of Organic-Rich Longmaxi Shale Using Argon Gas Adsorption

Abstract

Physical gas adsorption is a classical technique to study pore structure characteristics (e.g. pore size distribution, pore volume, and specific surface area) of porous solid materials, and the most common adsorbates are carbon dioxide and nitrogen. However, carbon dioxide adsorption in 273 K can only measure pores less than 1 nm accurately, and nitrogen adsorption in 77 K is applicable for the measurement of mesoporous materials. Furthermore, the quadrupolar nature of the nitrogen molecule will result in the specific interaction with surface functional groups and exposed ions of the adsorbent, which will affect the orientation of nitrogen molecule on the adsorbent surface and the micropore filling pressure. In contrast to nitrogen, argon is spherical molecule, which does not exhibit specific interactions with surface functional groups, and the argon molecule at 87 K can fill narrow micropores at significantly higher relative pressures than nitrogen in 77 K, which means the argon adsorption at 87 K can measure both micropores (less than 2 nm) and mesopores (2-50 nm) of porous materials. In this study, 15 organic-rich shale samples were selected for pore structure investigation using FIB-SEM, argon adsorption at 87 K, and mercury intrusion capillary porosimetry. These samples were collected from two evaluation wells in the national shale gas demonstration zone in Sichuan Basin, where the annual shale gas production increased rapidly from 12.8×108 m3 in 2014 to 64.6×108 m3 in 2016. The target layer is the lower part of Longmaxi Formation, which is the most important shale gas producer in China. The FIB-SEM images show that the fundamental storage spaces for hydrogen are nano-scale pores in organic matter. Argon low-pressure adsorption and mercury intrusion capillary porosimetry are combined to quantify the pore size distribution of organic-rich shale samples. The result of argon adsorption suggests that the pore size of shale samples are mainly distributed below 10 nm, and micropores provide most of the specific surface area for gas adsorption. The modified argon low-pressure adsorption experiment can obtain continuous nano-pore size distribution of the shale samples quantitatively and accurately.