--> Relationship Between Pore Characteristics and Occurrence State of Shale Gas: A Case Study of Lower Silurian Longmaxi Shale in the Upper Yangtze Platform, South China

AAPG ACE 2018

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Relationship Between Pore Characteristics and Occurrence State of Shale Gas: A Case Study of Lower Silurian Longmaxi Shale in the Upper Yangtze Platform, South China

Abstract

We use focused ion beam-Helium ion microscopy (FIB-HIM) and low pressure N2 adsorption to investigate the pore characteristics of Lower Silurian Longmaxi shale. These results, combined with the molecular potential energy theory reveal the relationship between pore size and occurrence state of shale gas. The results show that: (1) Both the pore volume and the specific surface area of Lower Silurian Longmaxi shale are mainly contributed by the pores with diameter less than 10 nm. Nano-scale organic matter pores are predominant, and the pore surface is not generally smooth and has fractal characteristics. (2) When the distance between the methane molecule and the pore wall in organic matter pores is limited to 2 nm, there is a strong interaction force between them, and the methane molecule is affected by the interaction force and is in the adsorbed state accordingly. When the distance between them is greater than 2 nm, the interaction force can be ignored, and the methane molecule is not affected by the interaction force and is in the free state accordingly. (3) In the pores having radius greater than 2 nm, the adsorption capacity is positively correlated with the specific surface area; while in the pores having radius smaller than 2 nm, the average gas concentration is related to the pore radius. (4) The adsorption zone volume increases firstly and then decreases with increasing pore diameter. When the pore diameter ranges from 1 to 6 nm, the adsorption zone volume is significantly larger than the free zone volume. Within this range, the adsorption volume nearly equals to the pore volume. When the pore diameter ranges from 6 to 60 nm, the adsorption volume gradually decreases, while the free volume increases, which equals to the adsorption volume when the diameter reaches 15 nm.