Organic Matter Pore Characterization of the Wufeng-Longmaxi Shales from the Fuling Gas Field, Sichuan Basin: Evidence from Organic Matter Isolation and Low-Pressure CO2 and N2 Adsorption

Zhuo Li,Hailong Yu,Zhenxue Jiang,Zhikai Liang

doi:10.3390/en12071207

Abstract

Organic matter (OM) pores are significant for shale gas accumulation and flow mechanisms. The pores of Wufeng-Longmaxi (W-L) shale in the Sichuan Basin, China have been extensively characterized, however, the proportion of OM pores in this shale have not been adequately discussed. In this study, the contribution of OM pores to the total pore volume of W-L shale was quantitatively studied through the analysis of OM isolation, field emission scanning electron microscopy (FE-SEM) and low-pressure CO2 and N2 adsorption (LPGA). FE-SEM images showed abundant OM pores, interparticle pores and intraparticle pores with various shapes and widths in the W-L shales. The pore size distribution (PSD) of the isolated OM from five shale samples showed a consistent, unimodal pattern. The pore volume of isolated OM was greater than that of the bulk shale samples, suggesting that OM is more porous than the inorganic compositions in shales. The average contribution of OM to the volumes of micropores, mesopores and macropores was 58.42%, 10.34% and 10.72%, respectively. Therefore, the pore volume of the W-L shale was dominantly related to inorganic minerals. This was probably due to the small weight ratio of OM in the shale samples (1.5 wt%–4.2 wt%). The findings of this study reveal the different effects of OM and minerals on pore development, and provide new insights into the quantitative contribution of OM pores to the total pore volume of the W-L shale.

Highlights

Organic shales commonly contain complex pore systems with various pore types, pore geometry and multiscale pore widths [1,2,3]
The pore size distribution (PSD) of isolated organic matter (OM) and bulk shale samples were compared to understand the different impacts of OM and inorganic minerals on pore characteristics in shale
These results suggest that inorganic minerals host abundant pores and that mineral compositions may contribute mainly to the macropores in gas shale

Summary

Introduction

Organic shales commonly contain complex pore systems with various pore types, pore geometry and multiscale pore widths [1,2,3]. The pores in shale are commonly classified as interparticle (interP), intraparticle (intraP), and organic matter (OM) pores or microfractures, using direct imaging techniques [2]. Due to the nanoscale organic and inorganic pores within the shale formations, the solid–fluid interaction becomes non-trivial, causing the gas flow to derivate from classical Darcy’s law [5,6]. Researchers have reported some pore-scale studies of fluid flows in shale, most of them focused on a single nano-tube with the mean pore-throat radius. All the derived formulas are based on gas molecular dynamics, which might not be applicable to shale formations, especially

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