Pore Structure and Fractal Characteristics of Niutitang Shale from China

Zhaodong Xi,Jing Wang,Shuheng Tang,Yingying Guo,Kaifeng Wang,Junjie Yi

doi:10.3390/min8040163

Abstract

A suite of shale samples from the Lower Cambrian Niutitang Formation in northwestern Hunan Province, China, were investigated to better understand the pore structure and fractal characteristics of marine shale. Organic geochemistry, mineralogy by X-ray diffraction, porosity, permeability, mercury intrusion and nitrogen adsorption and methane adsorption experiments were conducted for each sample. Fractal dimension D was obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) model. The relationships between total organic carbon (TOC) content, mineral compositions, pore structure parameters and fractal dimension are discussed, along with the contributions of fractal dimension to shale gas reservoir evaluation. Analysis of the results showed that Niutitang shale samples featured high TOC content (2.51% on average), high thermal maturity (3.0% on average), low permeability and complex pore structures, which are highly fractal. TOC content and mineral compositions are two major factors affecting pore structure but they have different impacts on the fractal dimension. Shale samples with higher TOC content had a larger specific surface area (SSA), pore volume (PV) and fractal dimension, which enhanced the heterogeneity of the pore structure. Quartz content had a relatively weak influence on shale pore structure, whereas SSA, PV and fractal dimension decreased with increasing clay mineral content. Shale with a higher clay content weakened pore structure heterogeneity. The permeability and Langmuir volume of methane adsorption were affected by fractal dimension. Shale samples with higher fractal dimension had higher adsorption capacity but lower permeability, which is favorable for shale gas adsorption but adverse to shale gas seepage and diffusion.

Highlights

Shale gas status can be divided into three categories: adsorbed gas, free gas and solution gas [1,2].The strong heterogeneity of the characteristics and structure of shale porosity leads to different shale gas storage and transition mechanisms, which makes it difficult to predict the spatial distribution and availability of shale gas [3]
Amorphous sapropel indicates that the shale contains mainly Type II1 kerogen, supported by the results of the type indicates that the shale contains mainly Type II1 kerogen, supported by the results of the type index index (TI) (Table 1)
Ro values vary from 2.24% to 3.42%, with most values greater than 3%, indicating a a high over-mature stage of hydrocarbon development

Summary

Introduction

Shale gas status can be divided into three categories: adsorbed gas, free gas and solution gas [1,2].The strong heterogeneity of the characteristics and structure of shale porosity leads to different shale gas storage and transition mechanisms, which makes it difficult to predict the spatial distribution and availability of shale gas [3]. Shale is typically characterized as an intricate and heterogeneous porous media with complex pore systems, thanks to its wide-ranging pore size distribution (PSD), varied. Minerals 2018, 8, 163 pore types and multiple pore geometries [4]. The evaluation of shale pore structure is an interesting topic that is critical for shale gas exploration [5–7]. Minerals 2018, 8, x FOR PEER REVIEW. Many experimental methods, both qualitatively and quantitatively, have been used to characterize pores,pore such as scanning electron microscopy focused ion beam-scanning electron microscopy types and multiple pore geometries [4].(SEM), the evaluation of shale pore structure is an (FIB-SEM), mercury intrusion, gas adsorption, small-angle or ultra-small angle neutron scattering and interesting topic that is critical for shale gas exploration [5–7].

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