Investigation of Fractal Characteristics and Methane Adsorption Capacity of the Upper Triassic Lacustrine Shale in the Sichuan Basin, Southwest China

Abstract

To better understand the nanopore characteristics and their effects on methane adsorption capacity of shales, we performed fractal analysis of nine shale samples collected from the fifth member of Upper Triassic Xujiahe Formation in the Sichuan Basin, southwest China. [Formula: see text] adsorption results show that shales have different adsorption characteristics at relative pressure of 0–0.5 and 0.5–1. Two fractal dimensions [Formula: see text] and [Formula: see text] were calculated using the Frenkel–Halsey–Hill (FHH) equation. Results show that the methane adsorption capacity increases with the increase of [Formula: see text] and [Formula: see text], of which [Formula: see text] has a more significant influence on adsorption capacity than [Formula: see text]. Further studies indicate that [Formula: see text] represents the pore surface fractal characteristics caused by the irregularity of shale surface, whereas [Formula: see text] represents the pore structure fractal characteristics, which is mainly affected by shale components (e.g. TOC, clay minerals) and pore parameters (e.g. average pore diameter, micropores content). A higher [Formula: see text] corresponds to a more irregular pore surface, which provides more space for methane adsorption. While a higher [Formula: see text] indicates a more complex pore structure and a stronger capillary condensation action on the pore surface, which in turn enhances the methane adsorption capacity.

Nanopore Structure and Fractal Characteristics of Lacustrine Shale: Implications for Shale Gas Storage and Production Potential

“…Fractal concept was first proposed in 1975, by Mandelbrot [42], and widely used in porous media including shale [35,36,37,38,39,40,41,43,44,45]. Fractal theory is used to describe the roughness of pore surface and the complexity of pore structure for porous media.…”

“…The commonly used parameter is fractal dimension D. The larger the fractal dimension, the rougher the pore surface or the more complex the pore structure. It is a well-accepted method to use gas adsorption for fractal dimension calculation [35,36,37,38,39,40,41,64]. According to Pfeifer’s theory [65], the fractal dimension can be calculated by the FHH equation:lnV=Kln[ln(P0P)]+C…”

“…Among them, low-pressure N 2 adsorption has been proved highly effective in the pore structure characterization of shale. In recent years, more and more research have been conducted on the pore fractal characteristics of shale using N 2 adsorption isotherms [35,36,37,38,39,40,41].…”

Adsorption of Methane, Nitrogen, and Carbon Dioxide in Atomic-Scale Fractal Nanopores by Monte Carlo Simulation I: Single-Component Adsorption

“…(1) Atomic-scale geometrically heterogeneous surfaces: statistical self-similarity, a typical geometric feature of surface heterogeneity that can hardly be described by Euclidean geometry, is a ubiquitous characteristic of most materials in nature . Thanks to the fractal theory advanced by Mandelbrot, , the surface fractal dimension functions as an applicable measure of surface geometric heterogeneity. − The fractal pore morphology has been examined by mercury intrusion porosimetry (MIP), − the fractal Frenkel–Halsey–Hill (FHH) adsorption isotherm equation based on nitrogen adsorption, − ,− nuclear magnetic resonance, ,, small-angle and ultrasmall-angle neutron scattering, ,− ...…”

“…(3) Nanoscale average diameter: IUPAC refreshed the pore classification by size, defining nanopores as the pores with diameters to an upper limit of about 100 nm . Usually, the average pore diameter ( d ) is calculated from the cylindrical pore model ( d = 4 v / a ) with the specific pore surface area ( a ) and the specific pore volume ( v ) obtained in carbon dioxide or nitrogen adsorption experiments (see refs − , − , − , , , − , − , , , − , , , , ). The distribution indicates that the overwhelming majority of the average diameters lies in the range of 0–50 nm (Figure c), which conforms to the definition of nanopores.…”