Experimental and Fractal Characterization of the Microstructure of Shales from Sichuan Basin, China

Abstract

Comprehensive characterization and analysis of the microstructure characteristics of shale are significant for understanding its transport mechanisms and determining highly efficient production programs. In this study, composition analysis, scanning electron microscopy imaging, and gas adsorption−desorption measurements are first conducted on 29 marine shale samples from the Longmaxi Formation, Sichuan Basin, China, to obtain their petrophysical properties and microstructure characteristics. Then, surface fractal dimension, pore fractal dimension, lacunarity, and succolarity are introduced to analyze the microstructure of these shale samples. Finally, the correlations between total organic carbon (TOC) content and microstructures and the relationships among the microstructures are analyzed systematically. The results show that the complex pore network in these shales is constituted by microfractures, intergranular pores, intragranular pores, dissolved pores, and organic pores. The typical type IV adsorption isotherm and type H3 hysteresis loop are identified from nitrogen adsorption−desorption data. TOC content has positive influences on the adsorption capacity and nanopore distribution. The surface fractal dimensions under low and high relative pressure are similar in these marine shale samples, which is different from the two-section characteristic of continental and marinecontinental shale. The statistical self-similarity characteristic of the pore-size distribution in these shales is further confirmed based on the analytical fractal dimension equation. The normalized lacunarity can quantitatively characterize the heterogeneity of the poresize distribution of these shales in two-dimensional space. The succolarity analysis in two-dimensional space explains the low fluid transport capacity of these shales. Universal and logically explained correlations are found between TOC content and microstructures. Moreover, logically explained correlations also exist among the microstructures of the shale samples evaluated.

Controlling Factor Analysis of Microstructural Property and Storage Capacity of Deep Longmaxi Formation Shale in Sichuan Basin

“…The fractal parameters, such as fractal dimensions, lacunarity, and succolarity, are helpful to understand the complex storage and structural properties of the porous media. , Herein, the pore fractal dimension and surface fractal dimension are analyzed from the T 2 spectra for water-saturated samples and low-pressure nitrogen adsorption data, respectively.…”

“…Nuclear magnetic resonance (NMR) is a fast and nondestructive experimental method. The transverse relaxation ( T 2 ) spectrum can represent the overall fluid storage property of shale by monitoring the amount of hydrogen atoms in hydrogenous fluids (e.g., water and hydrocarbons). − The geometry analysis method, especially the fractal geometry theory, can reveal more structural information by quantified fractal parameters (e.g., fractal dimensions, lacunarity, and succolarity represent the complexity, heterogeneity, anisotropy of the fluid storage space, respectively) from the experimental data. − The application of these fractal parameters is a powerful supplement to experimental measurements and enriches the physical and geometrical connotation of the experimental data.…”

Techniques used to calculate shale fractal dimensions involve uncertainties and imprecisions that require more careful consideration

“…Many thanks to Kouqi Liu for clarifying aspects of the calculation methods used in (Liu et al, 2021) and Jianchao Cai and his associates (Tian et al, 2021) for providing some useful additional insights with respects of fractal dimensions and their analysis.…”

“…The interpretation of gas-adsorption isotherms provides a means to estimate fractal dimension values (Sahouli et al, 1996). The techniques are now widely applied to petroleumrich shales (Clarkson et al, 2013;Yang et al, 2014;Wood and Hazra, 2017;Hazra et al, 2018;Liu et al, 2021;Tian et al, 2021), and the influence of a shale's thermal maturity on fractal dimension recognised (Hazra et al, 2018;Gao et al, 2020). Shale fractal dimension values published have been derived almost exclusively based on the interpretation of low-pressure gas adsorption isotherms exploiting the Frenkel-Halsey-Hill (FHH) model (Halsey, 1948;Hill, 1952;Greggs and Sing, 1982).…”

Improvement of Emulsion Stability and Plugging Performance of Nanopores Using Modified Polystyrene Nanoparticles in Invert Emulsion Drilling Fluids

“…Currently, research on wellbore instability in shale oil and gas formations has mainly focused on shale hydration inhibition mechanisms ( Huang et al, 2018a , 2018b ; Du et al, 2020 ; Ma et al, 2021 ). Due to the increased usage of OBFs, research into the controls of shale wellbore collapse has been extended to studies of the micro-structures of shale and other formations ( Arif et al, 2021 ; Tian et al, 2021 ). The infiltration and pressure transmission of drilling fluids due to the development of micro-pores and fractures in shale formations, most of which are nanoscale ( Chen et al, 2021 ), are important factors leading to the instability and collapse of wellbores.…”