Nanoscale pore structure and fractal characteristics of the continental Yanchang Formation Chang 7 shale in the southwestern Ordos Basin, central China

Abstract

Shales from the Yanchang Formation Chang 7 Member in the Ordos Basin are among the most important shale reservoirs in China and have been investigated due to their great potential. Knowledge of pore structure is important for understanding the storage capacity and flow mechanism in shale reservoirs. In this study, eight shale samples were collected from the Yanchang Formation Chang 7 Member in the southwestern Ordos Basin, and their geochemistry, mineral compositions, pore structure, and fractal characteristics were investigated based on X‐ray diffraction (XRD) analysis, total organic carbon (TOC) analysis, low‐pressure adsorption/desorption analysis, thermal maturity analysis, and fractal analysis. The results indicated that the TOC content ranged between 0.48% and 2.37%, and the Ro values varied from 0.826% to 1.217%. The major mineral compositions were quartz and clay minerals. Nitrogen adsorption/desorption analysis indicated that the isotherms were similar for all collected shale samples from the Chang 7 Member and resembled the type IV isotherm. Narrow slit‐like pore was the dominant pore type, and the pore size distribution appeared to be unimodal with its peak mainly around 40 nm. Investigation of factors for pore structures showed that the TOC content was the controlling factor for the Chang 7 shales. The Frenkel‐Halsey‐Hill (FHH) method was applied to determine fractal dimensions, which were calculated as the D1 (relative pressure >0.96), D2 (0.96 >relative pressure >0.45), and D3 (0.45 >relative pressure) values, ranging in the intervals of 2.788~2.854, 2.547~2.688, and 2.410~2.567, respectively. The relatively high fractal dimensions indicated that pore structures were complicated, and the higher D1 values suggested that pores with larger sizes showed a rougher pore surface and more complex pore structure. Fractal dimensions showed positive correlations with the contents of TOC and clay minerals, and a negative relationship with the quartz contents.

Supercritical Methane Adsorption on Shale over Wide Pressure and Temperature Ranges: Implications for Gas-in-Place Estimation

“…According to petrophysical test results, the shale physical model can be simplified into two parts: mineralogical components and pore space (Figure b). Mineralogical components include brittle minerals (quartz, feldspar, carbonate minerals, and pyrite), clay minerals, and dispersed organic matter, ,, in which organic matter and clay minerals are regarded as the main carriers for shale gas adsorption. ,,, Pore space covers the pores and fractures developed within and between various minerals. ,,, Some pores are isolated, which the fluids (formation water, liquid hydrocarbon, and gas) cannot access. , In the accessible pore space, with the subtraction of the fraction occupied by formation water and oil, the remaining is the available space for shale gas storage in the form of adsorbed gas and free gas.…”

Multifractal characteristics of shale and tight sandstone pore structures with nitrogen adsorption and nuclear magnetic resonance

“…Li et al (2017) studied the pore structures of Chang 7 tight sandstones using HPMI combined with fractal theory, and the value of calculated fractal dimensions changes from 2.2520 to 2.7875. Ju et al (2019) studied the pore structure and fractal characteristics of Chang 7 shale were combined with N 2 GA, and found that the fractal dimensions increase with the increase in organic matter and clay mineral content.…”

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, ,− ...…”

“…These findings indicate marked surface geometric heterogeneity at the atomic scale. (2) Slitlike shape: the pore shape can be determined from the shape of the hysteresis loop of the nitrogen adsorption and desorption isotherms (see refs − , , , , − , − , − , − , − , , , − , − , , , , ) according to the classification by the International Union of Pure and Applied Chemistry (IUPAC) or by the field emission scanning electron microscope (see refs , , , − , , , , , , − , , , , , , , , , , , , , ) and MIP. ,,, The summary statistics reveal that slit pores constitute the largest...…”

“…(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.…”