Scale-Dependent Fractal Properties and Geological Factors for the Pore Structure in Shale: Insights from Field Emission Scanning Electron Microscopy and Fluid Intrusion

Abstract

Cost-effective pore characterization is essential for understanding gas storage and flow in shale reservoirs. Herein, we thoroughly investigated the pore structure of Qiongzhusi shale from Northeastern Yunnan, China, utilizing field emission scanning electron microscopy, high-pressure mercury intrusion porosimetry (HP-MIP), and low-pressure gas adsorption (LP-N 2 /CO 2 GA). Scale-dependent fractal properties were identified to evaluate the applicability of fluid intrusion methods. Furthermore, geological factors controlling the pore structure were analyzed. Fractal analysis via the Menger sponge model and the Frenkel−Halsey− Hill model revealed scale-dependent fractal features that act as a "fingerprint" to identify sequential stages during measurements. Mercury intrusion was identified in three stages: pore volume filling, matrix compression, and pore structure damage. Matrix compression appeared at approximately 25 MPa, corresponding to 50 nm in pore size; thereby, HP-MIP is unreliable for characterizing pores below 50 nm. Likewise, N 2 adsorption was identified in four stages: micropore filling, monolayer adsorption, multilayer adsorption, and capillary condensation. As the fractal dimensions from the micropore filling stage are invalid for certain samples, LP-N 2 GA is inadequate for micropore characterization. To cover full-scale pore size distribution (PSD), LP-CO 2 GA, LP-N 2 GA, and HP-MIP were employed to characterize micropores (<2 nm), mesopores (2−50 nm), and macropores (>50 nm), relatively. The resulting PSDs are unimodal or bimodal, with peaks at 0.3−1.0 and 5−20 nm. The Qiongzhusi shale contains interparticle pores along brittle mineral edges and within clay interlayers, intraparticle pores within pyrite framboids and dissolutionrelated minerals, and organic matter (OM) pores with varying morphologies influenced by surrounding minerals. OM and quartz were found to promote pore development, with quartz-based frameworks protecting internal OM pores from compaction-induced damage. However, clays demonstrate a multifaceted impact on pore structure, with interlayer pores between illite−smectite mixed layers contributing to pore volume, while illites negatively affect pore volume. This study provides valuable insights into full-scale pore characterization and shale reservoir evaluation.

Matrix Compressibility and Multifractal Nature of Nanoporous Shale

“…Differences in the fractal characteristics signify alterations in the mechanism of mercury intrusion . Several researchers also found similar stages for fractal analysis on pore structures in coal samples. , In addition, previous research has viewed matrix compression as elastic deformation under high-pressure stages, with mercury intrusion volumes generally exhibiting a linear increase relative to the intrusion pressures, ,− which is consistent with the results observed in all the Yanchang shale samples in the current study (Figure b). Therefore, we employed the fractal segmentation and linear region in the intrusion curve to determine the matrix compression during mercury injection.…”

“…Notably, all of the values of D B exceed 3.0, which is the upper limit for fractal dimensions, making the D B values nonphysical. This anomaly is attributed to matrix compression occurring in the shale samples under high pressures. ,− Compression-induced deformation of the matrix results in intrusion volumes that are larger than the actual PVs, leading to inaccuracies in the calculation of both PVs and fractal dimensions.…”

“…Clay content seems to have no clear relationship to pore heterogeneity and connectivity. This outcome can be explained by the multifaceted influence of clays on the nanopores in shale reservoirs . From one perspective, uniformly distributed interlayer pores in clay aggregates can promote pore quantity, thereby enhancing pore connectivity.…”

“…The above operation is repeated, and we can finally get a Menger sponge, and its fractal dimension is 2.78. Presently, the Menger sponge framework effectively serves to simplify and represent the complex nanopores in shale, , and the fractal dimensions, D m , of shale samples were further derived using eqs and based on the MICP data. log ( normald V P d P ) ∝ K · log 0.25em P D m = K + 4 where D m is the fractal dimension; V P is the intrusive volume, mL/g; P is the injection pressure, MPa; and K is a constant.…”

“…Mercury injection capillary pressure (MICP) measurements with the combination of low-pressure N 2 adsorption (LPN 2 A) were employed to evaluate compression deformations in porous materials. − Liquid mercury can penetrate the nanopores in dense rock samples due to its extremely low compressibility, making it more resistant to compression compared to the solid matrix. , With the increasing intrusion pressure, the matrix skeleton would be compressed by high-pressure mercury, , and the volumetric deformation from matrix compression would provide noticeable intrusion volumes . Matrix compression is considered as elastic deformation at high intrusion pressures, suggesting that the deformed volumes show a linear correlation with pressure increments. ,− Friesen and Mikula earlier highlighted the phenomenon of matrix compression in MICP testing on coal samples. According to scale-dependent fractal properties, Feng et al effectively distinguished between pore volume filling and matrix compression during the MICP tests on shale samples.…”