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

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 dissolution-related 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.

Automated summary

Cost-effective pore characterization is crucial for understanding gas storage and flow in shale reservoirs. This study thoroughly investigated the pore structure of Qiongzhusi shale from Northeastern Yunnan, China, using various techniques. The resulting insights into full-scale pore characterization are valuable for shale reservoir evaluation.