Geotechnical geological disasters occur frequently in China. Especially under complex environmental conditions, the failure mode of rock and the change mechanism of its mechanical properties are not clear. Although some progress has been made in recent studies on the physical and chemical properties of rocks and the microscopic mechanism of action, there is still a lack of systematic understanding of the change mechanism of calcite under different temperature, pressure and humidity conditions. In this paper, the influence of these environmental factors on the expansion behavior and elastic modulus of rock is deeply analyzed by constructing a calcite supercell model. The results show that the lattice parameters a, b and c of calcite increase by 0.45%, 0.45% and 0.44%, respectively, when the temperature increases from 300 K to 1000 K. At the same time, the bulk modulus, shear modulus and Young‘s modulus decreased by 6.45%, 3.63% and 3.92%, respectively. When the pressure increases from 0.1 GPa to 0.5 GPa, the volume of calcite crystal decreases by 1.10%, while the bulk modulus, shear modulus and Young’s modulus increase by 2.74%, 9.36% and 8.66%, respectively. The bulk modulus, shear modulus and Young‘s modulus decreased by 15.6%, 18.5% and 18.1%, respectively, when anhydrous calcite was transformed into 50 water molecules. This study clarifies the degradation mechanism of calcite under the action of temperature, pressure and hydration, and provides an important theoretical basis and guidance for the prevention and control of geotechnical geological disasters.

To address the challenges of conflicting macroscopic mechanical tests and the inability to reveal complex nanoscale mechanisms during deep shale thermal modification, this study comprehensively investigates the microstructural and nanomechanical evolution of Longmaxi shale under heat treatment. This involves the innovative combination of gas adsorption, atomic force microscope high-resolution mapping, and multi-level spatial statistical analysis to systematically elucidate the spatial evolution of the pore structure of shale, surface morphology, and nanomechanical properties. The findings reveal a unique hardening-softening-rehardening three-stage pattern: From 25-400 °C, the average reduced modulus increases due to dehydration; from 400-600°C, organic matter pyrolysis significantly decreases the modulus, with intense atomic force microscope topographic uplift; from 600-900 °C, the modulus slightly increases again due to structural collapse and pore network regeneration. Local indicators of spatial association analysis shows that this macro evolution stems from synergistic microscopic phase space reshaping. Crucially, the mesopore volume increases most significantly in the 400-500°C range, and it exhibits its most notable increase. Considering energy efficiency and feasibility, this study demonstrates for the first time that 400-500 °C is an ideal temperature window for effective organic matter pyrolysis and nanopore optimization from a nanoscale spatial distribution and geometric stability perspective. This work provides crucial micromechanical mechanism support and precise temperature guidance for deep shale thermal modification, significantly outperforming the temperature ranges from traditional macroscopic experiments and filling the gap in macro-nanoscale mechanical discrepancies and spatial feature analysis. Document Type: Original article Cited as: Chen, Q., Tang, X., Shi, Y., Zhang, R., Shang, F. Nanomechanics and pore structure evolution in organic-rich shale reservoirs during high-temperature treatment: A multi-scale analysis of microscopic stability. Advances in Geo-Energy Research, 2025, 17(3): 241-255. https://doi.org/10.46690/ager.2025.09.06

In order to study the adsorption process and adsorption characteristics of shale oil at the macro scale, the isothermal adsorption experiments of illite and kerogen on a heptadecane (oil phase) solution were carried out by infrared spectrophotometry and gas chromatography–mass spectrometry. Based on the adsorption isotherm model and adsorption thermodynamic model, the characteristics of heptadecane adsorbed by illite and kerogen at different temperatures and oily solution concentrations were studied. The experimental results show that the concentration and temperature of the alkane solution help to enhance the adsorption and increase the saturated adsorption capacity. The difference is that the concentration will have a certain effect on the adsorption rate, while the temperature will not. Based on the three adsorption isotherm models, it was found that Langmuir and Freundlich were more suitable for describing the adsorption process of the heptadecane solution by illite and kerogen, and the adsorption characteristics of heptadecane molecules at different temperatures and adsorbents were evaluated. Heating leads to an increase in the collision efficiency between adsorbate molecules and adsorbents, thereby accelerating the migration rate of alkanes. Therefore, increasing temperature helps to enhance the adsorption capacity of rocks and increase the saturated adsorption capacity of minerals. The research results clarify the adsorption characteristics of shale oil heavy components from the macro level and fill the research gap in the application of solid–liquid isothermal adsorption physical experiments on the adsorption and occurrence of shale oil.

Summary: The pore structure in shale oil reservoirs exerts a crucial control over hydrocarbon accumulation and migration. Taking medium-low maturity shale (R0 = 0.71–0.90%) from the Beibuwan Basin as the research object, we study thoroughly investigate the dynamic evolution of multiscale pore structures under high-temperature conditions (0–1,000°C) and their optimization mechanism for fluid percolation and transport efficiency. A comprehensive approach, combining gas adsorption experimental [low-temperature nitrogen (N2) adsorption and carbon dioxide (CO2) adsorption), scanning electron microscopy (SEM), X-ray diffraction (XRD), and high-pressure mercury injection experiments, along with quantitative indicators such as Shannon entropy and Lempel-Ziv complexity (LZC), was utilized to reveal the pore network reconstruction process from the perspectives of full pore size range and spatial heterogeneity. The results indicate that the evolution of shale pore structure exhibits two critical temperature thresholds: The first threshold (400–500°C) is where organic matter pyrolysis leads to a significant increase in pore volume and specific surface area, accompanied by an increase in Shannon entropy. However, the LZC value fluctuates minimally, and pores are mostly isolated and dispersed, resulting in limited improvement in connectivity. The second threshold (500–600°C) is where high-pressure mercury injection data show a sharp increase in permeability, and the LZC value reaches its peak, indicating that the pore-fracture network achieves efficient connectivity, forming a highly complex, interwoven multiscale percolation pathway. At higher temperatures (>600°C), mineral phase transformation further induces stress fractures, which synergistically interact with organic matter pyrolysis fractures (microcracks formed by the pyrolysis of organic matter and mineral phase transformation) to construct a pervasive permeable network within the matrix. This study reveals that the evolution of the shale pore network follows a dynamic process from “pore increment but limited connectivity” to “interconnected pore-fracture and efficient transport,” with the synergistic response between 500°C and 600°C being key to significant optimization of pore connectivity. This structural optimization enhances the pore-throat matching and effective fluid pathway development, providing a crucial micromechanistic basis for evaluating the fluid transport potential and enhanced hydrocarbon recovery in high-temperature shale reservoirs.