The sandstone is in a state of dry–wet cycle under the repeated action of rainfall, and its mechanical properties are deteriorated to varying degrees, which causes cracks in the sandstone. Therefore, it is of great significance to study the mechanical properties and fracture propagation of sandstone under the action of dry–wet cycles. Currently, there are limited studies using numerical simulation methods to study the fracture extension of rocks under various dry and wet cycling conditions.Therefore, in this paper, the effects of different amounts of dry and wet cycling on the mechanical properties and fracture behavior of sandstone are investigated through uniaxial compression tests and numerical simulations of fracture extension. The findings indicate that the deformation stage of sandstone remains unchanged by the dry–wet cycle. The uniaxial compressive potency and coefficient of restitution gradually diminish as the quantity of cycles rises, while the Poisson's ratio exhibits the opposite trend, and the impact on the mechanical performance of sandstone wanes with cycle increments, and the correlation coefficient surpasses 0.93, signifying a substantial influence of the dry–wet cycle on sandstone's mechanical performances. The discrepancy between the numerical simulation and experimental results is minimal, with a maximum error of only 3.1%, demonstrating the congruence of the simulation and experimental outcomes.The mesoscopic examination of the simulations indicates that the quantity of fractures in the sandstone specimens rises with the escalation of dry–wet cycles, and the steps of analysis linked to crack inception and fracture propagation are accelerated, and the analysis steps from fracture initiation to penetration are also reduced.

Summary: High-temperature pore reconstruction technology is a reservoir reconstruction measure that has emerged in recent years. It is of great significance to study the variation in pore structure characteristics of shale under high temperature for reservoir reconstruction. To study the effect of high temperature on shale pores, scanning electron microscopy (SEM) experiments and fluid injection experiments were used to analyze the variation of pore structure characteristics under high temperature. Studies have shown that temperature has a great influence on the morphology and distribution characteristics of shale pores. In particular, there is a temperature between 300°C and 400°C that is suitable for modifying pores. The distribution characteristics, surface area, and volume of pores vary dramatically under this temperature threshold. The pore morphology and distribution characteristics changed from small and sparse to large and dense. The Brunauer-Emmett-Teller (BET) surface area increased by 95%. The cumulative surface area of Barrett-Joyner-Halenda (BJH) adsorption and desorption increased by 71.7% and 72%, respectively. The pore volume of the 2-nm to 20-nm pore size increased by 63.2%. The pore volume of pore sizes greater than 20 nm increased by 191.6%. The pore variation characteristics were in line with the typing law, and the fitting result R2 ranged from 0.92201 to 0.99882.

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

This study employs transformative nanoscale-spatial methodology, which reveals the complete thermal damage evolution mechanism of sandstone from 25 to 700 ◦C, fundamentally challenging the conventional linear degradation paradigm. Spatial autocorrelation analysis is innovatively applied to atomic force microscopy data, establishing that thermal damage follows a non-monotonic pathway governed by spatial heterogeneity evolution rather than simple progressive weakening. Four critical temperature thresholds are identified: 200 ◦C marks the initiation of localized damage through expansion of weak zones with 61.4% stiffness reduction; 400 ◦C features unexpected transient hardening due to clay mineral dehydroxylation and pore restructuring, reaching the peak adhesion force; 500 ◦C represents not only increased damage but also the thermal decomposition of the 400 ◦C hardened structure, evidenced by maximum spatial heterogeneity and negative adhesion; and the 700 ◦C following quartz phase transition establishes a completely reconfigured structure with a novel "harder-in-lower-areas" pattern. Crucially, this research demonstrates that the spatial clustering of mechanical properties evolves from balanced high-low zones at 200 ◦C to a percolating low-stiffness network at 500 ◦C, establishing spatial reorganization as the governing principle of thermal damage. The findings provide a quantitative framework that can accurately predict rock behavior in geothermal systems and underground energy storage applications. Document Type: Original article Cited as: Yan, H., Zhou, T., Zhou, X., Liu, X., Tang, X. Non-monotonic evolution and spatial reorganization mechanism of thermally induced micro-damage in sandstone. Advances in Geo-Energy Research, 2025, 17(2): 135-148. https://doi.org/10.46690/ager.2025.08.05