Microstructure and damage evolution in red sandstone under real-time load-high temperature-water cooling cycles: A low-field nuclear magnetic resonance investigation

In enhanced geothermal systems (EGS), in situ stress substantially influences reservoir rock properties. We employed nuclear magnetic resonance (NMR) to examine pore-structure evolution, damage, and permeability in red sandstone exposed to 200 °C, cyclic loading at 0.3 times peak stress, and subsequent water cooling. We refined the damage-coefficient calculation using MRI bitmap data, enabling visualization and quantitative analysis of the two-dimensional spatial distribution of damaged microns. Results demonstrated that prestress reduced the threshold cycles for porosity and damage changes (15 for Group W, 10 for Group WF) and increased their rates. This effect altered the medium-pore distribution: in Group WF, 50–200 nm pores decreased while other pores expanded; in Group W, only pores >2000 nm increased. The pore fractal dimension increased before 15 cycles—reflecting greater complexity—and declined thereafter, indicating reorganization and simplification. Prestress accelerated homogeneous pore damage and promoted clustered microcrack growth; beyond 15 cycles, it fostered pore redistribution and uniform microcrack clustering. Overall permeability—including macropores—increased by over 65%. Ten cycles marked the permeability transition threshold for micropores and mesopores, with macropores contributing more substantially. Additionally, determining the damage coefficient via MRI bitmap data circumvented over-limit calculations based on the T 2 spectrum peak-area weighting. This study elucidates macro-micro damage mechanisms under thermo-hydro-mechanical coupling in EGS reservoirs and provides a theoretical basis for reservoir optimization.