To investigate the fracture and permeability characteristics of sandstone-type uranium ore under cyclic impact, a Hopkinson bar experimental system was used to load sandstone samples by cyclic impacts. The dynamic mechanical properties of the sandstone samples were measured after 3, 6 and 9 impacts. Subsequently, the impacted sandstone samples were subjected to CT scanning, and the crack images obtained from the scans were reconstructed in three-dimensions to measure the changes in pore and fracture parameters. The internal structures and damages in the impacted samples were then analyzed. Furthermore, a microscopic seepage simulation was performed to analyze the permeability of the samples, revealing the changes in the simulated permeability. Finally, permeability tests were conducted on the impacted samples to measure the variations in the actual permeability. Results show that cyclic impacts cause cumulative damage in the specimens, reducing their dynamic mechanical properties. As the number of impacts increases, energy in the specimens accumulates and releases cyclically. This cyclic accumulation and release of energy lead to a process of crack "expansion, compaction, re-expansion, re-compaction". During the cyclic impact process, small and isolated cracks inside the specimen gradually develop into larger, interconnected fractures. Simultaneously, medium-sized cracks exhibit dual effects of faulting and connectivity, presenting nonlinear characteristics. Cyclic impacts induce more complex fractures in the specimens, leading to an increased number of fluid seepage pathways and a larger scale of seepage. When subjected to three cycles of impact, the sample forms a single crack, resulting in a permeability increase of 340.91%−380.00%. After six cycles of impact, the cracks begin to connect, leading to a permeability increase of 1468.18%−2893.33%. With nine cycles of impact, a connected network of cracks forms, resulting in a permeability increase of 4718.18%−9380.00%. The cyclic impact significantly enhances the permeability of sandstone, with crack propagation and connectivity being the key driving factors for the increase in permeability.

Rapid and accurate in-situ permeability testing is extremely important during the in-situ leaching of low-permeability sandstone-type uranium deposits. The current permeability testing methods rely on laboratory tests and the inversion of core or debris samples, which cannot reflect the true permeability of uranium deposits under their occurrence conditions. Therefore, this paper proposes a testing device based on the pressure pulse method for the in-situ permeability and corresponding automatic calculation software, and establishes the testing process. Specimen tests on a concrete model are carried out, and the testing results show consistency with the laboratory results and the micro-seepage numerical simulation results of uranium deposit cores in terms of magnitude and governing laws. However, due to factors such as the specimen tests not considering confining pressure, the uneven pouring, and the local cracking of the specimen caused by pulse pressure, the measured permeability deviation is between 7.14% and 21.47%. The permeability test results are related to the mineral stacking structure, the testing system, and the testing process. The permeability of uranium deposits with local gravel and basal cementation mode is relatively small. The main factors affecting the permeability test results are the deformation and friction of the high-pressure water storage tank and cable, the loose connection of various components, the integrity of the wellbore casing or the wellbore wall, and the installation position of the measuring section system. This study presents a rapid and accurate insitu permeability testing technology for low-permeability sandstone-type uranium deposits, providing technical support for site selection and effect prediction in in-situ learning.

To decipher the mechanical response mechanisms of coal seams with multi-scale pore-fracture structure to supercritical CO₂ (ScCO₂) injection, two coal samples from different mines of the Ordos Basin, North China, were first selected for conducting ScCO₂-water-coal reaction experiments. Subsequently, the pore-fracture structure, macroscopic and microscopic mechanical behaviors of samples with different reaction times were analyzed, and the evolution patterns of pore-fracture parameters and relationships between the macroscopic and microscopic mechanical parameters were finally elucidated. The results showed that the ScCO₂-water-coal reaction modifies the pore-fracture structure in coal. Originally filled fractures re-open, original micro-fractures expand, new fractures form, and pores evolve from small- to large-sized. After the ScCO₂-water-coal reaction, the evolution of the compaction stage, the macroscopic mechanical parameters and the energy dissipation during loading corroborate the weakening effect of the ScCO₂-water-coal reaction on coal. The changes observed in the microscopic mechanical parameters align with those in the macroscopic mechanical parameters; however, due to the strong heterogeneity of coal and the inability of microscopic parameters to reflect the component and pore-fracture distribution, certain characteristics of the change amplitude of macroscopic and microscopic mechanical parameters of coal are inconsistent. The ScCO₂ extraction effect, the chemical dissolution, the different-sized pore-fracture evolution, the coupling effect of geostress, reservoir pressure, and swelling stress are the main factors to consider during the process of ScCO₂ sequestration in deep coal seams at the micro-, meso- and macro-scales, as they are responsible for potential safety issues.