Investigation on the reactive transport characteristic of uranium-bearing sandstone during the in-situ leaching mining uranium

The multi-phase flow in dynamic pore-fracture is crucial to the safety and efficient uranium mining using the in-situ leaching method. In this study, the pore-fracture evolution and multi-phase flow morphology was investigated using the online low-field nuclear magnetic resonance (NMR) system, and the relationship of pore structure, permeability transformation and chemical reaction was analyzed. The results show that the adsorption pore (0.1 ms < AP < 10 ms,46.4%), seepage pore (10 ms < SP < 100 ms,36.6%) and migration pore (100 ms < MP,17.0%) composed the pore-fracture structure, and the chemical corrosion increased the AP, the decreased SP was mainly controlled by the carbonatite precipitation, and MP was enhanced by the dynamic fluid pressure and decreased by the carbonatite precipitation. Compared with the increased permeability of uranium dissolution-dependent pore structure, the carbonatite precipitation-dependent pore structure dominated the decreased permeability during the in-situ leaching. The increased effect on permeability and mineral dissolution for dynamic pressure was gradually weakened by the carbonatite precipitation. Thus, the uranium dissolution was mainly occurred at AP, the uranium-bearing solution migration was controlled by SP, and carbonatite precipitation was occurred at MP. The transformation between the AP, SP and MP was integral dominated by the corrosion dissolution, carbonatite precipitation and fluid kinetics, resulting in the decrease of SP and permeability. The variation in uranium concentration from 0.1 MPa to 0.7 MPa indicates that uranium dissolution and migration in the in-situ leaching process are primarily governed by the interplay of pressure, fluid dynamics, and carbonate precipitation. At lower pressures, enhanced uranium dissolution facilitates greater mobility. However, as pressure increases, carbonate precipitation intensifies, significantly hindering uranium migration and resulting in a marked decrease in uranium concentration. Thus, the conductivity of the uranium-bearing sandstone characterized by the permeability was directly dominated by the SP. The finding provides significant insight into safety and efficiency in-situ leaching uranium mining.