Alkaline in-situ leaching of uranium in low-permeability sandstone: An experimental study using online low field nuclear magnetic resonance (LF-NMR) spectroscopy on multiphase response

Uranium leaching was influenced by multiphase flow and geological structure during in-situ recovery of uranium (ISRU). In this study, the characteristics of multiphase flow and pore structure responding to the alkaline ISRU was investigated, using a real-time triaxial coupling nuclear magnetic resonance experiment system. The component of leaching solute, flow regime and pore structure evolution was monitored online through the T₂ spectrum and magnetic resonance imaging (MRI), and corresponding coupling correlation of uranium recovery and pore conductivity was analyzed. The result demonstrates that the uranium sandstone was composed of micropore (1–10 ms), mesopore (10–100 ms), and macropore (>100 ms) corresponding to different relaxation times (ms). The uranium migration was mainly dominated by mesopore, and mineral dissolution and precipitation were influenced by micropore and macropore. Uranium recovery based on colloidal tetravalent and hexavalent uranium particles was exponentially correlated with the volume of leaching solution passed through the solute (solid phase/pores) and slightly contributed by the seal pore and matrix-bearing uranium. The physical migration, mineral dissolution, carbonate precipitation, and alternation by reactive transport occurred and was influenced by the hydrodynamic pressure. The dissolution of U, Ca, and Si experienced a dynamic period (0–5 PV), buffer period (5–37 PV), transition period (37–200 PV), and stable period (200–319 PV); peak concentration of U occurred at 5 PV, and 73.4% of uranium was recovered in the scope of 0–37 PV. Here, 1 PV represents the sample pore volume of 3.95 cm³. The porosity and permeability were enlarged under the influence of the chemical dissolution and physical migration, but decreased with the precipitation of CaCO₃ and SiO₂ colloids. Permeability and porosity exhibited a positive correlation with the volume of solution passed through in the range of 0–100 PV, but a negative correlation was observed in the range of 100–319 PV. The findings provide significant insight into ISRU engineering practice.