Selective Subsurface Flow Regulation by an HPAM/Aluminum Citrate Weak Gel in Heterogeneous Sandstone Uranium In-Situ Leaching: Laboratory Experiments and Field Application

Preferential flow through high-permeability pathways is a major challenge in uranium in situ leaching (ISL) because it can induce hydraulic short-circuiting and reduce the effective sweep of the leaching solution within heterogeneous sandstone aquifers. In this study, a weak-gel system composed of partially hydrolyzed polyacrylamide (HPAM) and aluminum citrate (AlCit) was investigated for selective hydraulic regulation in heterogeneous porous media, followed by field-scale application. The HPAM/AlCit system exhibited time- and temperature-dependent gelation behavior, enabling low-viscosity injection followed by delayed in situ crosslinking. In parallel sand-pack experiments with initial permeabilities of 9.06 and 2.87 μm2, 350 and 91 mL of gelant entered the high- and low-permeability columns, respectively, demonstrating permeability-driven preferential placement. After gelation, permeability was reduced by 99.59% in the high-permeability medium but by only 31.01% in the low-permeability medium, while the corresponding residual resistance factors were 245 and 1.45. Field injection further confirmed the injectability and hydraulic-regulation capability of the system. The injectivities of two treated wells decreased by 61.4% and 83.3%, respectively, while the average hydraulic conductivity of the test zone decreased from 25.1 to 17.2 m/d, corresponding to a reduction of 31.5%. An inspection well located 3.0 m from the injection well recovered gel-containing cores predominantly within the target sandstone interval, providing direct evidence of meter-scale lateral gel propagation. Overall, the results demonstrate a cross-scale mechanism involving permeability-driven preferential migration, delayed in situ gelation, selective permeability reduction, and hydraulic-resistance redistribution. The proposed approach provides a potential strategy for controlling preferential flow in heterogeneous sandstone aquifers during uranium ISL.