Up-scaling of radon migration in rock media based on multi-resolution CT pore modeling and volume averaging method

The macroscopic radon migration behavior in rock media is inherently dominated by its microscopic pore structure and transport properties. However, the lack of accurate correlation methods between microscopic transport characteristics and macroscopic parameters has long been a bottleneck in revealing radon migration mechanisms. To address this issue, this study proposes an integrated approach combining multi-resolution CT pore modeling and the Volume Averaging Method. 3D pore models of rock specimens were constructed at resolutions of 48.9 μm/voxel, 7.3 μm/voxel, and 1.1 μm/voxel using CT scanning and 3D visualization and analysis software. Through thresholding, connectivity analysis and REV analysis, key pore structure parameters (porosity and connectivity) and optimal REV sizes were determined. The results indicated that connected pore channels in the Z direction were only formed at the highest resolution (1.1 μm/voxel) with a connected porosity of 1.92%. On this basis, radon diffusion and seepage simulations were performed on the identified REVs to obtain the radon migration parameters. The Volume Averaging Method was innovatively adopted to realize the up-scaling of REV parameters to macroscopic ones, yielding the up-scaling diffusion coefficient (9.57 × 10^-9 m²/s) and permeability (2.19 × 10^-15 m²) in the Z direction, while parameters in disconnected directions were close to 0. Furthermore, the up-scaling parameters were extended to unsaturated and non-isothermal conditions by coupling the unsaturated non-isothermal radon diffusion coefficient model and Brooks-Corey model. Experimental verification showed that the deviation between the up-scaling radon diffusion coefficient and the experimental value was only 1.83% at 20 °C, and the deviation between the simulated radon exhalation rate (based on the up-scaling parameters) and the experimental value ranged from 10.38% to 31.12% when water saturation varied from 0% to 70%. This study presents a feasible technical approach for the accurate up-scaling of radon migration parameters in rocks, which helps to narrow the disconnect between microscopic and macroscopic research.