3D quantitative characterization and flow simulation of granite residual soil based on CT scanning

Abstract

<p indent="0mm">This paper aims to reveal the water transport mechanism in soils with macropores. Based on the combination of the X-ray CT scanning and image processing technologies, a three-dimensional (3D) equivalent pore network model (PNM) with macropores (&gt;0.15 mm) is constructed, and the size of the optimal representative volume unit (REV) is determined to be 300×300×300. Then, the quantitative characterization of the topological structure of the macropore space is carried out followed by the PNM single-phase flow and the 3D macropore water-gas two-phase flow simulation. The results show that there are more pores with smaller pore diameters, and more than 80% of the pores are concentrated within <sc>0.15–1.00 mm.</sc> The pore topological space structure is quite different, and the coordination number is mainly less than 20, but some can reach more than 60. The average coordination number of pores is between 3–7, and the average connected pore radius is greater than <sc>2.00 mm.</sc> The single-phase flow simulation find that the complexity of the pore structure and the resolution of X-ray CT scanning resulted in a certain difference between the simulated and measured permeability, but the difference of the main seepage direction is only 3.81%. The two-phase flow simulation find that the front edge of flow is almost linear at the beginning of seepage, then it tend to be fingering, and finally, it become irregular. The residual gas in the pores is mainly distributed in blocks at the edges of pores, dead ends, and sudden changes of pore throats. With the increasing seepage time, the saturation of the water phase shows an upward trend, while the saturation of the gas phase shows a downward trend, then gradually tend to be stable, and finally, the saturation of the water phase and the residual gas phase maintain at 94.92% and 5.08%, respectively. This study can better reveal the meso-seepage mechanism of reconstructed soil, and provide a certain reference for further understanding the law of pore flow in soil.