Permeability-controlled transition of slurry diffusion patterns in water-rich filled fractures via a coupled multiphase flow approach

The fluid physics of slurry transport in water-rich filled fractures remains unclear because existing models poorly capture the coupling among slurry–water interface evolution, effective filling resistance, and time-dependent rheology. A coupled multiphase flow approach investigates the permeability-controlled transition of slurry diffusion patterns. It combines volume-of-fluid interface tracking, Darcy–Forchheimer effective resistance, and a transport time equation for slurry viscosity evolution. The model was evaluated against a physical experiment at the macroscopic scale and used to compare effective permeability conditions and slurry types. As effective permeability decreases, downstream elongation weakens, while lateral spreading and upstream diffusion are enhanced, changing the diffusion pattern from slender to elliptical and nearly circular forms. At 25 s, from the unfilled condition to the lowest permeability condition, the aspect ratio of cement slurry decreased from 8.38 to 1.13, while the upstream to downstream diffusion ratio and convex hull circularity increased from 0.006 to 0.835 and from 0.300 to 0.988, respectively. This transition is accompanied by faster pressure buildup, stronger velocity attenuation, and weaker downstream slurry transport. Time-dependent viscosity growth suppresses downstream transport and promotes slurry accumulation. Compared with cement slurry, cement–sodium silicate slurry produces a rounder diffusion pattern, wider pressure distribution, higher peak pressure, and faster dynamic water blocking under the same permeability. These results indicate that slurry diffusion patterns within the adopted equivalent resistance framework are influenced by slurry–water interface evolution, filling medium resistance, and slurry viscosity evolution. This work provides physical insight into slurry transport and blocking under dynamic water grouting conditions.