Dynamic Pore-Scale Model of Drainage in Granular Porous Media: The Pore-Unit Assembly Method

Area of Science:

• Pore-scale physics
• Multiphase flow in porous media
• Geological engineering

Background:

• Understanding drainage dynamics is crucial for various subsurface processes.
• Existing models often simplify pore-scale interactions, limiting accuracy in dynamic scenarios.
• Numerical simulations offer a powerful tool to investigate complex fluid flow at the pore scale.

Purpose of the Study:

• To develop and validate a dynamic pore-scale model for simulating water drainage in sphere packings.
• To investigate the mechanisms of air-water displacement during drainage at the pore scale.
• To assess the influence of pore structure heterogeneity on drainage patterns.

Main Methods:

• Developed a dynamic pore-scale model using regular triangulation to represent pore space.
• Constructed pore units approximated by volume-equivalent regular shapes.
• Employed a pore-scale Implicit Pressure Solver and Explicit Saturation Update (IMPES) method for unsaturated flow simulation.
• Validated the model against quasi-static capillary pressure-saturation data from a sand packing.

Main Results:

• The dynamic model accurately reproduced quasi-static drainage curves for a sphere packing with similar properties to sand.
• Simulations revealed finger-like infiltration of air into the pore space, driven by pore structure heterogeneities.
• Observed pressure differences between air and water exceeded capillary pressure during dynamic drainage.
• Investigated the impact of averaging, boundary conditions, domain size, and viscosity on flow behavior.

Conclusions:

• The developed dynamic pore-scale model is capable of accurately simulating drainage under low flow rates.
• Pore structure heterogeneity significantly influences drainage patterns, leading to finger-like air intrusion.
• The model provides insights into the complex interplay of capillary forces and dynamic effects during drainage.