Modeling Water Retention Characteristic of Porous Media: A New Triangular Pore Space Model.

Quantifying the permeability of a porous medium has been a great interest in the fields of hydrology, environmental engineering or petroleum engineering. Water retention characteristic is an important hydraulic property related to pore structure (i.e., size and connectedness of pore spaces). Modeling water distribution and flow in partially-saturated soils requires knowledge of the water retention characteristics, therefore plays a critical role in water management and in prediction of solute and contaminant transport in the environment. In this work, we proposed a modified triangular pore space model aimed at improving the modeling of water retention curve. A porous medium is described with a series of triangular capillaries and with a statistic distribution of sizes (gamma function), and a scaling-up scheme from the single pore modeling was used. We considered the roughness features on the sides of triangles, which largely increased the water film length and thus could improve the prediction of water retention function at low water saturation condition. We also evaluated the parameters in the gamma function, and the result showed that heave-textured soils typically featured a smaller shape factor than fine-textured soils. Two improvements were made with the proposed model. First, the modified model reduces the number of adjustable parameters with reasonable physical consideration and can still reach considerable prediction accuracy. Second, the modified model adapts an advanced optimization program CMA-ES to compute the non-linear parameters in the calculations. The ongoing modeling of permeability based on pore-scale flow modeling will benefit from the above improvements of water retention model.