From aperture characterization to hydraulic properties of fractures

A methodology is suggested to measure the aperture and estimate the hydraulic properties of desiccation and tectonic fractures in heterogeneous soils. The aperture of desiccation fractures is quantified by the fluctuating distance between two opposite faces and is modelled as a network of parallel plates of variable opening. The aperture of tectonic fractures is modeled as a 2-dimensional network of elliptical channels with parameters the major and minor axis lengths. Scanning electron microscope (SEM) images of the 2-dimensional cross-sections of resin-impregnated fractures are employed to estimate the statistics of the fracture aperture parameters in both cases. This information is coupled with effective medium approximation for the calculation of the mean hydraulic fracture aperture which allows us to estimate approximately the permeability of the fracture networks over the various zones of the soil. The estimated values of permeability are comparable to corresponding ones measured with gas permeation tests on the field. Invasion percolation models are used to simulate the quasi-static and pressure-controlled immiscible displacement of oil/water drainage in single fractures and estimate the relevant capillary pressure and relative permeability curves, which are similar to corresponding ones resulting from the inverse modelling of two-phase flow experiments in planar glass-etched networks of elliptical pores.

Highlights► The aperture of tectonic and desiccation fractures is modeled by using BSEM images. ► Aperture models allow the estimation of hydraulic properties of single fractures. ► 2-D invasion percolation models provide the 2-phase flow properties of fractures. ► Conceptual models of fracture systems are needed to estimate soil permeability.