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5.1 Introduction
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suggested by Fatt [1956b], offer a more realistic approach for calculating multiphase
constitutive properties. The vast majority of PNMs consist of pore
bodies (or nodes) and pore throats (or channels), along with a selected topological
configuration which prescribes how pore bodies are connected via pore
throats. The pore bodies are meant to represent larger void spaces found in
natural porous media. The narrow openings that connect the adjacent pore
bodies are modeled by the pore throats, which are essentially capillary tubes.
The pore-network approach for modeling multiphase flow properties has been
employed extensively in the petroleum engineering literature [Chatzis and Dullien,
1977, 1985, Larson et al., 1981, Chandler et al., 1982, Wilkinson and
Willemsen, 1983]. In recent years, the pore-network approach has been also
explored in the fields of hydrology and soil physics [Ferrand and Celia, 1992,
Berkowitz and Balberg, 1993, Ewing and Gupta, 1993a,b] and upscaling of reactive
transport [Acharya et al., 2005a, Li et al., 2006b, Raoof and Hassanizadeh,
2010b].
Because of their ability to simulate the highly disordered geometry of pore
space and relatively low computational cost, PNMs hold promise as tools for
predicting multiphase flow properties of specific porous media. For example,
the dependence of capillary pressure on saturation is modeled by determining
the location of fluid-fluid interfaces throughout the network using the Young-
Laplace equation (e.g., Dullien [1991]). This is sometimes modified by other
pore-level mechanisms, such as snapoff during imbibition (e.g., Chandler et al.
[1982], Yu and Wardlaw [1986]). Also, the dependence of relative permeability
on saturation is determined by computing the resistance to flow in the connected
portion of a fluid. In these calculations, resistance to the flow within
the pore bodies is commonly ignored, assuming that conductance within the
pore bodies is much higher that the conductance within the pore throats (see
e.g., Joekar-Niasar et al. [2008a]). This means that fluid fluxes within the
pore bodies are not calculated. The significance and effects of this assumption,
however, have been never investigated.
5.1.2 Pore-network construction
The pore morphology of natural porous media is quite complex and its description
is a formidable problem. However, in many studies related to porous media,
geometrical features are crucial even though it is very hard to get detailed
information about them [Adler, 1992]. The morphology of a porous medium
consists of its geometrical properties (the shape and volume of its pores) and
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