A review of gas relative permeability (k_rg) studies of low-permeability sandstones indicates that they can be modeled using the Corey equation, but scarce data near the critical gas saturation (S_gc) limit k_rg modeling at high water saturations. Confined mercury injection capillary pressure and coupled electrical resistance measurements on Mesaverde sandstones of varied lithology were used to measure critical nonwetting saturation. Most of these data support the commonly applied assumption that S_gc 0.05. However, a few heterolithic samples exhibiting higher S_gc indicate the dependence of S_gc on pore-network architecture. Concepts from percolation theory and upscaling indicate that S_gc varies among four pore-network architecture models: (1) percolation (N_p), (2) parallel (N_//), (3) series (N), and (4) discontinuous series (N_d). Analysis suggests that S_gc is scale- and bedding-architecture–dependent in cores and in the field.

The models suggest that S_gc is likely to be very low in cores with laminae and laminated reservoirs and low (e.g., S_gc 0.03–0.07 at core scale and S_gc 0.02 at reservoir scale) in massive-bedded sandstones of any permeability. In cross-bedded lithologies exhibiting series network properties, S_gc approaches a constant reflecting the capillary-pressure property differences and relative pore volumes among the beds in series. For these networks, S_gc can range widely but can reach high values (e.g., S_gc 0.6). Discontinuous series networks, representing lithologies exhibiting series network properties but for which the restrictive beds are not sample spanning, exhibit S_gc intermediate between N_p and N networks.

Consideration of the four network architectures lends insight into the complications of heterogeneous lithologies at differing spatial scales and underscores the difficulty of upscaling laboratory-derived relative permeabilities for reservoir simulation. Analysis also indicates that for some architectures, capillary pressure and relative permeability anisotropy may need to be considered.