A review of gas relative permeability (krg) studies of low-permeability sandstones indicates that they can be modeled using the Corey equation, but scarce data near the critical gas saturation (Sgc) limit krg 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 Sgc < 0.05. However, a few heterolithic samples exhibiting higher Sgc indicate the dependence of Sgc on pore-network architecture. Concepts from percolation theory and upscaling indicate that Sgc varies among four pore-network architecture models: (1) percolation (Np), (2) parallel (N//), (3) series (N┴), and (4) discontinuous series (N┴d). Analysis suggests that Sgc is scale- and bedding-architecture-dependent in cores and in the field.
The models suggest that Sgc is likely to be very low in cores with laminae and laminated reservoirs and low (e.g., Sgc < 0.03–0.07 at core scale and Sgc < 0.02 at reservoir scale) in massive-bedded sandstones of any permeability. In cross-bedded lithologies exhibiting series network properties, Sgc approaches a constant reflecting the capillarypressure property differences and relative pore volumes among the beds in series. For these networks, Sgc can range widely but can reach high values (e.g., Sgc < 0.6). Discontinuous series networks, representing lithologies exhibiting series network properties but for which the restrictive beds are not sample spanning, exhibit Sgc intermediate between Np 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.
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Understanding, Exploring, and Developing Tight-gas Sands
The 2005 Vail Hedberg Conference was convened to gain a better understanding of the tight-gas sand resource life cycle by encouraging a free exchange of cross-disciplinary discussion among leading scientific and engineering experts. The results of the conference have led to improved exploration models and development and completion strategies required to exploit the vast North American tight-gas sand potential and emerging international tight-gas sand plays. This third volume in the AAPG Hedberg Series is recommended for geologists and engineers involved in exploring, developing, and appraising tight-gas sand plays for a comprehensive updated view of this important natural-gas resource.