Tight gas sands behave differently than conventional reservoirs. Two petrophysical properties stand out. The first is the strong stress dependence of permeability that is well documented. The second, involving relative permeability, is not widely known or documented but its effects are widely observed. Core data show that as absolute in-situ permeability drops from the millidarcy into the tens of microdarcies range, the critical gas saturation (Sgc, the gas saturation necessary for gas flow at measurable rates) increases and the critical water saturation (Swc, the water saturation necessary for water flow at measurable rates) also increases. Viewed in a common water saturation space the two critical saturations move apart with decreasing permeability, producing a widening range of water saturations at which both phases are effectively immobile. We informally call this no-flow region “permeability jail.”

The recognition of permeability jail has enormous implications for the basin centered gas model, and evaluation of resources. This model is interpreted by some to imply that gas is ubiquitous and production is limited only by technology and stimulation. If large sections of rock are low-permeability and in “permeability jail” the presence of gas may not translate to recoverable resource. Further, if higher permeability intervals are the carrier beds for gas production from adjoining low-permeability sections, then a consequence is that these intervals will exhibit lower Swc values and will be more prone to water production. “Sweetspot” exploration strategies may therefore have the unexpected consequence of having to deal with higher water production rates.

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