Laboratory and Field Validation of the Mechanism of Establishment of Very Low Initial Water Saturations in Ultra-Low Permeability Porous Media

The anomalous ability to produce natural gas from very low permeability (<0.05 mD in-situ) intercrystalline matrix sandstones and carbonates has been welldocumented on a worldwide basis. This paper illustrates that, in general, for a condition of economic gas production to occur, the initial water saturation that exists in the productive pay must be significantly lower than what would be expected from a normal capillary pressure equilibrium if the matrix under consideration were in dynamic contact with free water. A number of mechanisms for the establishment of these unique noncapillary equilibrium saturation conditions have been postulated over time. This paper presents the theory of long-term regional non-equilibrium gas migration and desiccation as an explanation for this phenomenon, and provides both field and laboratory data that corroborate this mechanism of subnormal initial saturation condition establishment.