Relative Permeability In Tight Gas Sandstone Reservoirs - The "Permeability Jail" Model

Permeability Jail is a useful concept for explaining typical two-phase (gas-water) relative permeability behavior in tight gas sandstone reservoirs. Although the basic behavior is typical of any two-phase flow system, where one fluid phase interferes with the ability of the other phase to flow freely, the combination of small pore throats in tight gas reservoirs with a reactive, polar, liquid phase (water) leads to some surprises. The concept of Permeability Jail was first developed in 1992 during an unpublished multi-client study of Mesaverde tight gas sandstones in the eastern Green River Basin of the U.S. Although used informally for over a decade, the idea was not published until 2004 as part of a larger conceptual study of low-permeability reservoir systems. Therefore the effective permeabilities to both phases are below 1 micro-Darcy. The Jail region varies by rock type and specifics of the pore geometry, but roughly occurs in the 55%–80% Sw range. Tighter rocks tend to have broader Sw range in Jail. Jail does not exist in more permeable rocks because the cross-over point between the gas and water relative permeability curves is at a higher Kr, often 5% or higher, such that one or the other or both phases can establish continuous tendrils through the rock and flow at measureable rates over the entire saturation range. Jail occurs because water is tightly held by capillary forces in the small pore throats of a tight gas sandstone, blocking the flow of gas through those throats, while at the same time the amount of free water connected through those same throats is extremely low and discontinuous over more than a few pore lengths. It is the absence of larger pore throats that are not completely blocked by pendular water that prevents the flow of gas through a rock in Jail.