Effective and Relative Permeability

Most low-permeability gas reservoirs are at “irreducible” water saturation and remain at that saturation during reservoir depletion because of low water relative permeability. Therefore, it is useful to characterize the gas flow properties of these rocks by determining effective gas
permeability at “irreducible” water saturation (k_eg,Siw). In high-permeability rocks, the value of keg,Siw commonly approximates the value of absolute permeability (k_i). However, in low-permeability rocks, k_eg,Siw can be significantly less than k_i because water occupies critical pore-throat space even at “irreducible” water saturation. For this reason, keg,Siw is more important than k_i as a control on gas production from low-permeability rocks. In Mesaverde-Frontier sandstones k_eg,Siw can be predicted from _i:

logk_eg,Siw = 0.415 _i - 6.41

where k_eg,Siw and _i are in md and % respectively.

Typical gas relative-permeability curves for low- permeability sandstones show that gas permeability decreases sharply at saturations above approximately 40-50%. Expressed as relative permeability (i.e., relative to insitu Klinkenberg absolute gas permeability), Mesaverde-Frontier sandstones exhibit a sharp decrease in gas relative permeability (k_rg,Siw) with increasing S_iw. The k_rg,Siw values are generally less than 5% at Siw greater than 60% following the general relation:

where k_rg,Siw and S_iw are expressed in percent. It is important to note that even at low k_rg,Siw and high S_iw, these sandstones produce water-free gas.

Work is still in progress on K_eg,Siw for Council Grove rocks. Relative permeability curves for a limited number of cores indicate that gas relative permeabilities are also low in Council Grove carbonates at high water saturations. Note the reference k for the low-k sample is 3% of the absolute permeability.