Conclusions

Comparison of carbonate and sandstone rocks indicate that many low-permeability petrophysical properties are “universal” - “tight gas” isn't just for sandstones anymore!

Previous research has documented low-permeability gas sandstone properties. Lower Permian Council Grove Group low permeability carbonates of the Panoma field, Hugoton Gas Area illustrate the universality of certain low-permeability rock properties and provide insight into lithologic controls on reservoir properties.

Like Mesaverde-Frontier Formation low-permeability sandstones and siltstones, Council Grove limestone porosity and permeability decrease with decreasing grain size (grainstone to mudstone) and increasing shaliness.

As with low-k sandstones, accurate measures of in situ properties in low-k carbonates require
consideration of confining stress, Klinkenberg gas slippage, and high water saturations. The effects of each of these variables in diminishing effective gas permeability increases with decreasing permeability.

The Council Grove low-k carbonates and Mesaverde-Frontier low-k sandstones can be divided into unique k- regions on the basis of lithofacies and for carbonates Dunham classification. Similarities between low-k carbonates and sandstones petrophysical properties include:

For any given porosity, Council Grove carbonates exhibit higher permeability than Mesaverde-Frontier sandstones.

In both groups in situ Klinkenberg permeability (k_i) exhibits similar increasing difference from
routine air permeability (k_air) with decreasing permeability. A nearly identical equation can be used to predict k_i from k_air values. Both also exhibit the characteristic exponential or cube root core permeability decrease with increasing confining stress from routine to in situ stress conditions.

The correlation between k and principal pore throat diameter (PPTD) is continuous for all
lithofacies including both carbonates and siliciclastics.

Both groups of low-permeability rocks exhibit significant increase in “irreducible” water saturation (S_wi) with decreasing k_i. Differences in the slope of a log Swi-logki correlation are consistent with differences in clay content and distribution.

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, k_eg,Siw can be more important than k_i as a control on gas production from low-permeability rocks. Unlike higher- permeability rocks, in both Mesaverde-Frontier sandstones and Council Grove carbonates k_eg,Siw is significantly lower than k_i and can be as much as two-orders of magnitude lower for rocks with absolute permeability below 0.1 md.

Low-permeability carbonates, like low-permeabillity siliciclastics, reinforce the axiom - “the good stay good and the poor get poorer”.

Read more about it - Visit the Hugoton web-site:
http://www.kgs.ku.edu/Hugoton/index.html