PERMEABILITY CORRELATIONS FOR CARBONATE & OTHER ROCKS

Carbonate rock systems have large variations in both the quantity of pores and in the ability to flow. This makes it difficult to find productive zones without excessive tests in non productive strata. Such variations in pore fabric may be why carbonate systems lack a generally reported permeability equation. Connate water, Sw-c, and porosity, p, are typical parameters for defining productive potential of a pore fabric. Connate water is residual water held to pore walls by combinations of weak molecular and capillary forces. Unless a zone is shown to contain oil at irreducible Sw, then a means of estimating Sw-c must be used. The Bulk Volume Water parameter, BVW=p*Sw-c, can estimate Sw-c, given porosity and lithology Given is a correlation of Sw-c to pore diameter, approximately: d( microns)=0.123/Sw-c. The correlation appears valid in either carbonate or sandstone rocks. Rocks containing clay or shale were not included with the core data reviewed by this study.

The form: k (md)=10p^1.5(1/Sw-c - 1)^1.9 (if k calc exceeds 200, use 1, not 10) was developed to improve description of potentially productive zones of carbonates. Figure 1 shows a comparison of permeability equations and core data points. By definition, rocks with Sw-c of unity cannot be productive. However, most permeability estimation equations use a form of p^n/(Sw-c)^m, which yields a finite permeability at Sw-c of 1. This is not physically possible and these equations are in-accurate at high porosity, low permeability cap rocks. The proposed equation is 93% effective at predicting producible zones and 83% efficient at predicting zones with k-air less than 1 md. A unique rating system for lower productive limits, expressed in BBL/AcFt of mobile oil is given. A lower limit of between 90 and 140 is suggested for common pore types. However, it is problematic to use Sw-c and/or BVW as a correlating parameter for Vuggy carbonate facies. Mainly because a small percentage of large diameter pores barely change Sw-c, but have a large impact on permeability, with good vug alignment. Finally, it is proposed to classify the upper Ordovician oolitic limestone section of western Latvia as a leaky carbonate cap rock, which in some places is underlain by a thin (<1 m) sucrosic, potentially productive layer.