Estimating Permeability from Porosity in the Alabama Ferry Field—The Rock Fabric Approach

By

Lucia, F. Jerry

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Characterizing fluid flow in carbonate reservoirs is difficult because of the robust mixture of pore types found in carbonates. The pore types that contribute the most to permeability are interparticle and touching-vug. The permeability from interparticle porosity is related to pore-size distribution, which can be linked to rock-fabric descriptions. Separate vugs may add to flow capacity but their contribution has been shown to be small. Core analysis and wireline logs measure total porosity, which in most carbonates combines interparticle and separate-vug porosity. In order to predict permeability from porosity, core and log measurements must be separated into interparticle and separate-vug porosity.

Samples from the Alabama Ferry Field were studied in an attempt to quantify the relationship between separate-vug porosity, intergrain porosity, and permeability. Thin sections were made from 28 perm-plugs of skeletal/ooid grainstone with varying amounts of grain molds and intragrain microporosity. The porosity and permeability data do not plot in the grainstone field of Lucia using total porosity. However, when interparticle porosity, determined by subtracting separate-vug porosity from total porosity, is used the data plots in the grainstone field. The scatter does not decrease, however, and this is attributed to the sampling bias of the thin sections. MRI scans of the perm-plugs show that porosity due to large pores is highly variable within the perm-plugs.