Using geophysical logs to estimate porosity, water resistivity, and intrinsic permeability

Abstract

Geophysical logs can be used to estimate porosity, formation-water resistivity, and intrinsic permeability for geohydrologic investigations, especially in areas where measurements of these geohydrologic properties are not available. The dual density and neutron porosity logs plus the gammaray log can be used to determine in-situ porosity and to qualitatively define lithology. Either a spontaneous-potential log or a resistivity log can be used to estimate relative resistivity of the formation water. The spontaneous-potential and the cross-plot methods were tested for their usefulness as estimators of resistivity of the formation water. The spontaneous-potential method uses measurements of spontaneous potential and mud-filtrate resistivity to estimate the formation-water resistivity. The crossplot method uses porosity values and observed resistivity of saturated rock to estimate the formation-water resistivity. Neither method was an accurate estimator. However, in areas of no data the methods can be used with caution. A review of the literature of the basic relations for formation factor (F), porosity (n), and cementation factor (m) implies that the empirical Archie equation is applicable to carbonates, coarse-grained elastics, and uniformly fractured media. The relation off, n, and m to intrinsic permeability (k) was also investigated. Merging the well-known Archie equation with the Kozeny equation establishes an equation for estimating intrinsic permeability. The resulting equation implies that intrinsic permeability is the function of a medium factor, (GISS 2), and a porosity factor -P, [nm + 2/(1-n) 2]. Where G is the Kozeny coefficient, 5S is the specific surface of grains per unit of volume of solids. Unfortunately, G and Ss values generally are impossible to determine from wireline geophysical logs. A plot of the porosity factor as a function of intrinsic permeability defined the following empirical equation for k, in millidarcies: /c=1.828x105 (-P1 ' 10), where .p= This equation for k was based on a selected data base of 10 sets, which included in-place measurements of permeability, in-place measurements of porosity from two different types of porosity logs, and a measurement of bulk resistivity of the rock and water from a resistivity log. The equation has a coefficient of determination (r2 ) of 0.90. The relation is applicable to permeable rocks in which surface conductance along grains is not dominant, such as most carbonate rocks, fractured rocks, and coarse elastics. Calculated permeability values for different lithologies using typical values of porosity and cementation factors compare well with typical permeability values of the different lithologies. Some data were available to support the equation, but considerably more data will be needed to better test the equation established for k.