Building the Wireline Database and Calculation of Reservoir Porosity
November 2004 Presentation in PDF Format
Integrated Geological and Engineering Characterization of
Fullerton Clear Fork Field in Andrews County, Texas
Reservoir Petrophysics

Accurate determinations of petrophysical properties in carbonate reservoirs (i.e., porosity, permeability, and saturation) require detailed knowledge of pore types. Traditional methods of estimating hydrocarbon reserves are also sensitive to variations in pore types. However, neutron and density wireline logging devices are assumed to indicate total porosity. In reservoirs in which moldic or separate-vug porosity exists, a distinction between moldic and interparticle porosity is especially important for accurate determination of petrophysical properties.

Conventionally, the sonic (acoustic) log is chosen to provide information on the volume of moldic porosity. However, because the sonic tool is commonly considered to be of secondary importance, acoustic porosity logs are uncommon in many data sets. Resistivity logs can also provide valuable information on the volume of moldic porosity in much the same way sonic logs do. Accordingly, a well-constrained methodology for determining moldic pore volume from resistivity logs offers great potential for improving the accuracy of reservoir-wide petrophysical calculations.

Such a methodology has been developed for the Fullerton Clear Fork reservoir. By combining a bulk volume water calculation derived from the Archie equation and a ratio water saturation computation, porosity can be calculated exclusively from resistivity logs. Comparisons of resistivity porosity calculations with thin-section point-count data of pore type abundance from cores and calculations of moldic versus interparticle pore volume made from combination porosity logging suites (sonic, neutron, and density) indicate that this resistivity log method is a good measure of interparticle porosity. This technique allows the user to get a more complete picture of the porosity and fluid distribution throughout the reservoir.

For more information, please contact Steve Ruppel, principal investigator. Telephone 512-471-2965; e-mail
January 2004