Determination of Primary and Secondary Porosity in Carbonate Formations Using Acoustic Data

摘要

A technique for quantitative estimation of crack or vugporosity using acoustic well-log data is proposed. Theestimation is based on a double-porosity rock model thatconsists of a solid matrix with primary porosity and inclusionswhich represent the secondary pore system. We applied themethod of effective medium approximation (EMA) tocalculate P- and S-wave velocities in the double-porosityformations. This method allows modeling multicomponentsystems composed by grains of different mineral components,primary pores and secondary inclusions. All these componentsare approximated by spheroids with different aspect ratios.The technique includes two steps. At the first step the matrixP- and S-wave velocities as functions of the primary porosityare calculated. For this procedure two groups of parametersare required: 1 - solid-skeleton elastic moduli that are definedby laboratory measurements or calculated using formationmineralogical composition; 2 - aspect ratios for grains andprimary pores. We determined these aspect ratios by theinversion of the published regression equations for P- and Swavevelocities to the microstructure parameters.At the second step, using the obtained previously matrixparameters, the value and aspect ratio of secondary pores aredetermined by the joint inversion of P- and S-velocities(minimizing the difference between the experimental data andvelocities predicted for the double-porosity model). Theobtained aspect ratio indicates the type of secondary-poresystem (cracks or vugs).Examples of secondary-porosity determination using acousticcore and well log data are presented.