Improved Reservoir Characterization and Delineation Using Acoustic Measurements on Cores
Authors: Ajufo Austin O. , Chapman Dennis J. , Kier Jerry S. 
Published: December 2007

A new tool and technique, Porosity-Sonic-Profile, PSP-300, has been developed for better evaluation of reserves, and reservoir characterization. Continuous compressional and shear-wave acoustic transit-time and velocities are obtained on whole or slabbed cores and are related to measured petrophysical properties, avoiding reliance upon matrix velocity and grain density. In thinly-bedded intervals where vertical resolution of the logging tool is too large, (reflecting only "average porosity"), this non-destructive method provides continuous porosity. Since measurements can be performed with little or no preparation of the core material, data can be obtained in time for completions decisions. The ability to measure shear and compressional wave transit time will enhance 3-D and 4-D seismic modeling, and combined with bulk density data allows determination of the classical elastic rock properties; Poisson's ratio, Bulk, Shear and Young's Moduli.

Introduction

Advances in acoustic wave and sonic logging technology have resulted in enhanced formation evaluation capabilities. Acoustic wave velocities in reservoir rocks depend on many parameters, for example, porosity, pore fluid properties and level of saturation, lithology, clay content, pore structure and geometry and other physical characteristics. The application of acoustic and sonic wave technology in the petroleum industry has been limited to measurement of compressional and shear waves. Compressional and shear waves correspond respectively to particle vibration parallel and perpendicular to the direction of wave travel. Acoustic wave measurements in the laboratory and in the field have been used for various applications ranging from determination of porosity, interpretation and calibration of sonic Logs, identification of lithology, determination of classical elastic rock parameters, enhanced seismic interpretation, micro fracture recognition, and formation damage evaluation.

Wyllie et al. introduced the use of acoustic wave measurements for porosity determination with the "time-average" equation, which empirically relates acoustic transit time or velocity to porosity as follows:

(1)

Since this pioneering work, other algorithms have been presented for converting transit-time data to porosity. Comparisons of porosities from the time-average equation and other sources such as core analysis, have indicated that the time-average equation is overly conservative in the 5 to 25% porosity range. One of the major problems associated with the transformation of acoustic transit time or acoustic velocity into porosity values using current models is the selection of proper matrix velocity. Sandstone matrix compressional-wave velocity values can range from 17,000 ft/sec to 19,000 ft/sec, limestone velocity may range from 21,000 to 23,000 ft/sec and dolomite from 23,000 to 26,000 ft/sec. This problem is compounded in mixed lithologies. Although Nations recommended a computational method for binary rock mixtures in consolidated sandstones, limestones, and dolomites, it however assumes a constant ratio of shear wave transit time to compressional wave transit time for a "pure" rock type, and equal distribution of porosities between the two mixed rock types.

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