CASE STUDY OF BOREHOLE SONIC DISPERSION CURVE ANALYSIS

摘要

Waveform dispersion analysis brings new understanding to borehole sonic data and its related applications. Fundamentally, the technique compares modeled acoustic dispersion curves for homogenous, isotropic formations to curves measured by borehole sonic tools. Key issues that can be identified are acoustic dipole slowness anisotropy and its cause (whether it be stress induced or intrinsic), near-wellbore alteration, and estimation of compressional slownesses in very slow formations. The technique can also be used as an excellent quality control indicator of slowness, and serve to increase confidence in borehole sonic measurements. Borehole sonic dispersion analysis can be used to identify the acoustic headwaves and modes that propagate in the wellbore environment. Most notably this includes compressional and shear headwaves in addition to the flexural, Stoneley, pseudo-Rayleigh, and LeakyP modes. The recognition of these headwaves and modes further enhances sonic log quality and validates the use of dispersion curves in related applications. This paper presents a review of the dispersion analysis technique and includes several field examples where it contributed to an increased understanding of both the acoustic measurements and the acoustic properties of the reservoir. It clearly identifies zones of near-wellbore alteration, stress-induced, and intrinsic anisotropy as well as LeakyP compressional and the various acoustic headwaves and modes. An understanding of borehole sonic dispersion analysis has applications in the areas of formation evaluation, wellbore stability, wellbore completion, and geophysical interpretation.