Applications of sonic waves in the estimation of petrophysical, geophysical and geomechanical properties of subsurface rocks

Sonic waves play an important role in estimating rock properties that are crucial in an efficient and safe production of oil and gas wells. An acoustic source in a fluid-filled borehole can generate both nondispersive headwaves as well as relatively stronger borehole guided modes. Processing of waveforms recorded with adequate spatial sampling yields sonic velocities in the surrounding formation over the receiver aperture. These velocities are then transformed into elastic moduli of the propagating medium. Elastic moduli of the formation provide many useful petrophysical, geophysical and geomechanical attributes of porous rocks that constitute hydrocarbon bearing formations. Petrophysical attributes of hydrocarbon bearing formations include porosity, pore pressure, and fluid mobility. Geophysical attributes of the formation deal with anisotropy characterization of formations on a seismic scale. Geomechanical properties of rock consists of estimating in-situ formation stresses and strengths as a function of radial position away from the borehole surface. Compressional velocity through a porous rock has been used to estimate porosity using Wyllie time-average equation whereby an interval transit time is decomposed into transit times in the solid and fluid components of the composite structure. Rock porosity can then be estimated using compressional velocity of the rock matrix and pore fluid in conjunction with measured velocity in the composite structure. There are well established correlations that help identify formation lithology in terms of the compressional to shear velocity ratio or the Poisson's ratio of the material. Plots of compressional to shear velocity ratio against compressional transit time help identify intervals containing limestone, dolomite, salt and quartz. Recent applications of elastic moduli of rocks in a reasonably uniform lithology are in the estimation of fluid mobility in porous rocks; formation stresses; and fracture characterization. The presence of a fluid-filled borehole in a tectonically stressed formation causes both radial and azimuthal heterogeneities in rock stresses. Formation stresses are estimated using an acoustoelastic model based on nonlinear continuum mechanics. This model predicts crossing dipole dispersions to be an indicator of stress differential in the borehole cross-sectional plane. In-situ rock strength can be estimated using radial variations of shear velocities obtained from the inversion of borehole dispersions. Estimates of rock stresses and strength help maintain wellbore stability during drilling and production.