Dipole dispersion crossover and sonic logs in a limestone reservoir

Analyses of sonic logs in a horizontal well provide new information about m echanical properties of rocks, made possible by recent developments in our understanding of acoustic wave propagation in prestressed formations. Most sections of this horizontal well exhibit azimuthal shear isotropy, indicati ng isotropic stresses in the plane perpendicular to the well trajectory, le ading to stable wellbore conditions. However, two sections show dipole disp ersion crossovers that confirm the presence of stress-induced shear anisotr opy caused by a difference between the maximum and minimum stresses in the plane perpendicular to the well trajectory. The two dipole dispersions are obtained by processing the recorded waveforms by a modified matrix pencil a lgorithm. The fast-shear direction is estimated from Alford rotation of the cross-dipole waveforms. One section of the well exhibits the fast-shear di rection parallel to the overburden stress as the maximum stress direction, whereas the other section has the fast-shear direction parallel to the hori zontal stress that is larger than the overburden stress. The cause of this change in the fast-shear direction is believed to be the well's penetration into a 3-ft-thick bed with lower porosity and permeability and significant ly higher elastic stiffnesses than those in the other part of the homogeneo us, high-permeability reservoir. A stiff bed is likely to have greater stre sses in its plane than perpendicular to it, which would make the horizontal stresses greater than the vertical.