Advanced sonic characterization of near-wellbore mechanical damage and formation anisotropy yields important new information for drilling and completion engineering as well as for geophysics. Damage in shales is a precursor to wellbore stability problems. Stress-induced mechanical damage in weak reservoirs indicates zones where selective vertical perforation can reduce the risk of sanding. Identifying stress-induced anisotropy enables orienting perforations consistent with the stress direction to minimize sanding problems and optimize production; and in the case of hydraulic fracturing, to minimize near-wellbore tortuosity. Proper well placement to maximize oil recovery from hydrofracing requires knowledge of the stress directions.
Sonic cross-dipole logging with advanced frequency-domain processing provides the unique capability to characterize the mechanical state of the formation around the borehole. Traditional processing is performed in the time domain and identifies whether the formation is isotropic or anisotropic. Recently, we showed that frequency-domain processing of cross-dipole data (i.e., slowness-frequency analysis or dispersion analysis) distinguishes intrinsic from stress-induced anisotropy. The unique identification of stress-induced anisotropy also yields the stress direction. Further processing can lead to estimates of stress magnitudes. These results are obtained even in the absence of breakouts or drilling-induced fracturing, the traditional borehole stress indicators.
The near-wellbore region can be significantly perturbed by the drilling process and the associated borehole stress concentrations, resulting in radial variation of the shear slowness. Slowness-frequency analysis of cross-dipole data shows that low frequencies sample at a distance into the formation that is equal to two to three borehole diameters, sensing the unaltered rock. High frequencies penetrate a distance equal to one borehole radius, probing the mechanically damaged zone. Deviation from the homogeneous model at high frequencies indicates damage. Analysis of dipole dispersion curves yields an estimate of the depth of the mechanical damaged zone.
Worldwide (Middle East, Africa, Gulf of Mexico, Southeast Asia) field examples in both overburden shales and reservoir rocks (sands and carbonates) highlight the additional information derived from cross-dipole dispersion curves. Dispersion analysis of these data sets enables identification of: 1) stress-induced anisotropy, stress direction, and stress magnitudes; and 2) near-wellbore mechanical damage in both sands and shales.
The key to this new understanding is the combination of wide-band acquisition of crossed dipole sonic data, advanced frequency-domain processing techniques, and good knowledge of acoustic wave propagation in boreholes.