Understanding Stress Effects on Borehole Acoustic Waves for Unconventional Shale Reservoirs

Acoustic velocities in many sedimentary rocks exhibit stress sensitivity. This behaviour has been validated through experiments, observed from field measurements, and is described by the acoustoelastic model. Inversion methods based on this model have been developed to characterize stresses, and provide the basis for non-destructive means to calibrate stress profiles. Observation of borehole sonic dipole dispersion cross-over signatures serve as an indicator of stress-indcued anisotropy – an effect that has been validated theoretically through 3D numerical modeling. Such modeling has been carried out for sandstone and cabonate rock and less so for shale rock. To understand the stress effects on sonic measurements in wells traversing unconventional reservoirs, we carry out simulations of the borehole sonic measurement in shale formations subjected to subsurface stresses. To this end, we have developed and used a new 3D modeling code based on the finite-difference time domain scheme in a cylindrical coordinate borehole system. The linear and nonlinear elastic constants of shale core samples from laboratory experiments are used as inputs to the modeling. Synthetic waveforms are processed using a modified matrix pencil algorithm to estimate the borehole sonic mode dispersions and their sensitivities to the stress-induced anisotropy. For a vertical well, our modeling results demonstrate new dispersion signatures associated with certain shale formations. The borehole flexural dispersions at the two canonical horizontal stress directions split at high frequencies whereas they overlay at low frequencies. The split at high frequencies is caused by near-wellbore stress concentrations and the overlay at low frequencies is owing to the typical shale laminated lithology. The modelled dispersion signatures were also observed from processing of field data acquired with both sonic and ultrasonic tools in a vertical well in a laminated unconventional shale formation. The ultrasonic tool measures compressional and shear slownesses azimuthally at radial depth of about 1 in. from the borehole surface. The presence of imbalanced stresses is confirmed in adjacent intervals from symmetric breakouts. In a 20-ft interval not exhibiting breakouts but surrounded by intervals with breakouts, the ultrasonic tool also measures the compressional and shear slownesses with an azimuthal quasi-sinusoidal variation caused by stress concentrations around the borehole. On the other hand, sonic waveforms recorded by cross-dipole measurements in the same interval show high-frequency splitting dispersions as reproduced by the modeling. Taken together, these results confirm the existence of a new sonic dipole signature caused by the subsurface stresses in vertical wells traversing unconventional shale reservoirs.