Mapping formation radial shear-wave velocity variation by a constrained inversion of borehole flexural-wave dispersion data

Abstract

We have developed a novel constrained inversion method for estimating a radial shear-wave velocity profile away from the wellbore using dipole acoustic logging data and have analyzed the effect of the radial velocity changes on dipole-flexural-wave dispersion characteristics. The inversion of the dispersion data to estimate the radial changes is inherently a nonunique problem because changing the degree of variation or the radial size of the variation zone can produce similar wave-dispersion characteristics. Nonuniqueness can be solved by developing a constrained inversion method. This is done by constraining the high-frequency portion of the model dispersion curve with another curve calculated using the near-borehole velocity. The constraint condition is based on the physical principle that a high-frequency dipole wave has a shallow penetration depth and is therefore sensitive to the near-borehole shear-wave velocity. We have validated the result of the constrained inversion with synthetic data testing. Combining the new inversion method with four-component crossed-dipole anisotropy processing obtains shear radial profiles in fast and slow shear polarization directions. In a sandstone formation, the fast and slow shear-wave profiles show substantial differences caused by the near-borehole stress field, demonstrating the ability of the technique to obtain radial and azimuthal geomechanical property changes near the wellbore.

Assessing rock brittleness and fracability from radial variation of elastic wave velocities from borehole acoustic logging

“…Because the drilling‐induced borehole cracks also affect the shear‐wave velocity, the shear‐velocity variation can be used to assess the drilling‐induced effects. The dipole flexural‐wave dispersion data are commonly used to determine the radial shear‐velocity variation (Tang and Patterson ; Su et al . ).…”

“…The underlying physics is that the low‐frequency wave penetrates deep into formation, and high‐frequency wave senses only the near‐borehole region. Thus, a constrained inversion technique is used to map the shear‐velocity variation from the flexural‐wave dispersion data (Tang and Patterson ; Su et al . ).…”

An equivalent-tool theory for acoustic logging and applications

“…Recent years have witnessed growing importance of inverting guided wave dispersion characteristics for formation acoustic parameter estimation, such as inversion of Stoneley wave for TI (transverse isotrop) estimation (Tang, 2003), inversion of dipole-flexural wave dispersion data for mapping formation radial shear velocity profile (Tang and Patterson, 2010;Sinha et al, 2005), and inversion of LWD quadrupolewave dispersion data for formation shear velocity estimation (Zheng et al, 2006), etc. In the inversion and interpretation of guided wave data from acoustic logging, the effect of an acoustic tool and its interaction with guided wave propagation have long been an important subject because of its theoretical signifi cance and relevance to practical applications.…”

Phase-based dispersion analysis for acoustic array borehole logging data

“…3 Acoustic logging tools typically measure the rate at which a pulse of sound moves across an array of receivers separated from a transmitter by an acoustic isolator. The transmitted pulse is characterized by short duration and high amplitude.…”