Inverting Formation Radial Shear-wave Velocity Profile Using Dipole Flexural-wave

The formation property near the borehole is often altered by drilling mechanical collision, stress release, fluid invasion or swelling clay. As a result, the acoustic velocity near the borehole is much different from that away from the borehole. To study the speed variation near the borehole in the above condition, we have developed a novel constrained method for estimating a radial shear-wave velocity profile away from the wellbore using dipole acoustic dispersion data. This new technique could obtain velocity profile of alteration formation which we called radial variation formation. Determining the radial property changes has been proven important for assessing borehole stability, estimation formation stress, detection fluid invasion and optimizing reservoir production, etc.For the formation near the wellbore with radial property changes, we have used concentric layered structure model to simulate the radial variation of formation near the borehole. In this model, we have carried on the theoretical derivation and numerical simulation for the sound field and dispersion property using the Thomson-Haskell propagation matrix method and the Equivalent Instrument model and studied the influence of the radial variation zone on the sound field and dispersion near the borehole. We have revealed that a low-frequency dipole wave has a deep penetration depth and a high-frequency dipole wave has a shallow penetration depth by sensitivity analysis.On the basis of the two layers equivalent theory and the above dipole acoustic dispersion characteristics, we have introduced the constrained inversion method which can obtain the formation radial velocity changes. To solve the problems with continuously varying formation, we have applied fitting function to fit the two layers equivalent model and verified its validity using numerical simulation. As the dipole flexural wave dispersion curve in high frequency is missing or inaccurate in the field logging, to improve the accuracy of constrained inversion method, we have constrained high-frequency data using theory dispersion and verified the effectiveness of the improved method using numerical simulation.We have studied the application effectiveness of the method by processing some in-situ examples. First of all we provided an example of ground stress anomaly and found the radial shear-wave velocity profile is consisting with ground stress. Then we showed an example of fracture and found the difference of the radial shear-wave velocity profiles in the fast wave and the slow wave. After that we offered an example in the irregular borehole and studied the radial shear-wave profiles in different orientations using three-dimensional tomography imaging technology. In the final example we found that the variable quantity of the radial shear-wave velocity is consisting with the brittleness of rock. The fracturing has been proven by anisotropy changes of pre-fracturing and post-fracturing and the gas production has been improved after fracturing.