Scattered Wave Characteristics O F Acoustic Velocity Variation Near The Borehole

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 primary-wave velocity profile away from the wellbore using d acoustic logging 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.Considering that,in the field of exploration seismology,scattering methods have been used widely as basis for linearized inversion.Most inversion methods invoke the Born approximation.So,we can apply Born approximation into the borehole acoustic wave field to effectively modeling the radial velocity profile near the borehole.We derive the integral expression for the perturbed acoustic wave field and obtain an approximate analytical solution for it.Compared with exact modeling,the approximate solution gives reasonably accurate results when Born condition is satisfied,providing a fast forward model for waveform inversion of radial velocity variation using borehole acoustic data.The characteristic of the scattered waveform is significant different for the two different models.The positive and negative polarities of the scattered wave correspond to the velocity increasing and decreasing situations.Finally,we apply the scattered theory and the resulting expression to acoustic logging data inversion.Discussing inversion conditions of different objective function,different acoustic source,different velocity perturbation model.According to the inversion results obtained under different conditions,we optimal selection and apply it handle the actual acoustic data,and achieved satisfactory results.