Assessment of mud-filtrate-invasion effects on borehole acoustic logs and radial profiling of formation elastic properties

Despite continued improvements in acoustic-logging technology, sonic logs processed with industry-standard methods often remain affected by formation damage and mud-filtrate invasion. Quantitative understanding of the process of mud-filtrate invasion is necessary to identity and assess biases in the standard estimates of in-situ compressional- and shear-wave (P- and S-wave) velocities. We describe a systematic approach to quantify the effects of mud-filtrate invasion on borehole acoustic logs and introduce a new algorithm to estimate radial distributions of elastic properties away from the borehole wall. Radial saturation distributions of mud filtrate and connate formation fluids are obtained by simulating the process of mud-filtrate invasion. Subsequently, we calculate radial distributions of the elastic properties using the Biot-Gassmarm fluid-substitution model. The calculated radial distributions of formation elastic properties are used to simulate array sonic waveforms. Finally, estimated P- and S-wave velocities for homogeneous, stepwise, and multilayered formation models are compared to quantify mud-filtrate- invasion effects on sonic measurements. We use a nonlinear Gauss-Newton inversion algorithm to estimate radial distributions of formation elastic parameters in the presence of invaded zones using normalized spectral ratios of array waveform data. Inversion examples using synthetic and field data indicate that physically consistent distributions of formation elastic properties can be reconstructed from array waveform data. In turn, radial distributions of formation elastic properties can be used to construct more-realistic near-wellbore petrophysical models for applications in reservoir simulation and production.