Radial profiling of shear slowness from borehole acoustic measurements acquired in thinly laminated formations

Because of their relatively shallow volume of investigation, borehole acoustic measurements can be affected by abnormal near-wellbore conditions such as irregular calliper, drilling-induced formation damage and mud-filtrate invasion, among others. Additionally, borehole-slowness measurements inherently deliver rock elastic properties spatially averaged across the length of the multi-receiver array included in the waveform acquisition system. The consequence is that the interpretation of borehole acoustic measurements needs to account for both radial variations of elastic properties and axial spatial averaging effects across thinly laminated formations before conducting seismic-well log ties and rock physics interpretations. We introduce an inversion-based interpretation method to estimate radial shear-slowness variations from frequency-dependent slownesses in vertical wells penetrating horizontally layered formations. The inversion procedure is efficiently implemented with an optimized two-dimensional fast-forward-modelling method that simulates borehole acoustic modes in the presence of invaded and thinly laminated formations. Furthermore, the inversion method consists of two sequential steps: Firstly, layer-by-layer dispersion slownesses are processed to mitigate axial spatial averaging effects on borehole acoustic measurements; secondly, radial variations of elastic properties are estimated from inversion results obtained from the first step. The new inversion-based interpretation workflow estimates a layer-by-layer single-radial-step shear slowness model, including altered and virgin radial zones. By considering both radial and axial averaging effects, the implementation of the two-step inversion-based interpretation method in thinly laminated and radially invaded formations consistently improves the definition of shear slowness between invaded and virgin radial zones compared to traditional radial-profiling methods. The accuracy of the estimated shear slowness heavily relies on the sensitivity of dispersion modes to radial variations in formation shear slowness.