MODELING OF LOW-FREQUENCY STONELEY-WAVE PROPAGATION IN AN IRREGULAR BOREHOLE

A fast modeling method is formulated for low-frequency Stoneley-wave p ropagation in an irregular borehole. This fast modeling method provide s synthetic waveforms which include the effects of two borehole irregu larities, diameter changes (washout), and formation property changes. The essential physics of the low-frequency Stoneley waves are captured with a simple 1-D model. A mass-balance boundary condition and a prop agator matrix are used to express Stoneley-wave interactions with the borehole irregularities. The accuracy of the proposed method was confi rmed through comparison with existing finite-difference and boundary i ntegral modeling methods that yielded cross-correlations greater than 0.98, Comparison of synthetic records calculated for an actual borehol e with field records showed qualitative agreement in the major reflect ions because of the washout zones, but showed some disagreements in th e reflections caused by the fractures; Since the synthetic records inc lude only information relating to the borehole geometry and the elasti c properties of formation, the reflection caused by the fracture will appear only in the field record. These results suggest the possibility of distinguishing Stoneley-wave reflections caused by fractures from those caused by borehole irregularities. Further, the fast computation al speed of this method-over 300 times faster than either boundary int egral or finite-difference methods-makes it quite suitable for field a pplication.