Prediction of P- and S-wave logs for seismic modeling
- Birkbeck College, London (United Kingdom); and others
The Xu-White clay-sand mixture model simulates the effects of porosity, clay content, water saturation and invasion fluids on sonic wave velocities. Its predictions compare well with observed P- and S-wave sonic logs. P-wave logs can be predicted from a suite of logs including at least {gamma}-ray, density and deep resistivity logs. The model provides three ways of predicting S-wave velocity: (1) from porosity and shale volume, (2) from the P-wave sonic log and shale volume, and (3) from the P-wave sonic log and porosity. Comparison of the results from these methods not only helps improve the prediction but also provides a method of quality control. This paper illustrates application of the model in improving the tie between a well-log synthetic seismogram and seismic data at a well. It is well known that the S-wave log from a full waveform tool is unreliable over soft-formation intervals where the S-wave velocity is lower than the P-wave velocity of the drilling mud. Comparison between the predicted and measured S-wave logs demonstrates that the full-waveform S-wave transit time is severely underestimated in soft shale formations. The V{sub p}/V{sub s} is then underestimated, which was found to significantly affect seismic modelling. The effects of water saturation and mud filtrate invasion were investigated. Comparisons between the measurements and our predictions show that the fit between measured P-wave sonic log and the prediction assuming a 100% mud filtrate saturation is better than that when water saturation has been considered, implying that the propagation paths of the measured P- and S-wave velocities are largely in the flushed zone. The well tie was considerably improved after editing of the sonic log guided by the predicted P-wave sonic and model-based correction of the log for the effect of fluid content. Use of the predicted S-wave sonic for the soft shale formation gave more diagnostic modelling of the pre-stack seismic data.
- OSTI ID:
- 542875
- Report Number(s):
- CONF-951013--
- Country of Publication:
- United States
- Language:
- English
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