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Reflection of drill-string extensional waves at the bit-rock contact.
Flavio Poletto1, Massimo Malusa
1Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy. fpoletto@ogs.trieste.it
The Journal of the Acoustical Society of America
|June 27, 2002
Summary
This study estimates the bit-rock reflection coefficient for drilling. The reflection coefficient depends on formation properties and drill-string/borehole dimensions, enabling rock impedance calculation.
Area of Science:
- Geophysics
- Drilling Engineering
- Acoustics
Background:
- Downward propagating extensional waves partially reflect at the drill bit-rock interface.
- Understanding these reflections is crucial for real-time drilling data acquisition regarding formation acoustic properties.
Purpose of the Study:
- To estimate the bit-rock reflection coefficient under specific assumptions.
- To analyze the factors influencing this coefficient.
Main Methods:
- Utilizing a low-frequency approximation where the drill string acts as a laterally free rod.
- Modeling the formation as a homogeneous and isotropic medium.
- Assuming a flat drill bit in perfect contact with the formation.
Main Results:
- The reflection coefficient for extensional waves is influenced by the formation's elastic properties.
- It also depends on the ratio of the cross-sectional areas of the drill string and the borehole.
- This relationship allows for the calculation of rock impedance.
Conclusions:
- The bit-rock reflection coefficient is a key parameter for characterizing drilled formations.
- The study provides a method to calculate rock impedance from measured reflection coefficients.
- This has implications for in-situ formation evaluation during drilling operations.