MEASUREMENT OF ELASTIC WAVE VELOCITIES IN COAL SAMPLES OF DIFFERENT RANK

Resumen

Laboratory experiment were carried out to measure: 1)compression (Vp) and shear wave (Vs) velocities in coal sampleswith different ranks (or vitrinite reflectance) from low rank to high rank coal, and 2) to measure Vp and Vs on coal underdifferent fluid saturation conditions including methane and water.  The coal samples were subjected to a range of confiningpressures and pore pressures that resulted in effective pressures in the range 0 to 3000 psi. Measurements wereperformed on: 1) fresh coal samples, under gas pore pressure 2) dry samples under gas pore pressure, and 3) watersaturated samples, under water pore pressure.   The gas used to pressurize the pore system was methane.    Coal samplesused in the experiments were: Four samples with vitrinite reflectance values (Ro) between 0.7 and 1.52%, from theBogotá Basin, and two low rank samples (with Ro < 0.5%) from the Powder River, and Washakie basins of Wyoming.The acoustic experiments on coal indicate that both compressional and shear wave velocities (Vp and Vs) are verysensitive to effective pressure, increasing with it. Most of the samples show a clear trend of increases for Vp witheffective pressure. Also, the Vp velocity increases with pore pressure, and become less sensible to effective pressure.The shear wave velocity also increases with effective pressure; however, Vs velocity does not show a clear trend withpore pressure. It was observed that both Vp and Vs velocities increase with coal rank (vitrinite reflectance), where thelow rank coal shows lower velocities than the higher rank coal. This trend is related also to coal porosity.   Ourexperiments should be useful indicators of the saturation stage in coalbed methane exploration. In addition these experimentaldata provide useful data for AVO (amplitude versus offset) modeling, and interpretation of seismic surveys carried outfor coalbed Methane exploration.

Key words: compression wave velocity (Vp); shear wave velocities (Vs)