The effect of different gases on the ultrasonic response of coal
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Copyright © 2011 Society of Exploration Geophysicists.
To gauge the potential of seismic methods for the estimation of gas content in coal, the ultrasonic response of a sample saturated in turn with He, N2, CH4, and CO2 has been investigated. Specifically, traveltimes were used to determine P-wave velocity as a function of the difference between confining pressure and pore pressure. After crushing the sample to powder, adsorption isotherms for CO2 and CH4 were measured and then used to estimate the bulk densities, P-wave moduli, and impedances during the traveltime measurements. The data suggest a significant difference in density, P-wave modulus, and impedance under CO2 relative to CH4 saturation. Though these findings are based on the assumption that adsorption capacity of the sample when confined was similar to that measured after crushing, they are also roughly supported by isostatic strain measurements taken during swelling. Two possible causes of this behavior are, first, the mechanical properties of the adsorbed phase may be more liquid than gaslike. Second, the swelling of coal under confining pressure should lead to the closure of soft pores, thus stiffening the frame.
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