Abstract
To identify the lithology of coal seam roof and explore the influence of these roofs on the enrichment of coalbed methane, low-frequency rock petrophysics experiments, seismic analyses and gas-bearing trend analyses were performed. The results show that the sound wave propagation speed in rock at seismic frequencies was lower than that at ultrasound frequencies. Additionally, the P-wave velocities of gritstone, fine sandstone, argillaceous siltstone and mudstone were 1,651 m/s, 2,840 m/s, 3,191 m/s and 4,214 m/s, respectively. The surface properties of the coal seam roofs were extracted through 3D seismic wave impedance inversion. The theoretical P-wave impedance was calculated after the tested P-wave velocity was determined. By matching the theoretical P-wave impedance of the four types of rocks with that of the coal seam roofs, we identified the lithology of the roofs. By analyzing known borehole data, we found that the identified lithology was consistent with that revealed by the data. By comparing and analyzing the coal seam roof lithology and the gas-bearing trends in the study area, we discovered that the coal seam roof lithology was related to the enrichment of coalbed methane. In the study area, areas with high gas contents mainly coincided with roof zones composed of mudstone and argillaceous siltstone, and those with low gas contents were mainly associated with fine sandstone roof areas. Thus, highly compact areas of coal seam roof are favorable for the formation and preservation of coalbed methane.
Highlights
Coalbed methane exists in coal rock, and coalbed methane enrichment remains a difficult research topic
By matching the tested velocity with 3D seismic data, we identified the coal seam roof lithology to explore the influence of the roofs on the enrichment of coalbed methane
By comparing the propagation velocity of sound waves in the four types of rocks, we found that both the P-wave and S-wave velocities successively increased at different frequencies
Summary
Coalbed methane exists in coal rock, and coalbed methane enrichment remains a difficult research topic. The theories regarding the propagation velocity of sound waves in rock at seismic frequencies have not been supported by experimental data, some have described the velocity and attenuation of sound waves in fluid-saturated rock at seismic frequencies (Ba, 2010; White, 1975; Biot, 1956). These theoretical models are mainly based on experimental data at ultrasonic frequencies (MHz). The propagation velocity of sound waves in four types of rocks was measured and studied using a low-frequency stress-strain method. We hope that this study will improve studies of the propagation velocity of sound waves at low frequencies and the enrichment trends of coalbed methane (Hungerford, 2013; Li & Wu, 2016; Liu, 2018)
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