P-wave and S-wave response of coal rock containing gas-water with different saturation: an experimental perspective

The acoustic response of gas and/or water saturated coal rock is fundamental for establishing the correspondence between the physical properties of the coal reservoir and the characteristics of the well-logging response, which is the technology essential for the geophysical exploration of coalbed methane (CBM). This acoustic response depends on water (S_w) and gas (S_g) saturation among other factors. In this study, we performed acoustic tests on dry and different gas-water saturated coal samples with different degrees of metamorphism and deformation, collected from several coal mining areas in China. These tests enabled us to analyze the influence of coal type and gas-water saturation on the acoustic response of CBM formations. Our results show that the acoustic velocity of P-wave and S-wave (V_p and V_s, respectively), and the relative anisotropy of and V_s, increased with increasing vitrinite reflectance, density, V_p and S_w. With S_w increasing from 0 to 100%, the growth rate of the acoustic velocity decreased with increasing vitrinite reflectance. The V_p/V_s ratio of tectonic coal was generally higher than that of primary coal. The growth rate of the relative anisotropy in tectonic coal was markedly higher than that in primary coal.