The effect of fluid saturation on the elastic properties of coal measure rocks under confining pressure

Coal measure gas is a hot spot for gas exploration at present, which mainly includes coalbed methane, shale gas and tight sandstone gas. A good understanding of the effect of water on the shear moduli of coal measure gas reservoir rocks, despite being essential for seismic exploration, has not yet been well studied and understood. Therefore, three major groups of coal measure rock types, including sandstone, mudstone and coal from the Qinshui Basin, are selected to investigate the effects of confining pressure and fluid on the elastic properties of the rocks. The P-wave velocities of dry coal measure rocks are more sensitive to confining pressure than the S-wave velocities, while the contrary situation occurs under saturated condition. The differential Kuster–Toksöz model can simulate the change in shear moduli caused by saturated water and can assume specific pore geometry, suggesting that the pore structure of coal measure rocks is affected by the combined effect of pressure and pore fluid. The change in shear modulus of coal measure rocks is sensitive to the confining pressure, the ratio of saturated to dry shear moduli decreases exponentially with the confining pressure. Further, the change in shear modulus after saturation is a very complex process. As confining pressure increases, coals and mudstone always display shear softening after saturation, while the shear moduli of sandstone could transit from stiffening to softening. Due to their importance in prediction of shear wave velocity, this complex variation pattern should be considered during the fracturing process.