Experimental study on coal and rock acoustic characteristics under the role of water drilling fluid

In order to study the coal and rock properties variation during the drilling process, a measurement and analysis was conducted on the longitudinal wave and transverse waves velocities and the first amplitude characteristics before and after the immersion time of the coal and rock under different surrounding pressure. The test results showed that, the longitudinal wave velocity of the coal and rock and the first wave amplitude would be increased while immersion time increased and the transverse wave velocity and the first wave amplitude would be decreased with immersion time increased.The elastic modulus after the coal and rock immersed in the water drilling fluid would be smaller, the Poisson ratio would be larger and would be obviously with the immersion time increased. The longitudinal wave velocity, transverse wave velocity and first wave amplitude value of the coal and rock would be increased with the surrounding pressure increased. The longitudinal wave velocity and first wave amplitude value after the immersion would be obviously increased. Before and after the immersion, the velocity difference would be decreased with the surrounding pressure increased. The difference value of the first wave amplitude value would be increased with the surrounding pressure increased. After the immersion, the longitudinal wave velocity and the first wave amplitude value would all be smaller than before the immersion. The difference value between the two would be increased with the surrounding pressure increased. According to the variation features of the coal and rock failure structure, the water absorption volume and elastic parameter, the analysis held that the drilling fluid would be penetrated in the joints of the coal and rock and the lubrication and support role would reduce the structure strength of the joints and cause the cracking failure of the coal and rock along the joints.