清水钻井液作用下煤岩声学特性试验研究

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

为了研究钻井过程中煤岩性质变化,测量并分析了不同围压下煤岩浸泡前后纵波、横波速度与首波幅值特征。试验结果表明:煤岩的纵波速度及首波幅值随浸泡时间增长而增大,横波速度及首波幅值随浸泡时间增长而减小;煤岩浸泡清水钻井液后弹性模量变小、泊松比变大,且随浸泡时间增长而愈加明显;煤岩纵波、横波速度与首波幅值均随围压增大而增大;浸泡后纵波速度及首波幅值明显变大,浸泡前后的速度差值随围压的增大而减小,而首波幅值差值随围压的增大而增大;浸泡后横波速度及首波幅值均小于浸泡前,且二者的差值随围压的增大而增大。根据煤岩失效结构、吸水量及弹性参数变化特征,分析认为:钻井液渗入煤岩层理面,其润滑及支撑作用会降低层理面结构强度,导致煤岩沿层理弱面破裂失效。

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.