煤岩吸水率对声波速度各向异性影响的实验研究

Experimental research on effects of water absorption on acoustic velocity anisotropy in coal rock

在煤岩超声波响应规律研究中,较少地考虑煤岩吸水率及各向异性对纵、横波速度的影响,这些问题对煤岩物性超声测试具有重要意义。为此,选取平煤八矿己_(16-17)煤层煤样,制备垂直层理(Z方向)、平行层理垂直面割理(Y方向)和平行层理垂直端割理(X方向)三类煤样,进行煤岩吸水和超声测试。实验结果表明:(1)在X、Y、Z方向,同一煤层在相同条件下吸附水的能力存在明显的各向异性,其中Y方向自然吸水率大于X、Z方向。(2)煤样超声响应具有明显的各向异性特征,其中Z方向的波速最大,Y方向的波速最小。(3)随着煤样自然吸水率的不断增加,纵波速度明显呈近似指数变化的非线性增长,在煤柱含水饱和度较小(低于60%)时,纵波速度随含水饱和度变化幅度较小;在含水饱和度大于60%(对应X、Y、Z方向的自然吸水率分别为1.3%、2.65%、0.72%)时,纵波速度增幅明显变大。(4)随着煤样自然吸水率的不断增加,横波速度也不断增大,但横波速度变化幅度远小于纵波速度;煤样纵、横波速度比随吸水率的增加基本呈先减小、后增大的现象。(5)分别利用空间平均模型和时间平均模型分析了孔隙流体对煤体超声波参数的影响,其中纵波对于煤体孔隙流体十分敏感,孔隙流体对横波速度几乎没有影响,可以利用这一性质探测煤岩中流体的存在和运动。

In the research of coal rock ultrasonic wave response characteristics,the influences of water absorption and anisotropy on longitudinal wave and transverse wave velocity are lack of comprehensive consideration.However,these issues have great significance for ultrasonic technology in coal physical characteristics tests.In our experiment,J_(16-17) seams in the Pingdingshan No.8 Mine are selected,the vertical-bedding samples(Z-direction),the par-allel-bedding and vertical-face cleat samples(Y-direction),the parallel-bedding and vertical-butt cleat samples(X-direction)are used for water absorption and ultrasonic experiments.The following experimental observations are obtained:(1) Under the same conditions the water absorption for the same coal seam in different directions are anisotropic,the Y-direction samples are greater than the X-direction and Z-direction samples;(2)The anisotropy of P-wave and S-wave velocity in coal layers is obvious,the maximum velocity is in the Z direction,and the minimum velocity is in the Y direction;(3)P-wave velocity shows approximately exponential variation with water content increase.In early stages,when the water saturation is less than 60%,the change of water absorption in coal is ignored,which may minimally affect P-wave velocity.When the water saturation is higher than 60%(corresponding to W_X=1.3%,W_Y =2.65%,W_Z=0.72%),P-wave velocity increases greatly in perpendicular to bedding and parallel bedding coal;(4)S-wave velocity increases with coal natural water absorption.But this change is much less than Pwave.The velocity ratio of P-/S-waves in coal samples decreases first and then increases with water absorption rate;(5)The influence mechanism of pore fluid on ultrasonic parameters is analyzed with the space average model and the time average model,P-wave is very sensitive to pore fluid in coals,but S-wave velocity can be ignored.This property can be used to detect the presence and movement of fluid in coal.