Study On Coal Body Deformation And Ultrasonic Response At Load Of Three Axis Stress

Acoustic wave characteristics of coal rocks are the basis of seismic geophysics exploration,and developing the ultrasonic response of coal body stress and strain under loading condition is important for determining the structure and material composition of coal rock by sonic logging technology.In order to simulate the actual stress conditions of different buried coal reservoirs,a three-axis loading experiment of coal body under low confining pressure high axial pressure,high confining pressure low axial pressure and high confining pressure.Through the experiment of ultrasonic-strain response characteristics of three-axis loading with different confining pressure conditions in different directions(Parallel bedding direction,including parallel cut X,parallel end cutting,Y and vertical bedding direction Z) of the original structure anthracite in Guhanshan mine,the general law of the response characteristics of ultrasonic anisotropy in anthracite under different stress conditions is analyzed,and combined with the volume strain and the fissure ratio model in the course of three-axis loading,the variation law and structure characteristics of coal seams from shallow to deep pore are further analyzed from the characteristics of the fissure anisotropy of the coal body,this paper provides an effective theoretical basis for the characterization of the structure of coal rock for the ultrasonic response characteristics under loading condition.Studies show that:(1)The longitudinal(V_p) and transverse(V_s) wave velocities and the quality factors of the longitudinal(Q_p) and transverse(Q_s) waves in the loaded coal have regular variation and the anisotropy is enhanced.At the beginning of loading,the confining pressure is low and the longitudinal and transverse wave velocities in the parallel bedding direction are similar,As the confining pressure increases,the anisotropy increases,but the vertical and transverse velocity of the vertical bedding direction is smaller than that of the parallel bedding direction;The anisotropy of the quality factor of the longitudinal and transverse wave of coal body is obvious:The longitudinal quality factor Q_p presents the pattern of X>Y>Z,cross-wave quality factor Q_s presents the pattern of Y>X>Z.It is shown that in the case of the same composition of coal sample,the internal structure difference of the coal sample in the direction of X and Y,which is the direction of its crack,is the reason of the change of coal seam ultrasonic characteristics.(2)Under low confining pressure and high axial pressure,with the increase of axial pressure,the coal sample goes through the compaction stage and the elastic stage,the increase of V_p in X direction is significantly higher than that of Y and Z;In the low axial pressure of high confining pressure,the V_p of the three-direction coal sample is growing slowly and irlinely;V_p increases linearly under high pressure and high axial pressure.Under different confining pressure,the V_s law of the parallel bedding direction is basically the same as that of V_p:Under high confining pressure and low axial pressure,V_s of Z-direction coal-sample showed linear growth;When high-pressure high-axial pressure,V_s presents a nonlinear slow growth.Under different loading conditions,the effect of the change of the pore structure on the V_p characteristics is obvious,and the effect on V_s characteristics is more complicated,which shows that Vs has a small variation range and has a certain dispersion.(3)In the whole process of three-axis loading,the Q_p presents the X>Y>Z rule,attenuation of longitudinal wave is Z>Y>X.The Q_p increases linearly regardless of the confining pressure,but the Q_p increases linearly with the axial pressure,however,with the increase of confining pressure,Q_P and V_P’s law of change is consistent,the greater the confining pressure is,the more uniform the pressure is,and the increase of Q_P in X and Y direction is greater than that of Z direction;If the pressure is low,when the axial pressure increases,the Q_s increases linearly in the three directions;If the pressure is high,when the axial pressure increases,the Q_s in the three directions basically tends to be stable first but there is a critical axial pressure value,when the axial pressure exceeds the critical value,the Q_s increases linearly with the axial pressure.(4)Under different confining pressure,the V_p-fissure ratio of three coal samples in three directions has a good negative linear independence,the average fitting degree of V_p-fissure ratio is Z>X>Y;And the linear fitting slope of V_p-fissure ratio in three directions is X>Y>Z.The fit degree of coal sample’s V_s-fissure ratio in three directions is low,the low of average fitting degree is different from that of V_p-fissure ratio:X>Y>Z;And V_S-fissure ratio linear fitting slope law is not obvious.It is shown that the fitting degree of V_P and the fissure ratio model is higher in the loading process,therefore,the variation law of V_P is more accurate than V_s to reflect the changing characteristics of coal pore crack under different confining pressure.(5)With the increase of confining pressure,the fissure ratio tends to decrease,Q_P and Q_s increase,the influence of confining pressure on fissure ratio is greater than that of axial pressure,and the coal sample’s fissure ratio reduction of X and Y direction is similar,which is less than that of Z direction.Under different confining pressure,the coal sample’s Q_p-fissure ratio of the thre directions has a good negative linear independence,the rule of average fitting degree is consistent with that of Vp-fissure ratio:Z>X>Y;The average fitting law of Q_s-fissure ratio is not obvious.The coal sample`s linear fitting slope of Q_p-fissure ratio in three directions is X>Y>Z,which is same anisotropy as that of the Vp and Q_p;The skew rate of Q_s-fissure ratio fitting line in X and Y direction is similar,which is larger than that in Z direction,but the slope of Q_p-fissure ratio fitting line is lower under the same condition,and it is the same anisotropy regularity as the V_s Q_p.To summarize,in this paper,the changes in pore and fissure of coal and anisotropic characteristics of ultrasonic are studied under different stress conditions through ultrasound-strain testing,which provides some theoretical support to study the physical properties of coal reservoirs by geophysics method.