循环荷载下深部煤层工作面顶板砂岩的渗透率演化规律

随着技术的进步以及浅部矿产资源的枯竭,深部矿产资源开发与利用将成为常态。深部岩石的物理力学性质复杂,往往处于循环加卸载的应力条件。深部地下水的压力较大,是深部煤矿的重大安全隐患之一。顶板砂岩是工程最为常见的岩石种类之一,研究其力学性质及渗透规律对深部空间开发利用及矿产资源开采等都具有重要意义。以埋深约1050m的平煤12矿顶板砂岩为研究对象,采用循环加卸载声发射渗透实验对其渗透率演化规律进行研究。从应力-应变及损伤特征、耗散能密度占比、累计声发射事件数的增长速度、不同围压条件下的破裂面特征4个方面进行分析,总结了循环荷载下深部工作面顶板砂岩不同应力阶段的渗透率演化特征。实验结果表明:循环应力对深部顶板砂岩的作用可以分为压密作用与压裂作用2种机制,应力水平较低时主要起压密作用,应力水平增大到屈服强度的60%以上时则表现为压裂作用。岩样的渗透率在应力的压密作用下降低,在应力的压裂作用下升高。逐级增大的循环应力作用下,岩样的损伤以及耗散能密度占比均表现为先因压密作用减小,再由压裂作用而增大的演化规律,并且两者与渗透率演化呈正相关关系。整个实验过程中围压对岩样起压密作用,且随着围压的增大,渗透率减小的程度更大。顶板砂岩的破坏形式对破坏时的渗透率具有显著影响。岩样的破裂面角度随着围压增大而减小,岩样破坏时产生的贯通裂隙有轴向与横向2种形式,产生轴向贯通裂隙时的渗透率远大于岩样的初始渗透率,而产生横向贯通裂隙时的渗透率变化较小。综合5个岩样的渗透率演化情况,得到岩石渗透率在逐级增大的循环荷载下具有4个明显的阶段特征。渗透率在较低应力的循环中因压密作用减小;随着循环应力的增大,在压裂及损伤作用下增大;在应力达到岩样抗压强度发生破坏时因破裂面的产生骤增;破坏后因大幅下降的应力的压密作用再次降低。

Permeability evolution of roof sandstone at deep coal seam working face under cyclic loading

With the progress of technology and the depletion of shallow mineral resources,the development and utilization of deep mineral resources will become reality.The deep rocks have complex physical and mechanics properties,and they are often in cycle loading stress condition.The larger pressure of deep groundwater is one of the main safety hazards in deep coal mines.As one of the most common rock types,it is significant to study the mechanical properties and permeability evolution of the roof sandstone for the exploitation of deep mineral resources.In this paper,the roof sandstone of Pingmei 12th Coal Mine with a depth of about 1 050 m has been investigated.The sandstone permeability evolution under stress is researched by cyclic loading acoustic emission (AE) permeability experiment.Based on the stress-strain and damage characteristics,dissipative energy density ratio,the growth rate of accumulated acoustic emission events,the features of fracture surface under different confining pressures,the permeability evolution of the roof sandstone under cycle loading has been summarized.Experimental results show that the effect of stress on rock can be divided into compaction and fracturing.The low stress mainly acts as compaction.The stress,which increases to more than 60% of the yield strength,mainly acts as fracturing.Permeability decreases under compaction of stress and increases under fracturing of stress.The dissipative energy density ratio and damage variable D of the rock sample decreases under compaction and increases under fracturing,which are both positively correlated with permeability evolution.During the whole experiment,the confining pressure acts as a compaction on the rock sample,and as the confining pressure increases,the permeability decreases to a greater extent.The failure mode of the roof sandstone has a significant effect on the permeability at the time of failure.The rupture angle of rock sample decreases with the increase of confining pressure.When the rock sample is destroyed,there are two forms of axial through crack and transverse through crack.When the axial through crack is generated,the permeability will be much larger than the initial permeability.The permeability of the transverse through crack will not change significantly.Based on the permeability evolution of five rock samples,rock permeability has four distinct phase characteristics under stepwise cyclic loading and unloading.The permeability decreases by compaction in the cycle of low stress.With the increase of cyclic stress,the permeability will increase under the fracture and damage.The permeability increases sharply,with the fracture face generated.The stress is greatly reduced after the failure,and the permeability decreases again due to the compaction.