卸荷砂岩渐进破坏及渗透特性的中间主应力效应研究

Study of intermediate principal stress effect on progressive damage and permeability characteristics of unloading sandstone

为了探究中间主应力对卸荷砂岩渐进破坏及渗流特性的影响,基于多功能真三轴流固耦合试验系统,以含气砂岩为研究对象,进行不同中间主应力条件下的卸荷试验;得到与裂纹体应变、损伤变量相关的指数型渗透率模型。结果表明:随着中间主应力的增加,裂纹闭合应力、起裂应力、损伤应力均呈现增加趋势,弹性段与微小裂纹扩展阶段在峰前应变中的占比增加;在微小裂纹扩展阶段,中间主应力方向上的变形受到约束,对最小主应力方向产生的压缩效应随着中间主应力的增大而增强;在宏观裂纹扩展阶段,损伤应力与峰值应力比较接近,试样破坏呈脆性特征;随着中间主应力的增加,初期渗透率减少量呈下降趋势,渗透率升高点发生后移。

In order to investigate the effect of intermediate principal stress on the progressive damage and permeability characteristics of unloading sandstone,the unloading test was carried out under different intermediate principal stress conditions based on a multifunctional true triaxial fluid-solid coupling test system with gas-bearing sandstone as the research object;the exponential permeability model related to the cracked volume strain and damage variable were obtained.The results show that the crack closure stress,crack initiation stress and damage stress increase with the increase of intermediate principal stress,and the percentage of elastic section and microcrack extension stage in the pre-peak strain increases.The deformation in the direction of the intermediate principal stress is restrained in the microcrack extension stage,and the compression effect on the direction of the minimum principal stress is enhanced with the increase of the intermediate principal stress;in the macroscopic crack extension stage,the damage stress is closer to the peak stress,and the damage of the specimen is brittle in character.With the increase of the intermediate principal stress,the initial permeability reduction shows a decreasing trend and the point of elevated permeability shifts backward.