高压流体注入对煤岩变形和破裂特性的影响

Effect of high-pressure fluid injection on deformation andfracture characteristics of coal

煤岩储层中孔隙压力、地应力和煤岩自身结构的耦合效应对煤体内裂纹的萌生、扩展以及破裂特性起着显著作用。以原煤为研究对象,进行了真三轴应力条件下高压流体致裂煤岩试验,以探究不同水平应力差(或中间主应力)和不同黏度流体耦合作用下煤体内裂隙扩展规律和破裂模式。得到流体压力峰值随中间主应力的增加而减小,且注水产生的流体压力峰值相比液态CO_(2)和N_(2)较大。随着中间主应力的减小,煤体破碎程度加剧。低中间主应力水平下,煤体内主要形成沿层理面及层理面附近扩展的拉伸裂隙。高中间主应力水平下,煤体内主裂隙呈斜穿层理结构的剪切破坏特性,形成了较大的煤块。孔隙压力增加过程中,裂隙扩展行为包括:①煤颗粒翻转;②高压流体作用下诱发拉伸裂隙所引起的层理平移;③偏应力作用下诱发剪切裂纹穿越层理而形成宏观剪切滑移面;④剪切裂纹在拉伸裂纹处中止扩展。因有效应力各向异性特性,煤体内孔隙压力增大使得最大偏应力也相应增大,最终导致煤体失稳破裂。基于此,提出修正裂纹滑动模型,获得了流体注入过程中裂纹密度参数随流体压力的增加而增大,随水平应力差的增加而减小,这与原煤的应变变化一致。由于水的黏度较大,裂纹密度参数在增压注水时相比注液态CO_(2)和N_(2)情况下较小,表明低黏度流体可以激活更多的孔隙裂隙。

The coupled effect of pore pressure,in-situ stress,and coal structure in coal reservoir has a significant effect on the initiation and propagation of cracks and the failure mode of coal.In this study,the coal was used as the research object,and the experiment of high-pressure fluid fracturing coal under true triaxial stress conditions was performed to investigate the crack propagation law and failure mode under the coupled action of different horizontal stress differences and fluids with different viscosities.The peak value of fluid pressure ppeak decreases with increasing intermediate principal stressσ2,and the ppeak produced by pressurized water injection is larger than that of liquid CO_(2)and N_(2).As theσ2 decreases,the degree of coal fragmentation increases.Under the lowerσ2 level,the tensile cracks extending along the bedding plane and near the bedding plane are mainly formed.Under the higherσ2 level,the main fracture in the coal shows the shear failure characteristics of diagonally crossing the bedding structure,forming a larger coal mass.Under increasing pore pressure,the fracture propagation behavior includes:①Reversal of coal particles;②Translation of bedding caused by tensile fractures under high-pressure fluid;③Formation of macroscopic shear slip plane caused by shear fractures under deviator stress;and④arrest of shear fracture propagation at tensile fractures.The coals show obvious effective stress anisotropy during injecting fluid,and the maximum deviator stress increases accordingly,resulting in the failure of the coals.The modified crack-sliding model is used to get the evolution of the crack density parameter before the coal failure.The crack density parameter increases with increasing fluid pressure under the condition of fixed far-field stress,which is consistent with the strain evolution of coal.Because of the higher viscosity of water,the crack density parameter induced by pressurized water injection is smaller than that in the case of liquid CO_(2)and N_(2)injection.Besides,the crack density parameter decreases with increasing horizontal stress difference.The modified crack-sliding model can also better characterize the stress-strain nonlinear behavior of rocks in the accelerated dilatation stage.This indicates that the fluid with low viscosity can activate more pores and fractures.