煤岩水力裂缝扩展规律试验研究

采用大尺寸（300 mm×300 mm×300 mm）真三轴试验系统，研究了地应力、天然割理裂缝、隔层及界面性质对沁水盆地高煤阶煤岩的水力裂缝扩展行为及形态的影响。结果表明：受天然割理、裂缝影响，煤岩压裂施工压力高，波动频繁，水力裂缝扩展不稳定，形态复杂。当水平主应力差较小时，水力裂缝在多个方向起裂，延伸产生多裂缝，主要沿天然割理、裂缝方向随机扩展；随着水平主应力差的增加，水力裂缝会主要沿垂直最小水平主应力方向扩展，形态相对单一。单纯的煤层与隔层之间的物性差异（弹性模量等）对水力裂缝穿入隔层扩展的抑制作用并不显著，隔层上的垂向压应力和界面性质是决定水力裂缝能否穿层的主要因素。垂向应力小，界面胶结强度低时，摩擦因数小，水力裂缝在界面上易产生横向滑移，难以穿入隔层扩展；垂向应力大，界面胶结强度高时，摩擦因数大，水力裂缝将穿越界面进入隔层扩展。

Experimental study on propagation mechanism of complex hydraulic fracture in coal-bed

Through several groups of large scale(300 mm×300 mm×300 mm)true tri-axial tests,influences of ground stress,natural cleat or fracture,interlayer and interface’s property on hydraulic fractures’ propagation and geometry in coal samples deprived from the south of Qinshui Basin were studied.The test results show that developed natural cleat or fracture in coal-bed results in high pumping pressure with frequent fluctuation when fracturing,and hydraulic fractures’ propagation remains unsteady with complicated geometry.When horizontal stress difference is small,hydraulic fractures can start to occur along multi-direction,usually forming multi-fracture system during propagation,and hydraulic fractures can propagate along cleat and natural fractures randomly.With the horizontal stress difference increasing,hydraulic fractures tend to happen along the vertical direction of minimum horizontal stress with relative simple geometry.The function of physical property difference between coal-bed and interlayer in controlling hydraulic fractures’ interface crossing is not remarkable.However,vertical compressive stress on the interlayer and interface property are the main elements to determine whether the hydraulic fractures can cross the interface into interlayer.In the situation of low vertical stress,low interface’s cementing strength and correspondingly small friction coefficient,hydraulic fractures can glide transversally along the interface,but can not cross the interface into the interlayer.On the contrary,hydraulic fractures can easily cross the interface into interlayer.