燃爆载荷冲击下五峰—龙马溪组页岩断裂韧性及破坏模式

Fracture toughness and failure mode of Wufeng-Longmaxi shale under explosion load impact

燃爆压裂是页岩气储层改造增产的一种无水压裂技术,弄清燃爆载荷冲击下页岩断裂韧性及破坏模式是甲烷燃爆压裂推广应用的关键。断裂韧性是页岩储层压裂效果评价的重要指标,实现燃爆载荷冲击下试样裂纹复杂程度的定量表征以确定页岩的破坏模式。选取四川省宜宾市长宁五峰—龙马溪组一段页岩样品,基于自主搭建的甲烷燃爆冲击和SHPB试验系统开展研究,结合动态断裂理论确定了一种计算各向异性页岩动态断裂韧度的方法,并探究燃爆载荷冲击作用下页岩的断裂韧性及损伤破坏特征。结果表明:在不同燃爆加载速率下,试样动态断裂韧度随层理角度的增加呈线性增加趋势,相比静态断裂韧性显著增大,90°层理页岩试样的断裂韧度最大达到4.98 MPa·m^(1/2);随着加载速率的增大页岩动态断裂韧度显著增大,如0°层理试样在157.57 GPa·m^(1/2)/s的加载速率下的断裂韧度值为4.93 MPa·m^(1/2),而加载速率为35.43 GPa·m^(1/2)/s时的断裂韧度为1.72 MPa·m^(1/2),降低了2.9倍;同时,受页岩层理角度的影响,高加载速率下页岩试样裂纹扩展方向发生偏移转向、顺层延申和切层现象,使得裂缝网络整体结构更加复杂;在燃爆载荷冲击试验中,页岩初始起裂角为0°~70.51°,依据岩石断裂理论可知属于I-II型复合断裂类型;随着燃爆载荷由20~25 MPa增长到66~71 MPa,裂纹的尺寸和数量显著增长,且起裂角分布范围明显增加;燃爆压力达到50 MPa以上时,试样裂纹数量和偏转模式增多,伴随着岩体的剪切滑移现象,能够形成复杂树状裂纹,对页岩储层复杂裂缝网络有着正向作用。

The explosive fracturing is a waterless fracturing technology used to stimulate shale gas reservoirs for improving gas production.The investigation of the fracture toughness and failure modes of shale under explosive loads is a key for the industrial application of methane-explosion fracturing technology.Fracture toughness is widely known as a critical factor for evaluating the effectiveness of fracturing technique.The quantitative characterization of the complexity of crack networks could provide a way to quantify the failure mode of shale after explosive impact.Some shale samples were collected from the Wufeng-Longmaxi Formation in Changning,Sichuan Province,China.Experimental studies have been carried out with a self-made methane-explosion fracturing setup and the SHPB system.According to the rock fracture theory,a method has been determined to calculate the dynamic fracture toughness of shale for investigating the evolution of dynamic fracture toughness and damage patterns of shale samples.The results indicate that the dynamic fracture toughness of the samples increases linearly with the increased bedding angle under different explosion loading rates.It is much larger than the static fracture toughness.The maximum fracture toughness of the 90° bedding shale is about 4.98MPa·m~(1/2).As the loading rate increases,the dynamic fracture toughness increases significantly.When a loading rate of157.57 GPa·m~(1/2)/s is used for the shale with a 0° bedding angle,the corresponding fracture toughness is about 4.93MPa·m~(1/2),while the fracture toughness for a loading rate of 35.43 GPa·m~(1/2)/s is 1.72 MPa·m~(1/2),reducing about 2.9times.Meanwhile,due to the influence of shale bedding plane,the initial cracking direction is tended to deflect away from,extend along and cut through the bedding plane.It could cause an increase in the complexity of the crack network.All initial crack angles of the tested samples are located within the range of 0°-70.51°,which indicates a mixed I-II fracture type according to the fracture theory.As the explosion pressure increases from 20-25 MPa to 66-71 MPa,the distribution range of initial cracking angles increases by 178%.When the explosion pressure passes over 50 MPa,more cracks and connected cracking patterns are generated.The appearance of shear-slip deformation in brittle shales can create tree-like cracks,which can easily create fracture networks in shale.The laboratory results prove that the explosive impact has an advantage in producing complex fracture networks in shale reservoirs,which provides a basic support to the theoretical work of explosive fracturing.