循环冲击载荷作用下页岩动力学响应及能量耗散特征

采用$varnothing $50 mm分离式霍普金森杆(split Hopkinson pressure bar,SHPB)实验系统开展页岩循环冲击实验,研究不同循环冲击载荷作用下页岩动力学响应及损伤演化特征,同时揭示了控制入射总能量不变条件下,不同气压梯度循环冲击页岩能量演化规律。随着冲击气压升高,试样破裂所需的冲击次数呈线性减少,峰值应力随循环冲击次数的增加先升高后降低,极限应变先减小后增大,试样在循环冲击下表现出先压密后损伤的力学机制。基于Weibull分布的统计损伤模型表明,升高循环冲击气压,试样损伤破坏形式由缓慢劣化逐渐转变为骤然破坏。入射总能量恒定的情况下,通过控制循环入射能量梯度能够产生不同的损伤效果,降压冲击和升压冲击下的能量吸收比均大于恒压冲击下的,且气压梯度的绝对值与能量吸收比呈现正相关性。

Dynamic response and energy dissipating characteristics of shale under cyclic impact loadings

The formation of complex fracture networks in the shale subjected to cyclic impact loading is an important scientific problem for water-free fracturing technologies of shale reservoirs, such as explosive fracturing and high-energy gas fracturing. Two cyclic impact experiments based on a split Hopkinson pressure bar (SHPB) system were conducted on the freshly exposed black mud shale taken from the Wufeng Formation-Longmaxi Formation in Changning County, Sichuan Province, to investigate the kinetic response and damage evolution characteristics of the shale under different cyclic impact gas pressure and different cyclic impact gas pressure gradients, respectively, and to reveal the energy evolution law of the cyclic impact shale using different impact gas pressure gradients under the condition of controlling the constant total incident energy. The main conclusions are as follows. With the increase in impact pressure, the number of impacts required to rupture the specimen decreases, and the fragmentation and peak stress increase. The specimen undergoes cyclic impact showing the mechanical response characteristics of compaction first and then gradual damage. The damage degree of the shale specimens during cyclic impact was calculated by a dynamic damage model based on the Weibull distribution, and the results show that the damage of the specimen gradually changes from slow deterioration to sudden damage by increasing the cyclic impact pressure. Different cyclic impact experiments with different impact gas pressure gradients were conducted. The results show that under the condition of constant total incident energy, different cyclic incident energy gradients could produce different damage effects, and the energy absorption ratio of the negative or positive gas pressure gradient of cycle impact is greater than that of the zero ones. The absolute value of the pressure gradient shows a positive correlation with the energy absorption ratio. It indicates that under the condition of constant total impact energy, increasing the absolute value of the cyclic impact gradient can produce a better damage effect. The findings of the shale cyclic impact experiments can provide theoretical support for the technological design of multi-stage pulsed high-energy-gas-fracturing.