The complexity of the fracture network in failure rock under cyclic loading and its characteristics in acoustic emission monitoring

The damage inside rock caused by its stress history has an important influence on the rock failure process. In underground gas storage engineering, the gas injection and extraction process damages the reservoir. The results of microseism monitoring show that many microseism events occurred during the injection and extraction of the gas, although the working pressure did not reach to the failure strength of the reservoir. Damage can also be done by cyclic injection hydraulic fracturing technology, where the working pressure reaches the maximum value of the previous loop; many microseism events can be monitored, and the final stimulated reservoir volume improved greatly. In this paper, we focus on the effect of stress history on rock failure, based on the stress curve response and the acoustic emission (AE) response obtained from a cyclic loading experiment on the tight sandstone bed. Combining the strain-stress curves with the AE Kaiser or Felicity effect, the rock damage characteristic and mechanism are researched. It is concluded that after equal value cyclic loading more local fractures can be formed during a core's final rupture. Also, the rupture path changes, the main rupture is suppressed and the number of fractures increases. Multistage cyclic loading can also change the rupture mode, but the number of local fractures is smaller than the number generated after an equal value cyclic loading action. The damage inside the rock caused by historical stress action can be divided into crack damage and plastic damage; crack damage can be obtained by AE monitoring, and plastic damage can be monitored by both AE and the stress-strain curve. The plastic damage which was stored in the stress history affects the tendency of the main fracture to be derived and increases the local rupture.