Mechanism and Control Method of Fault Activation by Multi-Cluster Fracturing of Shale Gas

The activation of faults, fracture zones, or weak bedding surfaces (collectively referred to as faults) induced by the large-scale multi-cluster fracturing in shale gas sections in Sichuan often leads to engineering problems such as casing deformation or frac-hits. However, the mechanisms of fault activation during multi-cluster fracturing and the corresponding effective control methods remain unclear. To address this issue, a statistical analysis of multi-source field data was conducted to explore the correlation between multi-cluster fracturing and fault activation. Additionally, a model was proposed to describe fault activation induced by the non-equilibrium propagation of multi-cluster hydraulic fractures. In the process of competitive propagation of multi-cluster hydraulic fractures, when the main fractures of one or a few clusters were connected with faults, they gradually developed into dominant channels, while other clusters were inhibited or even stopped, showing strong non-equilibrium propagation characteristics. The fault gradually developed into a major fluid channel, and with the continuous injection of fluid, the internal pore pressure increased and became activated after critical stress conditions were reached. On this basis, geomechanic principles were applied, and it was further concluded that the fault activation was internally caused by faults at critical stress state with high angle development and externally caused by high operation pressure. Finally, to preventing fracturing fluid from entering the fault, this paper proposed the concept of balanced fracturing to control fault activation. In addition, some new fracturing technologies were developed, including “short segments with few clusters”, “one blockage with one drainage”, and “fracturing cluster by cluster”, etc. This achievement could provide guidance for addressing casing deformation and frac-hits of shale gas wells in Sichuan.