Investigations of seismicity induced by hydraulic fracturing in naturally fractured reservoirs based on moment tensors

In oil and gas production, natural fracture (NF) activation and the seismicities induced by hydraulic fracturing can cause unpredictable damage and even catastrophic consequences to reservoirs, which seriously threatens the safety of construction personnel and equipment. In this paper, the seismicities induced by hydraulic fracturing in naturally fractured reservoirs were investigated based on the moment tensor. Firstly, an algorithm for evaluating seismicities was generated using an independently developed subroutine. Then, a systematic simulation was implemented to explore the characteristics of seismic events (SEs) produced by the interaction of hydraulic fractures (HFs) and natural fractures. Finally, a sensitivity analysis was performed to examine the effects of the stress difference, approaching angle (theta), and friction coefficient on seismicities under different working conditions. The results show that there are four modes of interaction between hydraulic fractures and natural fractures. In the Type II mode, an NF can combine with other NFs to form fracture networks, however SEs produced in this mode are the most dangerous. In the Type IV mode, SEs are fewer and smaller in magnitude compared to those in the Type II mode. Additionally, in the Type IV mode, HFs can also cross NFs and thus become interconnected with other NFs. When theta <= 30 degrees, the stress difference becomes the main factor controlling the seismicities. Meanwhile, when 45 degrees <= theta <= 60 degrees, the stress difference and the approaching angle jointly affect the interaction modes. The NF friction coefficient also plays a role in SEs at this stage, however its effect is not prominent. When theta >= 75 degrees, the modes produced by the HF-NF interactions belong to Type IV or are in a transition to Type IV.