Dynamic Geomechanical Modeling of the Induced Microseismicity During Hydraulic Fracturing

Abstract

The activation of natural fractures and associated microseismicity generation and radiation during hydraulic fracturing treatments are dynamic processes. However, most of the current hydraulic fracturing models are based on a quasi-static framework. Then, how significant are the dynamic stress perturbations during hydraulic fracturing treatments? Can they induce the activation of the horizontal bedding planes (BPs), which could be the source of some specific patterns of microseismic events? What are the characteristics and the predominant frequencies of the induced microseismic signals during hydraulic fracturing? How is the geometry (i.e., orientation and length) of the induced microseismic clouds correlated with that of the hydraulic fracture (HF)? We apply a dynamic finite element geomechanics method to address these important questions. We compare the dynamic and static stress perturbations and find the dynamic stress perturbations could cause more instability around a propagating HF. BPs could be more easily activated when the HF crosses them by a short distance compared with when the HF approaches them but is still a short distance away. Fracturing fluid penetration into BPs could weaken the BPs and facilitate the activation. The rupture propagates bilaterally along the BPs at different speeds. The study on the induced microseismicity during hydraulic fracturing in a fractured reservoir indicates that rupture patterns along the natural fractures (NFs) affect the signal spectrum. The spectrum could either have multiple predominant frequencies or be relatively flat over the investigated frequency range. Injection rate doesn’t affect the predominant frequencies obviously. A higher Young’s modulus could shift the predominant frequency higher. The correlation between the geometry of the hydraulic fracture and the induced microseismic cloud depends on the inclination of the NFs with respect to the maximum horizontal principal stress direction. When the inclination is either high or low, not so many MS events would be generated, and they are close to the HF but quite asymmetric about the HF. The MS cloud has small discrepancy with the HF in length but large discrepancy in strike. It is the opposite when the NF inclination is nearly optimal.