Description: Energies, Vol. 10, Pages 1619: Effect of Injection Site on Fault Activation and Seismicity during Hydraulic Fracturing Energies doi: 10.3390/en10101619 Authors: Zhaohui Chong Xuehua Li Xiangyu Chen Hydraulic fracturing is a key technology to stimulate oil and gas wells to increase production in shale reservoirs with low permeability. Generally, the stimulated reservoir volume is performed based on pre-existing natural fractures (NF). Hydraulic fracturing in shale reservoirs with large natural fractures (i.e., faults) often results in fault activation and seismicity. In this paper, a coupled hydro-mechanical model was employed to investigate the effects of injection site on fault activation and seismicity. A moment tensor method was used to evaluate the magnitude and affected areas of seismic events. The micro-parameters of the proposed model were calibrated through analytical solutions of the interaction between hydraulic fractures (HF) and the fault. The results indicated that the slip displacement and activation range of the fault first decreased, then remained stable with the increase in the distance between the injection hole and the fault (Lif). In the scenario of the shortest Lif (Lif = 10 m), the b-value—which represents the proportion of frequency of small events in comparison with large events—reached its maximum value, and the magnitude of concentrated seismic events were in the range of −3.5 to −1.5. The frequency of seismic events containing only one crack was the lowest, and that of seismic events containing more than ten cracks was the highest. The interaction between the injection-induced stress disturbance and fault slip was gentle when Lif was longer than the critical distance (Lif = 40–50 m). The results may help optimize the fracturing treatment designs during hydraulic fracturing.

Description: Hydraulic fracturing is an important method to enhance permeability in oil and gas exploitation projects and weaken hard roofs of coal seams to reduce dynamic disasters, for example, rock burst. It is necessary to fully understand the mechanism of the initiation, propagation, and coalescence of hydraulic fracture network (HFN) caused by fluid flow in rock formations. In this study, a coupled hydro-mechanical model was built based on synthetic rock mass (SRM) method to investigate the effects of natural fracture (NF) density on HFN propagation. Firstly, the geometrical structures of NF obtained from borehole images at the field scale were applied to the model. Secondly, the micro-parameters of the proposed model were validated against the interaction between NF and hydraulic fracture (HF) in physical experiments. Finally, a series of numerical simulations were performed to study the mechanism of HFN propagation. In addition, confining pressure ratio (CPR) and injection rate were also taken into consideration. The results suggested that the increase of NF density drives the growth of stimulated reservoir volume (SRV), concentration area of injection pressure (CAIP), and the number of cracks caused by NF. The number of tensile cracks caused by rock matrix decrease gradually with the increase of NF density, and the number of shear cracks caused by rock matrix are almost immune to the change of NF density. The propagation orientation of HFN and the breakdown pressure in rock formations are mainly controlled by CPR. Different injection rates would result in a relatively big difference in the gradient of injection pressure, but this difference would be gradually narrowed with the increase of NF density. Natural fracture density is the key factor that influences the percentages of different crack types in HFN, regardless of the value of CPR and injection rate. The proposed model may help predict HFN propagation and optimize fracturing treatment designs in fractured rock formations.