Hydraulic fracture extension in a naturally fractured reservoir: a numerical study using the cohesive zone method based on a geological model

To quantify how hydraulic fractures (HFs) extend in a fractured reservoir has proven challenging, in part due to the complex distribution patterns of existing natural fractures (NFs). However, current models could not describe structural characteristics of fractured reservoirs and therefore fail to reveal extensions of HFs. In this paper, a geological model for one fractured sandstone reservoir in the Ordos Basin is built based on the technology of unmanned aerial vehicle photogrammetry. Extensions of HFs in the reservoir is simulated using the cohesive zone method. The influences of pre-cut slit angles (θ) and injection schemes on HF complexity are explored. The results suggest that the length of HFs in case θ = 0° is the greatest, followed by cases θ = 45°, 135°, and 90°. It has been found that the fluid injection scheme with high rate at the early stage and low rate at the later stage is an effective way to increase the HF complexity, whereas the injection scheme with low fluid rate at the early stage and high fluid rate at the later stage strongly reduce the HF complexity. This provides valuable understanding for optimizing stimulation design in similar reservoirs. The results also suggests that, in general, the density values of hydraulic fractures in this fractured reservoir model, which is defined as the ratio of the total length of HFs to the model area, are all smaller than those in the rock matrix model, suggesting that NFs alleviate the HF complexity.