Numerical studies of interactions between hydraulic and natural fractures by Smooth Joint Model

The hydraulic fracturing technology is a key for enhancing the permeability of reservoirs during the production of natural gas. Significant progress has been made in hydraulic fracturing theory and practices, however, precise prediction of the fracture patterns in naturally fracture reservoirs still requires further study. The mechanisms influencing the interaction between hydraulic fracture (HF) and natural fracture (NF) should be well understood to achieve a wider application of hydraulic fracturing. In this paper, it is the first time to simulate the details of the interactions between hydraulic fractures (HFs) and natural fractures (NFs) using two-dimensional particle flow code (PFC2D) with embedded Smooth Joint Model (SJM). Firstly, the ability of SJM to emulate the natural rock joint was validated, and the fluid-mechanically coupled mechanism was introduced. Secondly, the interactions between a driven HF and a pre-existing NF were simulated considering fracturing fluid injection in a borehole. Lastly, the influence of approach angles and in-situ differential stress were studied. It is found that the model is capable of simulating the variety of interactions between HFs and NFs such as direct crossing, crossing with an offset and HFs arrested by NFs. Under high approach angles and high differential stresses, the HF tend to cross pre-existing NFs; on the contrary, a HF is more favorable to propagate along the NF and re-initiate at the weak point or the tip of NF. Moreover, our numerical results agree well with the analytical results based on the modified Renshaw and Pollards' criterion. Therefore, this numerical method may become a useful and powerful tool for optimizing fracture design in naturally fractured shale gas reservoirs.