Research on fracture propagation of hydraulic fracturing in a fractured shale reservoir using a novel CDEM-based coupled HM model

Abstract

Shale reservoirs are usually buried deep, and contain lots of complex natural fractures (NFs) under the influence of tectonic stress and faults. Previous studies usually focus on small-scale reservoir models and there is also a lack of research that completely focuses on NF characteristics. Using the continuous-discontinuous element method (CDEM), a Hydro-Mechanical (HM) coupling model was constructed to explore the propagation mechanism of hydraulic fracture (HF) and the effects of NF characteristics on HF propagation. The results show that as the HF approaches the NFs, the NFs preferentially tend to undergo shear failure, resulting in a decelerated HF propagation rate and an increased injection pressure. After water enters the NFs, the HF rapidly propagates, so the fracture area swiftly increases and the HF has a more tortuous fracture morphology. The NF orientation (α) and the internal friction angle (φ) exhibit a pronounced and distinct effect on HF propagation. Specifically, when α is 30°∼60°, the HF propagation is more tortuous and complex, so it is easier to form a larger transverse reconstruction range. When the NF density (η) is no less than 0.12, the HF tends to open the NFs and form a more tortuous morphology, but a higher η does not necessarily imply a higher fracture complexity. A longer NF (l > 12 m) benefits to improve the longitudinal reservoir reconstruction scope, but has little impact on the overall reconstruction effect. In the presence of stress shadow effect, it is improbable for the HF to completely open the intersecting NFs; instead, it typically opens only a segment or one of them. Consequently, it may be challenging to establish a fracture network with the simultaneous development of primary and branching fractures. Additionally, some water leakage into the inactive NFs gives rise to a decline in both the fracture length and fracture area. The findings can offer theoretical insights for refining the fracturing design in shale reservoirs with NFs.