Influence mechanisms of plasticity and horizontal stress difference on the fracture propagation in plastic reservoir rocks: a 3D XFEM-based plastic fracturing model

In deep unconventional oil and gas reservoir rocks, the degree of plasticity and in situ horizontal stress difference (HSD) significantly influences the behavior of hydraulic fracture propagation in plastic rocks. In this study, a three-dimensional (3D) extended finite element method (XFEM)-based model of plastic rocks for hydraulic fracture propagation was established. The concrete damage plasticity model was employed to determine the plastic characteristics of rocks in the 3D XFEM-based model. Spatial free propagation of hydraulic fractures in plastic rocks was realized using the stress field distribution near the fracture tip. The energy principle was employed to quantify the degree of plasticity in reservoir rocks. The effects of the degree of plasticity and in situ HSD on fracture spatial morphology and breakdown pressure were analyzed, and the interaction mechanisms between plasticity and in situ HSD were revealed. The results demonstrate that plasticity significantly inhibits fracture propagation. The rocks with a lager plasticity have a higher breakdown pressure and facilitate complex and wider fractures involving smaller extension ranges. Plasticity evidently reinforces the effect of in situ HSD on breakdown pressure and shadows the influence of in situ HSD on hydraulic fracture spatial morphology. This implies that plasticity weakens the contribution of in situ HSD to reservoir stimulation. This study reveals the fracture propagation mechanisms of hydraulic fracturing in plastic reservoir rocks, which provide a theoretical guidance for effectively implementing hydraulic fracturing technology and improving the recovery of deep unconventional oil and gas resources.