Abstract
The simulation of hydraulic fracturing by the conventional ABAQUS cohesive finite element method requires a preset fracture propagation path, which restricts its application to the hydraulic fracturing simulation of a naturally fractured reservoir under full coupling. Based on the further development of a cohesive finite element, a new dual-attribute element of pore fluid/stress element and cohesive element (PFS-Cohesive) method for a rock matrix is put forward to realize the simulation of an artificial fracture propagating along the arbitrary path. The effect of a single spontaneous fracture, two intersected natural fractures, and multiple intersected spontaneous fractures on the expansion of an artificial fracture is analyzed by this method. Numerical simulation results show that the in situ stress, approaching angle between the artificial fracture and natural fracture, and natural fracture cementation strength have a significant influence on the propagation morphology of the fracture. When two intersected natural fractures exist, the second one will inhibit the propagation of artificial fractures along the small angle of the first natural fractures. Under different in situ stress differences, the length as well as aperture of the hydraulic fracture in a rock matrix increases with the development of cementation superiority of natural fractures. And with the increasing of in situ horizontal stress differences, the length of the artificial fracture in a rock matrix decreases, while the aperture increases. The numerical simulation result of the influence of a single natural fracture on the propagation of an artificial fracture is in agreement with that of the experiment, which proves the accuracy of the PFS-Cohesive FEM for simulating hydraulic fracturing in shale gas reservoirs.
Highlights
As an important unconventional fossil hydrogen energy source, shale gas reservoirs have been widely developed in recent years
Rock samples from shale gas reservoirs were scanned by microseismic monitoring technology, and the results showed that there are multi-intersected natural fractures in the rock samples [12,13,14]
Zhao et al analyzed the tendency of a propagating angle and the impacts of propagating pressure of a complex fracture network influenced by natural fractures [15]
Summary
As an important unconventional fossil hydrogen energy source, shale gas reservoirs have been widely developed in recent years. The ABAQUS cohesive finite element method is introduced to simulate the intersection of artificial fractures with natural fractures and some scholars have carried out related research. Xu et al [23] studied the impact of an approaching angle and initiation pressure on the propagation of artificial fractures and the opening of a natural fracture by cohesive element method. While intersected natural fractures are widely distributed in shale reservoirs, the effective connection of a multi-intersected natural fracture, as well as the free propagation of artificial fractures in a rock matrix, has become necessary for numerical simulation of large-scale hydraulic fracturing in shale reservoirs. A new dual-attribute element of pore fluid/stress element and cohesive element (PFS-Cohesive) method for a rock matrix is presented based on the further development of a cohesive finite element, which can realize artificial fracture propagation along the arbitrary path. The results obtained can be used to explain the expansion mechanism of artificial fractures in shale reservoirs
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