Abstract
The shearing of natural fractures is important in the permeability enhancement of shale gas reservoirs during hydraulic fracturing treatment. In this work, the shearing mechanisms of natural fractures are analyzed using a newly proposed numerical model based on the displacement discontinuities method. The fluid-rock coupling system of the model is carefully designed to calculate the shearing of fractures. Both a single fracture and a complex fracture network are used to investigate the shear mechanisms. The investigation based on a single fracture shows that the non-ignorable shearing length of a natural fracture could be formed before the natural fracture is filled by pressurized fluid. Therefore, for the hydraulic fracturing treatment of the naturally fractured shale gas reservoirs, the shear strength of shale is generally more important than the tensile strength. The fluid-rock coupling propagation processes of a complex fracture network are simulated under different crustal stress conditions and the results agree well with those of the single fracture. The propagation processes of complex fracture network show that a smaller crustal stress difference is unfavorable to the shearing of natural fractures, but is favorable to the formation of complex fracture network.
Highlights
Shale gas production relies heavily on hydraulic fracturing stimulation due to the low permeability of shale gas reservoirs
During the fracturing treatment of shale gas reservoirs, several mechanisms can lead to permeability enhancement [1], including the propagation of hydraulic fractures (HFs), the opening of natural fractures (NFs) and the shear stimulation
We exported the fractures states when the strength of the natural fracture is assumed to be infinite, i.e., the displacement discontinuities (DDs) equals zero at the fracture parts that are not filled with fluid
Summary
Shale gas production relies heavily on hydraulic fracturing stimulation due to the low permeability of shale gas reservoirs. During the fracturing treatment of shale gas reservoirs, several mechanisms can lead to permeability enhancement [1], including the propagation of hydraulic fractures (HFs), the opening of natural fractures (NFs) and the shear stimulation. The microseismic events during fracturing treatment are generally only associated with shear fractures [2,3]. Both experiments and modelling have demonstrated that the slip on pre-existing natural fractures is important to the effectiveness of the hydraulic fracturing in shale gas reservoirs [4]. WillisRichards et al [6] presented a model in which the amount of shear displacement depends on the fracture shear stiffness and on the amount of “excess”
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