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Abstract
This thesis deals with modeling of an induced hydraulic fracture and a natural fracture in a poroelastic medium and study their interaction. A finite element based numerical model is developed for this purpose. The model integrates a wellbore, an induced hydraulic fracture, a natural fracture and a reservoir in a fully coupled manner and simulates the interaction between induced hydraulic fracture and a natural fracture. A half reservoir model is used to take advantage of symmetry. In order to have control over the entire grid and element numbering, an innovative mesh generator was developed as part of this study. Fracture propagation is modeled based on KGD fracture mechanics.
The numerical studies have shown that a natural fracture has a profound effect on the induced fracture propagation. It has been observed that in most cases the induced fracture crosses the natural fracture at high angles of approach and high differential stress. The width of the induced fracture decreases as it propagates. Once the induced fracture crosses a natural fracture and it propagates further into the formation fracture width increases. At low angles of approach and low differential stress the induced fracture is more likely to be arrested (at least short time) and/or breaks out from the far end side of the natural fracture. Results also showed that in the case of high angle of approach the hydraulic fracture always crosses the natural fracture and the differential stress has no significant effect on the propagation of hydraulic fracture. It has been also observed that propagation of induced fracture is stopped by a large (>10m) natural fracture at high angle of approach. If the injection rate, however, is increased the induced fracture crosses the large natural fracture at high angle of approach. At low angle of approach the induced fracture deviates and propagates along natural fracture. Crossing of natural fracture and/or arrest by the natural fracture is controlled by shear strength of the natural fracture, natural fracture orientation and in-situ stress state of the reservoir.
From the results of this study it has been found that this model has a potential application in the design and optimization of hydraulic fracture treatments in naturally fractured reservoirs including tight gas reservoirs and enhanced geothermal systems. The model can also be used in the design of hydraulic stimulation of naturally fractured reservoirs based on shear dilation principle.