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Abstract:
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Large volumes of natural gas exist in tight fissured reservoirs . Hydraulic fracturing is one of the main stimulating techniques to enhance recovery from these fractured reservoirs . Although hydraulic fracturing has been used for decades for the stimulation of tight gas reservoirs , a thorough understanding of the interaction between induced hydraulic fractures and natural fractures is still lacking . Recent examples of hydraulic fracture diagnostic data suggest complex , multi -stranded hydraulic fracture geometry is a common occurrence . The interaction between pre -existing natural fractures and the advancing hydraulic fracture is a key condition leading to complex fracture patterns . Large populations of natural fractures that exist in formations such as the Barnett shale are sealed by precipitated cements which could be quartz , calcite , etc . Even though there is no porosity in the sealed fractures , they may still serve as weak paths for fracture initiation and /or for diverting the path of the growing hydraulic fractures . Performing hydraulic fracture design calculations under these complex conditions requires modeling of fracture intersections and tracking fluid fronts in the network of reactivated fissures . In this dissertation , the effect of the cohesiveness of the sealed natural fractures and the intact rock toughness in hydraulic fracturing are studied . Accordingly , the role of the pre -existing fracture geometry is also investigated . The results provide some explanations for significant differences in hydraulic fracturing in naturally fractured reservoirs from non -fractured reservoirs . For the purpose of this research , an extended finite element method (XFEM ) code is developed to simulate fracture propagation , initiation and intersection . The motivation behind applying XFEM are the desire to avoid remeshing in each step of the fracture propagation , being able to consider arbitrary varying geometry of natural fractures and the insensitivity of fracture propagation to mesh geometry . New modifications are introduced into XFEM to improve stress intensity factor calculations , including fracture intersection criteria into the model and improving accuracy of the solution in near crack tip regions . The presented coupled fluid flow -fracture mechanics simulations extend available modeling efforts and provide a unified framework for evaluating fracture design parameters and their consequences . Results demonstrate that fracture pattern complexity is strongly controlled by the magnitude of in situ stress anisotropy , the rock toughness , the natural fracture cement strength , and the approach angle of the hydraulic fracture to the natural fracture . Previous studies (mostly based on frictional fault stability analysis ) have concentrated on predicting the onset of natural fracture failure . However , the use of fracture mechanics and XFEM makes it possible to evaluate the progression of fracture growth over time as fluid is diverted into the natural fractures . Analysis shows that the growing hydraulic fracture may exert enough tensile and /or shear stresses on cemented natural fractures that they may be opened or slip in advance of hydraulic fracture tip arrival , while under some conditions , natural fractures will be unaffected by the hydraulic fracture . A threshold is defined for the fracture energy of cements where , for cases below this threshold , hydraulic fractures divert into the natural fractures . The value of this threshold is calculated for different fracture set orientations . Finally , detailed pressure profile and aperture distributions at the intersection between fracture segments show the potential for difficulty in proppant transport under complex fracture propagation conditions . Whether a hydraulic fracture crosses or is arrested by a pre -existing natural fracture is controlled by shear strength and potential slippage at the fracture intersections , as well as potential debonding of sealed cracks in the near -tip region of a propagating hydraulic fracture . We introduce a new more general criterion for fracture propagation at the intersections . We present a complex hydraulic fracture pattern propagation model based on the Extended Finite Element Method as a design tool that can be used to optimize treatment parameters under complex propagation conditions . |