Numerical Simulation Study Of Hydraulic Fracture Propagation In Shale And Tight Reservoirs

Shale gas has rich and widely distributed reserves but with low abundance.It relies on hydraulic fracturing to carry out commercial exploitation.Shale generally contains lots of natural fractures,resulting in that it's difficult to control the propagation direction of hydraulic fracture and to estimate the scope of volume fracturing,thus having an impact on the accurate evaluation of its capacity and fracture optimization design.Conventional fracture model is not applicable to the shale and tight reservoirs anymore,so it is necessary to establish a set of fracture network propagation theory and method for such reservoirs.In this study,the main research contents and methods are listed as follows.First of all,the interaction model between hydraulic fracture and natural fracture is established with consideration of fluid pressure,and interaction curve between them is constructed.Numerical test of shale cracking with preset natural fractures has been carried out by using particle flow model,and the influence mechanism of natural fractures on hydraulic fracture propagation is revealed.Secondly,the numerical method to simulate hydraulic fracturing of shale based on extended finite element method is constructed.Mathematical model coupling rock deformation and fluid flow is established.Stress filed of rock mass is solved using extended finite element method and fluid pressure field is solved by traditional finite element method.The implicit algorithm is constructed based on fracture tip asymptotical solutions.Numerical cases of fracture non planar extension,hydraulic fracture propagation with effect of natural fractures and simultaneous propagation of multi-cluster fractures have been conducted and the influences of rock mechanical parameters,heterogeneity,fracturing parameters on fracture propagation have been analyzed.Thirdly,the numerical method to simulate fracture network propagation of shale based on displacement discontinuity method is constructed.The model of segmented multi-cluster hydraulic fracturing of horizontal well with consideration of stress shadow is established and implicit iterative algorithm based on fracture tip asymptotical solutions is constructed.The influences of fracture spacing,stress difference on propagation path,length and flow rate of fracture have been analyzed.Based on discrete fracture network,the model of hydraulic fracturing of naturally fractured reservoir is established and the distribution of fracture network is obtained by simulation.The influences of natural fracture property and distribution on the morphology of fracture network are analyzed.The model of simultaneous fracturing of multi-well in the mode of multi-well pads and factory drilling is established and the influences of fracture spacing,well spacing on hydraulic fracture are analyzed.Finally,the theory of fracturing optimal design of shale reservoirs based on stress interference and gas flow interference is established.Based on the model of hydraulic fracture propagation of shale,the fracture induced stresses are calculated and the influence of fracture spacing on stress reversal region is analyzed.The hybrid model of shale gas flow with consideration of stimulated reservoir volume is established.The model is solved by finite element method and the influence of fracture parameters on shale gas production capacity is analyzed.Summing up,a set of method to simulate fracture propagation and optimize fracturing design of shale reservoirs is developed in this dissertation,which provides a theoretical basis and technical support for the efficient development of shale and tight reservoirs.