Research Of Hydraulic Fracturing Dynamic Propagation In Fractured Reservoirs

Large volumes of oil and natural gas exist in low permeable fractured reservoirs. Hydraulic fracturing is one of the main stimulating techniques to enhance recovery from this kind of reservoir.In recent years,in the process of development and research of this kind of reservoirs,people find the hydraulic fracture of hydraulic fracturing well in fractured reservoirs is not a single plane fracture generated in homogeneous reservoirs but complex,nonplanar,multi-stranded hydraulic fracture.Ofen,there is a positive correlation between complexity of the hydraulic fracture and the production. A large number of hydraulic fracture diagnostic date show that the interaction between pre-existing natural fracture and the advancing hydraulic fracture is a key condition leading to complex fracture patterns.The hydraulic fracture simulation in heterogeneous reservoirs such as fractured reservoi-rs requires modeling of fracture intersections,the change of hydraulic pressure after meeting with natural fractures,diverting of a hydraulic fracture and so on.Using the classical theory of the hydraulic fracturing cannot adapt to these heterogeneous reservoirs. Some related theor-y and research are also not so mature,a thorough understanding of mechanisms of interaction between a hydraulic fracture and a natural fracture is still lacking.Further research on the hydraulic fracture dynamic propagation in fractured reservoirs is needed to promote the dev-elopment of expolitation and theoretical study system of similar reservoir.This paper forms a systematic research method and idea for studying the hydraulic fracture dynamic propagati-on in fractured reservoirs with the interaction between a hydraulic fracture and a natural fracture as its core.(1)According to the virtual work principle, the week form of hydraulic fracturing equilibrium equation is derived in this paper,which provides the basis theory to simulate the hydraulic fracture by the extended finite element method(XFEM).(2)According to the correlation theories of the extended finite element method, the displacement pattern after meeting with a natural fracture is proposed.The displacement pattern provides the basis theory to establish the extended finite element method model to simulate the interaction between a hydraulic fracture and a natural fracture.(3)Start with possible paths of a hydraulic fracture’s extension,the extension of a hydraulic fracture is classified into extension in bedrock and extension after meeting with a natural fracture, the maximum circumferential stress criterion and the maximum energy release rate criterion are adopt to judge the orientation of extension, the criterion for a hydraulic fracture’s extension in the fractured reservoirs is formed.(4)According to the correlation theories of fluid mechanics,considering fluid exchange between a hydraulic fracture and a natural fracture,the model of hydraulic pressure distribution in a hydraulic fracture after/before meeting with a natural fracture at different hydraulic fracturing parameters,properties of the fracturing liquid,the natural fracture approaching angles and in-situ stresses is created.The equivalent node force loading model of hydraulic fracturing extend finite element is established to form the force vector of entire equilibrium equations.(5)A method to track a hydraulic fracture and judge element types is presented,and then the implementation of enrichment of different types of elements is derivated.Different types of elements are subdivided by subdomain integration algorithm, this paper gives the gauss point strategy and the computing methods of the element stiffness matrix.Based on the theories and methods mentioned above,the factors of the propagation of hydraulic fractures in fractured reservoirs are analyzed. A natural fracture may be opened and sheared before meeting with a hydraulic fracture because of disturbance stress caused by the hydraulic fracture, meanwhile, a hydraulic fracture is also attracted by a natural fracture and turn to the natural fracture. A hydraulic fracture is more likely attracted by a natural fracture when the in-situ stress difference is small,on the contrary,the hydraulic fracture extends in the direction of the original.A hydraulic fracture propagates easily at high pumping rate because of the high net pressure. A hydraulic fracture is more likely to form a disorderly because of its deflexion when the viscosity of fracture fluid is low.A hydraulic fracture is most likely to cross a natural fracture at high approach angle,and diverts into the natural fracture at low approach angle. There is a fracture energy threshold of a cemented natural fracture for a hydraulic fracture to divert into at different approach angles, the threshold values decrease with approach angle’s increase.