Study On The Law Of Crack Propagation In The North Part

The north district of Z23reservoir located in the north direct of intertidal swamp zone in He Kou, Dong Ying, Shan Dong Province. The reservoir is classical multi-thin layers which have low porosity, low permeability and many thin layers. The properties of layers and barriers are anisotropic. The exploitation of the reservoir is difficult. As a result, general fracturing and multi-staged fracturing are employed to the stimulation of the north district of Z23reservoir. Vertical fractures are formed after fracturing. The reservoir exists the problems of excessive fracture height propagation, inaccurate prediction of fracture morphology and unclear of multi-fracture interference mechanism. Therefore, it’s necessary to analyze the fracture propagation law to predict the fracture morphology so that the fracture design of the north direct of Z23reservoir can be directed to control fracture height and prevent the fractures get overlapped.Based on finite element method, the seepage-stress-damage coupling equations are deduced. The tangential and normal flow models in the fracture and the fracture initiation and propagation criterion are established based on Cohesive Zone Method. The damage factor D is used to characterize the degree of damage. Finally, the seepage-stress-damage coupling fracture propagation can be simulated. The multi-layer2D fracture dynamic propagation finite element model is established on commercial finite element software ABAQUS aiming at classical well X of the reservoir. The variation rules of fracture morphology, induced stress and multi-fracture interference are analyzed. Furthermore, the multi-layer3D fracture dynamic propagation finite element model is established. The variation rules of3D fracture morphology and multi-fracture interference are analyzed. Through the analysis of the fracture propagation law in the north district of Z23reservoir, the results and achievements are as follows:(1) Based on seepage-stress-damage coupling method, multilayer3D dynamic fracture propagation finite element model is established to precisely characterize the anisotropy of the properties of layers and barriers. Consequently, the real3D fracture morphological evolution can be analyzed.(2)Fracture morphology is mainly determined by crustal stress, tensile strength, Young’s modulus, critical energy release rate, pumping rate and fracturing fluid viscosity. Barriers with high min principle stress, high tensile strength, high critical energy release rate and low Young’s modulus and low pumping rate and fracturing fluid viscosity can control fracture height to prevent the fractures get overlapped.(3)The inter procedure of ABAQUS is modified to simulate the multi-fracture simultaneous or sequential propagation. General fracturing and multi-staged fracturing can be simulated. Based on multi-staged fracturing, the coordinates of grid nodes are adjusted to analyze the multiple fracture interference under different barrier thickness.(4)Compared with multi-staged fracturing, the interference between fractures after general fracturing is stronger, the fracture is higher and shorter and the induced stress is larger. The fractures get overlapped with each other at the early procedure after general fracturing. Multi-staged fracturing is recommended to control fracture height growth and prevent fractures getting overlapped.(5)During multi-staged fracturing, the fracture interference is larger and the fractures get overlapped while the barrier thickness is less than3.6m. On the contrary, the fracture interference is smaller and the fractures do not get overlapped while the barrier thickness is larger than5.6m. Consequently, the spacing of fracture segments can be optimized. Single staged fracturing should be implemented while the barrier thickness is less than3.6m and double staged fracturing should be implemented while the barrier thickness is larger than5.6m.A systematic research has been carried out for the fracture propagation of multi-thin layer in the north district of Z23reservoir. The research achievements gained in this paper can help to direct the optimal design of similar multi-layer reservoir.