Study On The Propagation Variation Of Artificial Fractures Under The Interference Of Natural Fractures In Coal Shale

Complex fracture structures which contained artificial fractures and its connected natural fractures are the core to improve the production of the shale gas.Different propagation modes are formed by artificial fractures propagation which are significantly disturbed by natural fractures.Effective stress inside the rock is the physical quantity to react the geostress and the pore pressure.Based on this relationship,this thesis combined the theoretical derivation with the laboratory test.Firstly,the initiation pressure formula was derived based on the mechanism of initiation pressure considering effective stress.Secondly,considering effective stress in the test,the hydraulic fracturing test was conducted,and the injection method at a low water pressure was used before the fracturing stage.Two factors of water injection pressure and geostress were determined to explore the artificial fractures propagation mode under the interference of the natural fractures.Clear water was used as the fracturing fluid and the acoustic emission was used to receive the information of fractures during the test.By analysing on the pressure curve,the theoretical value and the test value of the initiation pressure were compared.By analysing on the frequency domain and time domain of the acoustic emission,the criterion of the cracks enter the propagation stage from the initiation stage was given.According to different acoustic emission characteristics,a criterion of artificial fracture propagation mode under the interference of natural fractures was proposed.The following results were obtained.1.The theoretical analysis shows that with the increase of the wellbore azimuth and the borehole inclination,there are different stress dominant stages in the wellbore.The variation degree of initiation pressure is obviously affected by the minimum horizontal in-situ stress.Penetration,obstruction and turning of artificial fractures propagating along the crack surface occur under the interference of natural fractures.The main fracture modes are as follows:The single transverse fracture,simple fracture propagation model of the branch and the transverse fractures,the complex fracture propagation model of the"T"shape fracture and the"?"shape fracture.2.The results of analysis on acoustic emission frequency domain shows that micro-cracks are formed in the effective stress transformation stage,and the macro-cracks are formed in the fracturing stage.With the interference of natural fractures,considering the interaction of the water injection pressure and the geostress,it is easy to form the single transverse fracture mode when the main frequency is 60KHz-110KHz in the effective stress transformation stage and when the main frequency is 20KHz-25KHz in the fracturing stage.It is easy to form the complex fracture propagation model of the"T"shape fracture and the"?"shape fracture when the main frequency is 130KHz-135KHz in the effective stress transformation stage and when the main frequency is 14KHz-25KHz in the fracturing stage.3.In the test,the optimal water injection pressure is 3.0MPa and the optimal geostress is?_v=5.5 MPa,_H?=4.3MPa,?_h=3.7 MPa.The average of the acoustic emission energy is defined as energy rate k.With the interference of natural fractures and the interaction of the water injection pressure and the geostress,the decreasing amplitude of k is more than 35.09%.When the artificial fractures enter the propagation stage from the initiation stage and the decreasing amplitude of k is 40%-42.47%,it is easy to form the complex fracture propagation model of the"T"shape fracture and the"?"shape fracture.