Research On Shale Hydraulic Fracturing And Microseismic Release Mechanism

The study of the shale initiation,fracture propagation and fracture network under the water pressure can provide theoretical support for shale gas reservoir reconstruction.The microseismic monitoring can evaluate the law of fracture propagation and provide technical support for reservoir reconstruction effect evaluation.The mechanism of fracture propagation and microseismic release are complex,so it is difficult to have a unified understanding in theory,which led to a lot of controversy among domestic and foreign scholars.Therefore,based on the traditional fracture mechanics and microseismic theory,the physical simulation experiments and numerical analysis methods was used to study the microseismic mechanism and laws of shale hydraulic fracture propagation.It is of great practical significance to study shale performance,evaluate of shale gas reservoir reform,optimize fracturing construction parameters and to improve the recovery rate.The main contents and achievements of this study are as followed:(1)The basic theories of hydraulic fracturing and microseismic releasing were summarized,and the criterion of rock fracture under pore water pressure was studied.Based on elastic-plasticity mechanics and fluid-solid coupling theory,a mathematical model of horizontal well and perforation was established,and the direction,pressure and propagation of perforation initiation were analyzed.It is found that the fracture pressure in the direction of the maximum principal stress is the lowest and it is easier to crack.(2)A large shale triaxial hydraulic fracturing and acoustic emission monitoring test for shale was designed.It is showed that the direction of main hydraulic crack is perpendicular to minimum principal stress.Hydraulic fractures bifurcation,steering and fracture network are related to the weak structure planes and internal micro-fractures.The hydraulic fractures formed by the repeated fracturing technique can effectively connect to form the fracture network.(3)The acoustic emission signals monitored in the test are characterized by double main frequency bands.The main frequency signals are generated by macroscopic fracture and steering,and the micro-fracture mainly produce low dominant frequency signals.The low main frequency ratio of the shale hydraulic fractures is very high,and the energy release is dominated by low main frequency.(4)Judging from the initial motion of acoustic emission signals,the internal micro-fracture caused by shale hydraulic fracturing are mainly tensile failure and shear failure.Shear failure is also found in some aspects,such as rock edge,fracture turning,bifurcation and natural fracture.By the clustering of the value of Rising time/Amplitude(RA)and Average Frequency(AF),we can roughly predict that there are obvious fractures.The smaller the stress difference in the hydraulic fracturing process,the more complex and uneven the stress conditions,the more shear failures will occur.(5)The Extended Finite Element Method was used to simulate shale hydraulic fracturing process.The hydraulic fracture direction is perpendicular to the direction of the minimum principal stress,and it is easy to expand in the weak structural layer.Some cracks farther from the wellbore extend through the weak structural layer along the original direction.There is larger displacement in the center of fractures,and the form of "middle" fracture is shear failure,and the form of "tip" failure is tensile failure.The inhomogeneity of shale may cause complex failure under hydraulic press.(6)The implicit dynamic analysis was used to simulate the microseismic releasing process.The particle vibration velocity near the borehole is much higher than that in other areas.With the increase of time,the velocity of the source decreases rapidly.The frequency and energy of the signal released from the initiation crack of shale reach the largest quantity.Acoustic scattering and boundary effects have a great influence on micro-vibration.The waves easily propagate along the direction of the defect,and energy attenuation through the defect signal is large.The microseismic waveforms near the defects have the characteristics of high energy,high main frequency,fast energy attenuation and frequency offset.