The Simulation And Analysis Of Krauklis Wave In Fluid-filled Fractures

The Krauklis wave is a special guided wave,propagating in fluid filled fractures,whose propagating characteristics are decided by fluid filled in fractures and fracture geometry.This guided wave can be used for volcano or earthquake forecasting,fracture reservoir characterization and hydraulic monitoring.This thesis mainly studies the numerical and physical simulation methods of Krauklis wave field,and discusses the propagation characteristics and main influencing factors of Krauklis waves.Firstly a strategy is proposed,based on which the dispersion equation is solved from high frequency to low frequency iteratively.The equation is solved within sufficiently large frequency range,and the phase and group velocities are obtained.Based on the solved solution,the dispersion,attenuation and resonance of Krauklis waves are discussed.Then a highly accurate and stable finite element scheme is proposed to simulate the Krauklis wave in fluid filled finite fracture.Under this scheme,the numerical methods for loading seismic source outside fracture simulating P-wave alone or S-wave alone and seismic source inside fracture simulating hydro-fracturing are proposed.This scheme is not only suitable for different excitation mechanisms,but also adapts to variation of frequency and fracture geometry parameters.The wave field of Krauklis waves for different surrounding rock,fluid and fracture geometry parameters,as well as the frequency and excitation mechanisms,is simulated.The influence of these factors on the propagation characteristics is analyzed.What’s more,a method for estimating velocity of Krauklis wave in fluid-filled finite fracture is proposed and the influence of the geometry parameters on the velocity is analysed.At last,a physical modeling and data processing analysis scheme for single fracture synthetics rock samples is proposed.The results of the physical modeling are compared with the numerical simulation results,based on which the corrrelation between the radiated body wave of the Krauklis wave and the coda is discussed.This paper clarifies the wave field characteristics and main influencing factors of Krauklis waves through the methods and schemes metioned above.Firstly,the analysis results of the dispersion curves show that the Krauklis wave phase and group velocity converge to the phase velocity for the surface wave along the fracture wall in the high frequency limit,and the Krauklis wave dispersion has a scale effect.These two phenomena verify the guess about formation mechanism for Krauklis wave.Secondly,the discussion of the effects of fluid filled in fracture and fracture geometry parameters on the propagation velocity,amplitude and dominant frequency of Krauklis waves shows that the energy of Krauklis wave is the strongest in gas-filled fractures,and the energy of oil-filled fractures is slightly stronger than that of water-filled fractures;the influence of fluid type on the dominant frequency of Krauklis wave signal is restricted by initiation mechanism.The fracture geometry parameters determine the Krauklis wave velocity and amplitude.The velocity increases as the fracture width increases,the velocity in rectangular fractures is higher than that in elliptical ones,and the fracture length has no effects on velocity.The amplitude of the Krauklis wave varies along fracture.The width,shape and length of the fracture can affect the amplitude distribution,and the influence on the amplitude of the Krauklis wave is restricted by initiation mechanism.Thirdly,the analysis of the dominant frequency of the source shows that when the dominant frequency of the seismic source decreases,the energy of the Krauklis wave becomes stronger and the radiation range becomes larger.When the dominant frequency is constant,the excitation method becomes the key factor in determining the dominant frequency of the Krauklis wave signal.The dominant frequency doesn’t restrict the influence of the fracture width and shape on the velocity and amplitude,but it restricts the influence on the dominant frequency.Finally,the physical modeling seismic data of single fracture synthetic rock samples is compared with the results of the numerical simulation.The analysis finds that the energy intensity and travel time of the coda signal of the physical modeling seismic data can be affected by the inclusion type in fracture,the increasing fracture length can weaken the energy of the coda,and the energy will be stronger when the fracture inclination is about 45 degrees and 60 degrees.These three points are consistent with the results of the numerical simulation.The findings and analysis conclusions above can provide a basis for subsequent Krauklis wave field research and practical applications.First of all,the excitation mechanism is the premise and basis for the discussion and application of Krauklis waves.There are differences in the influence of various factors on Krauklis waves in different excitation mechanisms.Secondly,Krauklis waves and the radiated body waves excited by seismic source inside fracture,such as sonic logging,cross-well seismic and microseismic,are more obvious.Krauklis wave provides a new way to identify and describe fractures in the borehole.Moreover,compared with P-wave source outside fracure,the phenomenon of Krauklis waves excited by S-wave source outside fracure is more obvious.Therefore,S-wave seismic exploration has a better prospect in its practical application.Finally,the fracture width and length can be extracted from the Krauklis wave and its related signals,but the fracture shape also needs to be taken into account in order to obtain a more accurate result.