Processing Method And Application Of Cross-dipole Anisotropy In Frequency Domain

Cross-dipole acoustic logging is widely used to measure cicluar anisotropy in formation.In the formation with cicluar anisotropy,flexural-waves transmited by dipole source exist the phenomenon of "shear-wave splitting",namely two different flexural-waves propagating at different speeds in the propagation direction will be produced,low-frequency component of those two different flexural-waves propagate at the speed of shear-wave which exist in formation,by calculating the difference between the two flexural-waves velocity we can describe the size of anisotropy in formations quantitatively.In some cases(irregular borehole and first arrival of the flexural-wave with a big disturbance),anisotropy calculated with existed methods will have a big error,even false anisotropy appears.The parameters of anisotropy provided at this time will have a big effect on the fracture evaluation,stability analysis of the borehole wall,the construction program of well completion.Firstly,three dimensional finite difference in time domain is used to simulate the response characteristics of the flexural-wave of cross-dipole source in uniform anisotropic formation and isotropic formation whose borehole wall is collapsed in this ariticle.The result of the simulation shows that there is shear-wave splitting phenomenon in the above two cases.Especially,fake circular anisotropy's appearance in borehole irregular formation is proved with the model of isotropic formation whose borehole wall is collapsed.To overcome this problem,based on the principle of low frequency sonic wave of deep penetration,this article presents a frequency domain method to calculate anisotropy of the formation far from borehole by using lowfrequency part of the flxural-wave dispersion curves.This method is used to deal with simulated date and actual data,and we verify the superiority of the method from the comparation of it with conventional waveform matching method.The anisotropic processing has been done in casing wells before and after fracturing with this method.The result shows that this method can evaluate the fracturing effect well;further,we apply this method in irregular borehole cross-dipole data processing,and have verified its advantages in distinguishing fake anisotropy caused by irregular borehole;finally,we apply this method in the exploration of cicluar isotropy in shale gas formations,providing strong evidence for the detection of fracture which can transport hydrocarbons in shale gas formations.The results from processing of the above three kinds of actual data show that the application of this processing method for cross-dipole anisotropy which is in frequency domain in this article has been well verified.