Research On Dipole Shear-wave Anisotropic Data Processing Methods And Its Applications

Cross-dipole array acoustic logging is an effective method for extracting shear-wave anisotropy of formation.It is very important for exploration and development of oil and gas industry.The anisotropy detected by cross-dipole acoustic logging can be used to detect fracture,to evaluate fracturing effect and to determine in situ stress,it can also be used to guide the seismic data inversion.In the formation with circular anisotropy,sonic signal transmitted by dipole source will be transformed to two different flexural-wave propagating at different speeds in the propagation direction,namely the phenomenon of "shear-wave splitting".Based on the difference between the two flexural-waves slowness,we can quantitatively calculate the size of anisotropy(HTI)parameters.In the waveform inversion of cross dipole anisotropy,usually an objective function is firstly constructed and then searched using a 2D global optimization algorithm to find the optimal solution of formation azimuthally shear-wave anisotropy parameters.However,the 2D search is usually inefficient and has low resolution for low-anisotropy formations.Aiming at these existed problems,this paper presents a modified method for shear-wave anisotropy inversion.Firstly,the real axis integration(RAI)and the finite difference method is studied and used for forward modeling of multiple array sonic logging.The perfectly matched layer absorbing boundary(PML)condition is also used for the finite difference to eliminate the reflection wave of the outer boundary.The former simulates the elastic wave dynamic characteristics by using the real axis integration(RAI),the latter uses the 3D finite difference in time domain to simulate the response characteristics of the flexural-wave of cross-dipole source in uniform anisotropic formation.The result of the simulation shows that there is shear-wave splitting phenomenon in this case.Secondly,this paper shows the influence factors of Alford rotating method on measurement of the azimuth and relevant correction scheme: the more difference of the transversal wave amplitudes in the two directions,the more scattered of the distribution of the azimuth calculated in the different time,and it is not easier to obtain the accurate value,in addition,response time difference affects the value of the azimuth,and has more effect on its distribution.The anisotropic objective function is constructed using waveform matching inversion method.By combining with Alford rotation and the waveform inversion method,based on empirical law,this article replaces the 2D search with two 1D searches respectively along the azimuth and anisotropy parameter axes.The 1D search method uses the Brent algorithm to optimization search the anisotropy parameters efficiently.Using the new method to process cross dipole acoustic logging data,we obtain similar results compared to those from the existing 2D method.The advantage of new method is that it has substantially improved the processing efficiency and robustness of the result.Finally,the method in this paper is used to process some field data.Cross-dipole shear anisotropy is used to analyze present stress in block,which could estimate effective cracks.This anisotropic processing has been done in tight gas wells before and after fracturing with this method.The result shows that this method can evaluate the well reconstruction effect after fracturing.Moreover,we apply this method to invert circular isotropy in shale gas formation,providing evidence for detecting fractures which can transport hydrocarbons in shale gas formation,and then preparing drilling location.Through these practical examples,the method in this paper can meet the requirements of field anisotropic data processing,which is significant to applications.