Identification And Estimation Of Abnormal Principal Horizontal Stresses In Isotropic Formation Using Borehole Sonic Data

Stresses are widely found in crustal rocks,and it is very important to understand state of them in engineering,such as hydraulic engineering and petroleum exploration and development.The stresses are composed of the maximum horizontal principal stress,the minimum horizontal principal stress and the vertical stress,and the vertical stress can usually be determined accurately by the density curve of well logging.Therefore,people generally focus on the magnitude and orientation of the maximum and minimum horizontal principal stress.Although the traditional formation stress detection methods have high precision,the measurement data points are few and the measurement cost is high,which can not be widely used.With the improvement of the performance of acoustic logging instruments,more and more attention has been paid to the use of array acoustic logging for abnormal stresses identification and inversion.In this paper,the waveform is simulated for the stressed istropic formation firstly.The disepersion curves from the simulated waveforms agree well with the analytic dispersion curves,which validated that the stiffness matrix of the stressed formation can be simplified as traditional one.The dispersion curve is useful in anisotropy inversion and stress inversion by acoustoelasticity theory.This paper analyzes pseudo-solutions existing in dispersion curves,and presents a reliable method to remove pseudo-solutions.The paper also validates that the cut-off frequency of flexural dispersion curve does not exist in conventional VTI formation and the cut-off frequencies of flexural and screw dispersion curves do not exist in special VTI formation.To determine stress by radial profiles of the fast and slow shear moduli,the horizontal shear moduli of the far borehole formation is needed known firstly.In this paper,the Thomsen parameters inversion of VTI formation in the absence of stress is studied,and it is considered that it is possible to obtain the wrong inversion results by using the flexural dispersion curve.Other modes must be combined to effectively invert.Then,this paper analyzes the possibility of using this method to invert the horizontal shear moduli of the stressed formation,and holds that this method can not solve this problem.In order to determine the azimuth of maximum horizontal stress,this paper studies the three methods of anisotropy inversion of HTI formation in frequency domain,eliminates the assumptions used in the traditional time domain anisotropy inversion processing methods,and presents a reliable quality indicator for inversion results.The HTI formation type is distinguished by the azimuth of the fast flexural wave in the whole frequency domain,which is more effective than the traditional methods,such as the crossover phenomenon of the fast and slow dispersion curves.At the end of this paper,three methods are studied to invert abnormal horizontal stress.In this paper,it presents the poor performance in abnormal stress inversion by using the fast and slow flexural dispersion curves.Then this paper analyzes two methods of inversion of stress and third-order elastic parameters of formation using the radial profiles of the fast and slow shear moduli,and presents the detailed derivation process and stress inversion method.It's possible to obtain the magnitude of horizontal stresses by processing the the radial profiles of the fast and slow shear moduli with elastic parameters of the formation in natural state unknown.