Abstract：The depth of investigation of resistivity images is usually several centimeters,and the ultrasonic imaging data can only provide the sound amplitude or arrival images of the borehole wall. Therefore,it is difficult to eva-luate the distribution of fractures of the borehole wall in the formation. Acoustic logging generally uses elastic anisotropy to characterize the geological stress field and fracture characteristics. Generally,the intrinsic anisotropy of rocks,high-angle strata,fractures,or unbalanced stress compression can lead to the apparent anisotropy of elastic waves. Therefore,it is often difficult to determine the cause of anisotropy. Previous researchers have conducted acoustic forward modeling for a single-azimuth two-dimensional fracture model,but the model is not sui-table for elastic wave simulation of wellbore media with multiple sets of arbitrary azimuthal fractures. To this end,we first collect the information of three types of structural features (natural fractures,bedding fractures (or thin layers),and drilling stress-induced fractures) that lead to acoustic apparent anisotropy in imaging logging of the borehole wall,quantify bedding structure information (such as dip,azimuth,etc.),and separate and group fractures. Then,the slowness of fast and slow shear waves and the anisotropic orientation of the formation are extracted from the acoustic full wave train. Next,on the basis of borehole wall imaging and modeling of rick and fracture information,the acoustic properties of the rock are simulated through the additional fracture compliance model,pseudo layer fracture model,and elastic wave theory of nonlinear stress field. Finally,by comparing the predicted model results and measured results of acoustic velocity and anisotropic orientation,the minimum error fitting inversion is used to determine the fracture types and the causes of anisotropy for characterizing borehole wall images. In addition,the characteristics of dispersion curves are compared and analyzed. In order to verify the rationality of the proposed method,the method is applied to the production test well B in a fractured gas field on a granite buried hill. The actual simulation results show that the sensitivity of wave velocity to the cause of anisotropy is not as high as that of fast shear wave orientation,the fitting error of wave velocity is generally small and easy to converge,and the fast shear wave orientation can better distinguish and judge the anisotropic types of the formation and effectively indicate the fracture types. Well B formation is divided into upper and lower zones,and the lower zone (2950~3000 m) develops pseudo layer fractures,drilling-induced fractures,and a few effective fractures,and the productivity is low,while the upper zone (2900~2940 m) develops effective fractures,and the productivity is high.
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