The Research On Radiation Characteristics Of The Far Acoustic Field Outside Casing Well

Recently, the dipole single-well shear-wave imaging technique has made great improvement, and it fills the gap between the conventional acoustic well logging and the cross-well seismic on the vertical resolution and the radial investigation depth. And we have made some progress in using the dipole shear-wave reflection imaging technique to investigate the oil and gas structures alongside the borehole, and yet it has not been widely used in the cased hole. To apply the dipole single-well shear-wave imaging technique to the cased hole, we need to make some detailed research on this technique. In this paper, we compare the exact solution of the dipole radiation acoustic field and the asymptotic solution of the far field to verify the correctness of the far-field asymptotic solution. And then we use it to analyze its radiation characteristics of the dipole acoustic field of the open hole or cased hole in different formation and different acoustic source frequency, and we conclude that the formation type and the acoustic source frequency have great effect on the radiation characteristics of the dipole acoustic source. Besides, for the cased hole, we only give the far-field radiation characteristics of the SH wave of the dipole acoustic source, and analyze its rules in the cased hole of different coupling conditions. And in this paper we also give the difference of the far-field radiation characteristics of SH wave between the open hole and the cased hole, and although the energy of the SH wave radiated to the formation has reduced to some extent in the cased hole, it still can be used in the dipole shear-wave reflection imaging.The dipole single-well shear-wave imaging technique is affected by many factors, such as the radiation of the dipole acoustic source, the reception of the incident wave, the reflection of the elastic waves in the formation and so on, and therefore, it is very complicated and difficult to simulate the far-field of the dipole acoustic source. To simplify the complicated process, we have verified the reciprocity between the acoustic field radiation in the borehole and the reception in the borehole, and based on this, we can get the reception response of the incident waves by calculating the radiation factor of the dipole acoustic source in the borehole, and this greatly improve the computational efficiency. To study the correctness of the quick simulation method used in this thesis, we compare the simulation results with the Three-dimensional finite difference simulation results, and the two results fit well, and this suggests the method used in this thesis is correct and efficient. We can conclude from the simulation results that as a result of the modulation of the borehole on the acoustic field radiation and the reception, the contribution of SH wave is much greater than that of the SV wave in the dipole single-well shear-wave imaging, and this provide theoretical basis for the data processing and interpretation. And also, the simulation data and the imaging processing results prove the feasibility of the dipole shear-wave reflection imaging in the cased hole.