The Research And Application On The Interaction Of Elastic Wavefield With The Fluid-filled Borehole

The interaction of an elastic wave with a fluid-filled borehole,including the modulation on the radiating,receiving and scattering wavefield,has been concerned in the theory research for VSP,crosswell seismic and acoustic far detection techniques.Sing-well acoustic imaging technique utilizes the reflection wave,reflected by the reflectors around the borehole,to image the reflectors.The logging process of far-detection technique involves the modulation for the radiation wave,the scattering and receiving effects on the incident wave from the fluid-filled borehole.Thus,studying the effects of a fluid-filled borehole on sing-well acoustic imaging technique is greatly significant.Based on the above interactions,the theory of sing-well acoustic imaging technique was investigated in detail.We first calculated the flows along and away from the borehole generated by various borehole sources,including monopole,dipole and quadrupole sources.By comparing the energy flow of radiated waves away from borehole to that of guided waves along borehole,one method evaluating the radiation efficiency of the borehole source was presented.Using this method,the frequency variation characteristics of the radiation efficiency for a multipole source in a fluid-filled borehole were studied,and the optimal excitation frequencies for the monopole,dipole and quadrupole acoustic sources were selected.By analyzing the scattered and transmission wavefield,we gave the receiving directivity factor of the borehole with an incident elastic wave.Using a steepest decent method,we obtained a far-field asymptotic solution for the scattered wavefield,and proposed a fast and effective analytical method for simulating the reflected field in the adjacent wells.On this basis,the reflection wavefield from the fluid-filled borehole was analyzed.Through comparing the reflection signal with that of formation interface,the feasibility of the dipole shear-wave imaging technology in the adjacent wells was discussed.Our analysis results show that the energy flow characteristics and radiation efficiency are strongly frequency-dependent and are distinctly different for different types of sources.In the low-frequency range below a few kilohertz,the dipole source radiation efficiency is much greater than the monopole source efficiency,explaining why the dipole source is a better low-frequency source than the monopole.For the nominal borehole size,the scattered wave amplitude was comparable to the reflection wave amplitude from a geological boundary of infinite extent.Based on this,the feasibility of the dipole shear-wave imaging technology in the adjacent wells was predicted and validated both in theory and experiment.The results can be used to provide a theoretical foundation for testing downhole acoustic imaging tools for the shear-wave imaging application.In addition,the application of Sing-well dipole acoustic imaging technology in the LWD condition was discussed.We analyzed the elastic wavefield of an LWD dipole source.Using the wavefield solution,we calculated the source-generated energy flow along the borehole and the energy flow away from the borehole into the formation.By dividing the latter flow to the former,we obtained the radiation efficiency of the LWD source.In conjunction with shear-wave imaging application,we also studied the reflection wavefield for the LWD condition.Compared with wireline logging,the LWD acoustic measurement was made with a drill collar that occupies a large part of the borehole,which significantly influenced the radiation efficiency of the dipole acoustic source mounted on the drill collar.Our analysis results show that the energy flow characteristics and radiation efficiency of the LWD source are strongly affected by the drill collar.Relative to the wireline modelling result,the radiation efficiency of the LWD source and the optimum radiation frequency band are both reduced.Apart from these marked differences between LWD and wireline conditions,our modeling shows that an LWD shear-wave reflection survey in the optimum frequency band can still detect reflection signals from reflectors in formation.Our theoretical results can be used to provide a useful reference for the design and development of an LWD dipole acoustic imaging tool.