随钻偶极远探测声场的辐射特性

本文对偶极横波远探测技术在随钻测井中的应用进行了探讨.对于随钻远探测而言,井中声源向井外地层中的辐射性能是它的主控因素.本文从井孔内外声场出发,考察了随钻和电缆测井中偶极声源的辐射效率,即声源产生的辐射波能流与沿井传播的导波能流之比;分析了声源的远场辐射指向性;基于声源辐射与接收之间的互易性,快速模拟和处理了随钻测井下SH和SV横波远探测波形.研究结果表明,随钻条件下SH和SV横波的最大辐射效率受钻铤的影响较电缆的情况降低了70%左右,且横波最大辐射效率对应的优势激发频率也从电缆的3.5kHz降为2.5kHz左右;虽然随钻偶极的横波辐射效率明显降低,但由理论模拟的随钻远探测数据的成像结果可知,声源辐射到地层中的横波能量仍足以探测井外反射体,表明随钻偶极远探测技术在理论上是可行的,本文结果为随钻偶极远探测测井提供了理论依据.

Radiation characteristics of the single-well imaging field in while-drilling logging using an acoustic dipole source

Sing-well dipole acoustic imaging has recently emerged as an important geophysical well-logging technology to detect geologic structures several tens of meters away from a borehole. With growing needs in the logging while-drilling (LWD) environment, the application of this technology has received much attention. Compared with wireline logging, the LWD acoustic measurement is made up of a drill collar that occupies a large part of the borehole, which significantly influences the performance of the dipole acoustic source mounted on the drill collar. Thus a systematic study is needed to analyze the radiation characteristics of the LWD dipole source for the single-well imaging application. In this study, we analyze the elastic wavefield of an LWD dipole source. The wavefield consists of guided waves along the borehole and the radiated elastic waves into the formation. Using the wavefield solution, we calculate the source-generated energy flow along the borehole and the energy flow away from the borehole into the formation. The ratio of the latter to the former defines the radiation efficiency of the LWD source. In conjunction with shear-wave imaging, we also study the radiation and reflection of SH- and SV-waves for the LWD condition. Our analysis shows that the energy flow characteristics and radiation efficiency of the LWD source are strongly affected by the drill collar. Relative to the wireline modeling result, the SH- and SV-wave radiation efficiency in LWD is reduced by 70%, and the optimum radiation frequency band is shifted to 2.5 kHz from its wireline counterpart of 3.5 kHz. 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 information. Our theoretical results can be used to provide a useful reference for the design and development of an LWD dipole acoustic imaging tool.