圆弧片状声源向地层中辐射声场的有限差分模拟

提出了一种可用于随钻声波测井的非对称的圆弧片状声源,旨在发展一种能够在钻井过程中精确确定反射体位置的三维反射声波远探测方法。研究了在复杂的井孔条件(钻铤偏心、各向异性和不规则井孔形状)下,该声源向地层中辐射的声场的特征,进一步分析了不同井况条件对三维随钻远探测声波测井的影响。数值模拟结果表明,在圆形井孔、钻铤居中条件下,无论快速地层还是慢速地层,圆弧片状声源均能够向地层中定向辐射声波信号,其辐射指向性图主瓣3 dB角宽窄,旁瓣级低,方位分辨率较高,适用于随钻三维反射成像测井;井壁粗糙程度、地层各向异性等因素对声场特征影响不大,说明该方法适用于粗糙井壁和各向异性地层情况;钻铤偏心和井壁一侧的破坏对辐射声场的影响较大。对本文的模型而言,在破坏深度小于3 cm或者偏心距离小于2 cm时,声场仍有较好的方位特征,该方法仍然适用。而当破坏深度大于6 cm,声场指向性图出现多个幅值较大的角瓣,可能无法利用其确定反射体的方位。本文的研究结果为三维随钻反射声波仪的设计和研发提供了必备的理论基础。 

A finite difference modeling of acoustic wave field in the near-borehole formation excited by an arcuate source

We propose an acoustic logging-while-drilling (LWD) arcuate source, aiming to develop a three-dimensional acoustic reflection LWD tool, which is able to accurately locate tile reflectors outside of the fluid-filled borehole. The paper examines the acoustic field produced by the arcuate source under complex borehole conditions (collar eccentricity, anisotropic formations and irregular borehole), and discusses the influence factors of the method. Numerical results show that, the source is able to produce compressional wave field of good azimuthal resolution for most cases. In a circular borehole, the source is able to directional radiate compressional waves regardless of that the formation is fast or slow, and the directivity pattern of the compressional wave field includes a main lobe with a narrow 3 dB angle width and side lobes of low level. It suggests that the source is suitable for acoustic reflection LWD. The factors, including the formation anisotropy and the roughness of borehole sidewall, have little influence on the azimuthal characteristic of the compressional wave field. The tool eccentricity and the wellbore sidewall damage caused by the drilling operation greatly affect the compressional wave field. The acoustic field is of good azimuthal resolution when the damage depth is less than 3 cm and the eccentric distance l < 2 cm, whereas several side lobes of high degree appear in the radiation pattern when the damage depth is larger than 6 cm. The presented results provide necessary theoretical fundamentals for the development of the three-dimensional acoustic reflection LWD tool.