VTI地层随钻四极子声波测井数值研究

数值计算了四极子声源在含钻铤竖直向横向各向同性(VTI)地层井孔中产生的各种模式波的频散曲线、激发幅度以及对各地层、钻铤参数的灵敏度,合成了四极子阵列波形,研究了快、慢速地层井孔中地层各向异性的存在对四极子波场的影响.数值计算结果表明,地层各向异性对钻铤波的影响很小,对于地层波的影响较大,且影响因素非常复杂.仅在一些特殊的频率点处,地层模式波的控制因素相对较为简单,如螺旋波的截止频率处.在慢速地层井孔中,最低阶的地层四极子波对距离井壁约2个井孔半径内的地层有明显的响应,可以对该范围内的地层横波进行层析成像.在快速地层井孔条件下,可以考虑采用与横波到时相同的波包来评价地层横波信息.该波包包含了井孔折射横波、F1和F2模式与井孔折射横波速度相接近的部分;利用该波包获得的地层横波速度基本不受地层各向异性的影响.在慢速地层情况下,螺旋波受地层各向异性的影响较大,建议在实际数据处理时,考虑采用基于数据的处理方法进行频散校正.

Numerical study on quadrupole acoustic LWD in VTI formations

Acoustic logging while drilling(LWD)is able to provide real-time acoustic properties of rock formations near bits, and therefore has an incomparable advantage over acoustic wireline logging(WL)in saving wellbore occupation time and avoiding risks. Current quadrupole acoustic LWD is capable of measuring formation shear velocity in arbitrary rock formations. However, the quadrupole acoustic LWD theory is not complete yet, i.e., there is no clear understanding on some fundamentals, which required further study.We derive the dispersion equations for the waveguide consisting of a fluid-filled borehole surrounded by a VTI formation, and numerically calculate the dispersion, amplitude and sensitivity functions of the borehole modes excited by a quadrupole acoustic source. We also synthesize the array waveforms recorded by the receivers in the borehole. The influences of formation anisotropy on borehole quadrupole modes are investigated.Numerical results show that, the formation anisotropy has negligible effects on the collar modes, while it strongly affects the formation modes. Only at a few frequencies, the control factor is simple, such as at the cut-off frequency of screw waves. The formation quadrupole modes of the lowest order show significant response to the formation shear velocity within approximate 2 borehole radii away from the sidewall, and it can be utilized to perform the radial tomography of shear velocity in this radial range. For fast formations, the first arrival includes borehole refracted shear mode, part of F1 and F2 modes, the velocities of which are close to that of shear modes, and it can be utilized to accurately evaluate the formation shear velocity without the influence of formation anisotropy. In slow formations, the formation screw modes are highly influenced by anisotropy.It suggests that, in field data processing, for fast formations, the first arrived wave packet can be utilized to extract shear wave velocity, while for slow formations, the formation screw waves can be processed by data-driven methods to obtain true shear wave velocity of rock formations. Because the operation frequencies for fast and slow formations are different, broadband and high-powered transmitters and high-sensitivity receivers should be adopted in the design of an acoustic LWD tool, to increase the signal-to-noise ratio and to improve application range of the tool.