倾斜地层下的井孔声场分析及声速校正图版制作

理解声波在倾斜井孔下的传播规律,对于准确获得目的层真速度有着至关重要的作用。通过有限差分和几何声学分析方法对比了倾斜地层下纵横波首波速度的变化规律,旨在建立倾斜地层下的声速校正图版。研究结果表明,当倾斜地层速度"上低下高"时,从全波中得到的声波速度不再是地层的真实速度,均大于目的层真速度。尤其是当地层界面倾角大于45°时,即便是源距超过了4 m,仍对首波速度有影响,必须对其校正。相反,当倾斜地层速度"上高下低"时,任何源距和地层倾角下的视速度值都接近目的层真速度,可以不予校正。根据以上分析,建立了倾斜地层下的声速校正图版,并指出下层速度的变化对于校正图版有着较大影响,源距的变化对于图版影响很小,在不同源距下可以使用同一个图版。 

Wave-field analysis and velocity correction chart making of the tilted formation with a fluid-filled borehole

Understanding the propagation rule of acoustic wave in the tilted formation with a fluid-filled borehole plays an important role in getting the actual velocity of the target. This paper compares the change rule of P-wave velocity and S-wave velocity in a tilted formation by using 3D finite difference and geometrical acoustics methods, aims to establish a velocity correction chart of the tilted layered formation. The results show that, when the upper formation velocity has a greater value than that of the lower formation, the acoustic velocity from the waveform data is no longer the actual velocity of the target, and all are greater than the actual velocity. When the formation dip is more than 45°, even receiver offset is more than 4 m, it still affects the head wave velocity, so acoustic velocity must be corrected. On the contrary, when the lower formation velocity is greater, the apparent velocity value for any receiver offsets and any formation dips is close to the actual velocity of the target, without need of correction. According to the above conclusion, the acoustic velocity correction chart is established in a tilted layered formation, and this paper further points out that change of the lower velocity has a great effect on the correction chart. However, receiver offset basically has no significant effect on the correction chart, so the same correction chart can be used under different receiver offsets.