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.