Presented in this paper is a case study in South China Sea on formation pore pressure prediction ahead of bit by using a look-ahead vertical seismic profile (VSP). At the intermediate total depth (TD) of 2930m of the study well, wireline logs and VSP data were acquired, and VSP inversion conducted. A pore pressure model was calibrated based on VSP data and validated with the drilling and wireline data above the intermediate TD. The model was then applied to the VSP-inverted velocities below the intermediate TD to predict the pore pressure profile ahead of the bit. Following the successful drilling of the study well to the final TD, a second VSP, wireline logs and formation pore pressure measurements were acquired. The post-drill pore pressure evaluation revealed that the look-ahead pore pressure under-predicted the actual formation pore pressure by a small margin. A number of factors that would improve the accuracy of look-ahead velocity inversion, such as the presence of very low frequency range of the spectrum and application of constrain point(s), have been examined in the paper. Furthermore, it has been demonstrated that undercompaction is not the only overpressure mechanism in this study area, unloading may also have contributed to the overpressure. Both Bowers and Eaton models can be applied to predict pore pressure ahead of bit for the overpressure mechanisms in the area.

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