Predicting Drilling Target Ahead of Bit Using VSP Lookahead Technique that Help Mitigate Drilling Risk in Over Pressure Zone, a Case Example from South China Sea

Abstract Yinggehai basin is located in South China Sea; it is characterized with high geothermal gradient and high overpressure. Numerous pore pressure predictions with VSP look-ahead have been conducted in this area. As pore pressure prediction with this approach is very sensitive to the inverted VSP velocity, based on our post drill analysis of pore pressure prediction results with VSP look-ahead data, 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. An error of 5% in VSP-inverted velocity could generate the error in predicted pore pressure approximately 0.16SG based on Bowers model. Therefore, with experiences gained from VSP velocity inversion part, determining the top depth of the high pressure target with the inverted VSP velocity curve is proposed in this study block to allow for an optimal setting depth of the casing shoe. A case study with this approach is presented in this paper.