Pore Pressure Prediction Using Look-Ahead Vertical Seismic Profile VSP - A Case Study from the Red Sea, Saudi Arabia

Abstract

Abstract A case study is presented to illustrate the successful prediction of formation pore pressure ahead-of-bit using vertical seismic profile (VSP) data acquired from the Red Sea, offshore Saudi Arabia. Two look-ahead VSPs were collected at two intermediate depth (TD) levels. VSP data was analyzed through corridor stack and inversion to estimate the depth of potential reservoir and related interval velocity. Two strong reflections ahead of the bit were identified as having lower acoustic impedances and the deeper one was associated with a potential reservoir. After drilling for about 4000 ft from the bottom of the first VSP, a second VSP was acquired to better image the potential reservoir and to investigate any over-pressure zones ahead of the bit in an open-hole. The second VSP clearly revealed the potential reservoir and presence of velocity reversal ahead of bit, which could be associated with an increasing pore pressure. Further drilling for few hundred feet, events such as high torque and tight hole as indicators of over-pressure were encountered. These suggested that drilling to the target reservoir could greatly increase operational risk due to over-pressure. Therefore, the decision was made to call for final TD. The post-drill pore pressure evaluation revealed that the look-ahead pore pressure prediction was different only by a small amount from the actual measurements. This concluded that our look-ahead pore pressure prediction workflow can provide critical data for drilling engineers to call for final TD and successfully complete the well.