This paper highlights the value of seismic while drilling to successfully drill a vertical exploration well offshore Malaysia when faced with an expected pressure ramp in the shallow section and a gas chimney in the deeper section disrupting the surface seismic image data.

Using a dual tool Logging While Drilling (LWD) seismic configuration enabled real time Vertical Seismic Profile (VSP) imaging and velocity data for accurate casing point selection, hazard avoidance and successful well construction and enabled elimination of a planned casing string.

The seismic while drilling was conducted from seabed down to final well Total Depth (TD) to acquire a time-depth relationship for the entire wellbore. Acquisition was conducted during connections, an acoustically quiet period when the seismic shots are fired and downhole receivers in the seismic collar are recording. Transmission of the real time data starts when the connection is complete, and the mudpulse telemetry resumes with the drilling. Seismic waveforms sent to surface after each connection were processed for checkshots and VSP image successively after each seismic acquisition level.

Critical 20 inch casing point was accurately set within 20m of identified hazards, despite an initial uncertainty of 100m predrill. The VSP image data was superior to the surface seismic image, recording clear reflectors within a disrupted gas zone, with better phase and frequency content. Full velocity profile over the whole well interval with 15m definition was acquired and processed. The character differed from the initial predrill model and offset well information significantly.

Seismic while drilling removed the need for a pilot hole, and optimized the 20 inch casing point. Accurate real time velocity information allowed for accurate time to depth conversion for drilling decisions i.e. to assist in navigating the shallow pore pressure ramp, casing point setting and providing better seismic imaging in the gas chimney. This case study highlights one of the first applications of real time VSP data enabled by the use of two seismic tools placed 15m apart in the Bottom Hole Assembly (BHA).

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