Safe Landing of a Deviated Well with a Rig Source VSP Using RT Lookahead Seismic Technology

Abstract

Abstract This paper demonstrates how an operator mitigated a hazardous pore pressure drilling risk using a technology that was new to the operator. The Seismic While Drilling (SWD) increased safety and efficiency at lower cost over commonly used alternatives. In 2019 MOL Norge AS drilled a deviated exploration well in the southern part of the Norwegian North Sea. The large 300 bar pore pressure uncertainty in the reservoir made it necessary to land the 9 7/8-in casing shoe as close as possible to the reservoir to maintain a safe kick tolerance. The pre-drill depth uncertainty of +/- 75m at the reservoir made it necessary to find a reliable technology to significantly decrease this depth uncertainty. Borehole seismic technology enables measurement of seismic velocities, which is the main culprit in seismic depth uncertainty. Measuring the time taken for a seismic pulse from a surface airgun to reach a downhole sensor provides velocity information which can provide a significant reduction in depth uncertainty while drilling. The closer the seismic sensor is to the formation of interest then the more accurate the depth prediction of the formation top is. In addition, seismic reflections from the formation top can provide a lookahead capability extending to hundreds of meters. SWD technology coupled with 3D ray-trace modelling allowed a survey to be designed for this deviated well using rig deployed airguns rather than deployment from a costly additional vessel. The survey design included real time data during drilling to significantly reduce the depth uncertainty and also image the reservoir top. The 12.25in hole section was drilled with a seismic logging while drilling (LWD) tool in the BHA, acquiring data during the drilling connections. Vertical Seismic Profile (VSP) processing of the real time data allowed the drilling team to see seismic reflections from the reservoir top during the approach from over 600m away. The decision to stop drilling occurred when the lookahead image indicated the reservoir was at a depth of 3,426 m TVD (+/- 13m), 50m shallower than the predrill plan. The casing point was set at a depth of 3,402 m TVD, approximately twice the depth uncertainty above the lookahead prediction. The actual reservoir top was subsequently logged at 3,425.6m TVD, less than 1m from the predicted depth.