DEVELOPMENTS IN PORE PRESSURE PREDICTION USING SEISMIC-WHILE-DRILLING TECHNOLOGY

Abstract

Exploration and development drilling encounters many technical challenges. Knowing the depth to overpressured zones is essential for choosing a safe mud weight that will keep formation pore fluids in place and prevent a blowout. The danger of encountering overpressure and variations in rock strength with depth can require well plans to include many casing string changes, and without some idea of true depth to target, it becomes hard to plan a well efficiently.

The seismic-while-drilling (SWD) approach has offered an alternative seismic method to image geology ahead of the bit while drilling, so that overpressured zones would be detected aboard the rig. The technology involves recording the drill-bit noise using surface or seabed receivers, cross-correlating this with data recorded by an accelerometer mounted on the swivel, and producing interval velocities as well as a seismic section beneath and to the side of the bit.

Success at producing seismic sections while drilling has been limited. The industry now offers the technique as a check shot replacement for time-to-depth conversion, which in itself is a highly valued tool for casing setting. However, a walk-away section while drilling is still desirable. In April 1998, the Gas Research Institute of Chicago announced the funding of a scoping study to determine the status of the SWD technology, and the areas of technology which could be developed by industry. This paper presents the status of the technology, and indicates the direction in which research is going to develop a pore pressure prediction methodology while drilling ahead.