Drilling Optimization Using Drill-bit Seismic in the Deepwater Gulf of Mexico

Abstract

Abstract Drill Bit Seismic has been used successfully on land and in shallow marine environments for many years. The technique utilizes the acoustic energy radiated during the drilling process to provide vital information about the drilling operation. It provides time-depth and formation velocity information at the well site, allowing the position of the bit to be plotted on the surface seismic section as drilling progresses. By generating look ahead VSP images, the approach to critical horizons such as casing points or overpressure zones, can also be monitored, leading to significant cost savings and enhanced safety. The technique has recently been extended to water depths of almost 4000ft in the Gulf of Mexico, where time-depth data was obtained down to a depth of 12500ft, drilling in soft formations with a rollercone bit. This information can be used to calibrate pre-drill velocity models and helps to prevent unexpected occurrences during drilling. Introduction In the deep water Gulf of Mexico, drilling wells is a very expensive business. There are often relatively few existing wells and the velocity profile of the overburden is rarely accurately known, leading to considerable uncertainty in pre-drill estimates of target depths. This in turn can lead to unwanted surprises during drilling which can cause expensive delays and wasted rig time. It was considered that if the drill-bit seismic technique could be extended into this domain, it could make a valuable contribution to preventing such drilling surprises. In the past, a seabed cable has been the preferred technology for Drill-bit Seismic data acquisition and horizontal deployment of such cables in Vietnam and Indonesia for BP Amoco has been carried out successfully in water depths of up to 1600ft. However, in much deeper water (>3000ft), horizontal seabed deployment is no longer viable. The aim of this work was to extend the Drill Bit Seismic technique into the deep water Gulf of Mexico. This paper will describe the development of the deep-water method and show results from field experiments. Overview of Drill-bit Seismic method In some ways the Drill-bit Seismic method is similar to that of a Vertical Seismic Profile (VSP) acquired on land (fig. 1). The drill-bit replaces the vibrator as the seismic source and a similar cross-correlation approach is used to obtain travel time information. The key difference is that the drill-bit signal is unknown and needs to be measured for cross-correlation to be successful. Therefore the bit signal is monitored using accelerometers mounted on top of the drill string. This enables the source signature to be estimated.