Demonstrations of Real-Time Borehole Seismic From an LWD Tool

Abstract

Abstract A new technique provides conventional borehole seismic measurements in real time while a well is being drilled. The system consists of a logging-while-drilling (LWD) tool with seismic sensors positioned near the drill bit, a seismic source at the surface, and a measurement-while-drilling (MWD) system for real-time telemetry. The seismic source is fired during pipe connections, seismic data are processed downhole and check-shot information is sent to the surface. Borehole seismic tools conveyed by wireline cable have been used for a variety of applications including seismic time-depth mapping, and reflection and transmission imaging of subsurface structures. Although wireline borehole seismic services are mature, they are unable to provide all the benefits that are sought in real time while drilling. This is particularly the case when borehole seismic surveys are conducted to help make drilling and well construction decisions, such as steering and deciding on casing and coring points. The new service provides standard borehole seismic information in real time that can be exploited for drilling optimization, cost savings, and safety improvements. Check-shot (time/depth) data are used to place the bit on the seismic map to help well navigation and casing point selection and to avoid drilling hazards, such as pore pressure, seen on the seismic. Real-time seismic velocities can be used to update pore-pressure predictions from surface seismic. Ultimately, the real-time velocities will be combined with look-ahead images, formed from the seismic reflections from horizons ahead of the bit, to provide a direct measurement of the distance to a marker of interest. In 2000, a field-test campaign conducted with experimental tools in the Gulf of Mexico demonstrated the first known real-time check-shot technique with an LWD tool. We will show the results and further describe the status of the service. Introduction Many oil exploration and production companies have seen the value of real-time borehole seismic information provided by seismic-while-drilling (SWD) techniques. This information has been used to optimize the drilling process, resulting in savings of millions of dollars per well.1,2 Until recently, the only SWD technique available used the noise of the drill bit while drilling as a seismic source and receivers placed on the surface to record borehole seismic information in what was primarily a time-versus-depth measurement. The technique has been tested in most environments and the operating limitations are now well understood. However, it has been found that the technique using the bit as a source is generally ineffective in soft sediments, horizontal wells, and while drilling with polycrystalline diamond compact (PDC) bits.3 The alternative to SWD is intermediate wireline check-shot or vertical seismic profile (VSP) surveys, which are typically run to reduce the uncertainty in putting the drill bit on seismic images and sometimes to provide a velocity model for seismic reprocessing. Being able to make this VSP or check-shot survey while drilling can significantly reduce costs by eliminating wireline rig time. More importantly, it provides real-time information that places the well more precisely with respect to an event on the surface seismic, and thus eliminates potential sidetrack costs. Additionally, operators would like to have a technique that imposes no restrictions on the type of bit, well deviation, or lithology. Potential applications include pore pressure prediction and imaging ahead of the bit.