New Concepts for Seismic Surveys While Drilling
Authors: Althoff Gary , Cornish Bruce , Varsamis Georgios , Kavaipatti Balaji , Abbas Arian , Wisniewski Laurence T. , Blanch Joakim O. , Cheng Arthur C. 
Published: December 2007

Borehole Seismic measurements are extremely useful for correlating drilled depth with seismic depth, setting coring and casing points, avoiding drilling hazards, and identifying, over pressure zones. It is widely accepted that while these seismic surveys can be obtained while drilling, there are at least two main issues that need to be understood in order to make these seismic measurements accurately and reliably. The first issue relates to the fact that, as opposed to a wireline based measurement tool, the drilling operation may prevent direct coupling between the sensors and the formation. The coupling of the formation signal into the sensors depends on factors including the weight of the tool in combination with well deviation. Thus, in general, the quality of the coupling needs to be evaluated. In particular, the sensors which are tangential to the borehole, i.e., the nominal azimuthal and vertical sensors, need to be more thoroughly tested. It can be expected that the radial sensor may couple better than the other components for a large range of deviations.

The second issue for making accurate and reliable seismic measurement is accurate time keeping between the downhole and surface systems. Timing (a highly accurate clock) is far more complicated than the traditional wireline approaches which rely on direct cable communications to correlate the seismic source activation, and communicate it to the acquisition components. Instantaneous and direct means of communication are not currently available in the MWD environment.

To further complicate matters, the rigors of the drilling environment impose significant constraints on the design and mounting of the seismic sensors and the accuracy of the clock.

One mean to address the coupling issue is the deployment of multiple, multi-axis sensors, including ruggedized geophones, seismic accelerometers, and hydrophones. As illustrated by field examples, depending on the conditions the tool encounters, at least one sensor is able to reliably record the seismic signals. While in certain situations, the primary measurement of the seismic while drilling (SWD) tools is a reliable checkshot survey, field results indicate that traditional VSP-type surveys/data can also be obtained while drilling.

Previous approaches for coordinating the timing have relied on complex synchronization techniques between surface and downhole clocks. Traditional wireline checkshot accuracy is quoted to be around 1 millisecond. An MWD clock would then be required to maintain at least this accuracy over an extended period of time (say 200 hours), within a severe temperature range (-20°C to 150°C), and within a hostile drilling environment (shock and vibration issues). An accuracy of 1 ms over 200 hours requires a clock that does not drift more than several parts per trillion, an accuracy similar to those of atomic (rubidium based) clock standards.

This paper investigates the design issues to build an accurate and reliable SWD system and presents the results of field tests and operations in which seismic data were acquired using different types of sensors (geophones, seismic accelerometers, hydrophones, etc.), in various types of formations, and in deviated boreholes. Furthermore, the data illustrate that the downhole clock maintained the required accuracy for a commercial SWD service.