In concept, any type of mechanical vibration that is introduced into the Earth can be used as a seismic wavefield to illuminate and image subsurface geology. Seismic imaging does not always have to be done with controlled, sophisticated sources such as air gun arrays, vibrators or shot hole explosives.
One unique mechanical vibration that illustrates the principle of seismic imaging without the use of a conventional seismic source is the repetitive Earth impulses that are created by the teeth of a rotary-cone drill bit as a well is being drilled to reach a geologic target.
Drill-bit seismic technology was a topic of rather intense research and development in the 1980s and 1990s, and the application should not be forgotten.
The principle of geologic imaging with a rotary-cone drill bit is illustrated on figure 1.
The key to the imaging procedure is to position a reference sensor at the top of the drill string, near the swivel. This reference sensor records each impact of each tooth of a rotary-cone bit as rock strata are being drilled.
As shown in the diagram, the drill-tooth impulses propagate along direct paths to sensors deployed on the Earth surface (or on the seafloor if the well is offshore) at stations that allow specific target geology to be imaged. An imaging capability is created by the drill-bit wavefields that propagate downward and reflect upward from rock interfaces below the drill bit as depicted by the raypath diagram.
By continuously correlating the reference-sensor response with the responses of the surface sensors, a sequence of seismic traces can be created as the drill bit traverses equally spaced depth intervals during the drilling process. Usually thiscross-correlation between reference sensor and far-field sensor responses is done continuously as the drill bit penetrates a depth interval equal to one joint of drill pipe (30 feet, or nine meters).
Depending on rock type, bit quality and drilling parameters, the cross-correlation computation during the drilling of this 30-foot interval may span a time period of five minutes to one hour. The resulting output trace is equivalent to that generated by a seismic source having a vertical dimension of 30 feet and positioned across the 30-foot interval that was drilled.
Under appropriate conditions, the image created from drill-bit wavefields can be quite good.
Figure 2, in fact, is an example comparing a drill-bit image created as a well was being drilled, and an image made from vertical seismic profile (VSP) data produced by a conventional seismic source after the well reached target depth. In this instance, the drill-bit image has a signal-to-noise character equivalent to that of the conventional VSP data.
A second example, comparing a drill-bit image with surface-recorded seismic data across the drilled well is displayed on figure 3. At this latter well site, the drill-bit image was a good match to the surface seismic data.
Using a rotary-cone bit as a seismic source has several proven applications such as:
- Real-drill-time velocity check shot information.
- Guiding the bit to a target seen on surface-acquired seismic data.
- Real-drill-time imaging ahead of the bit.
- Real-drill-time depth-to-time conversion to know when the bit is reaching an important depth interval.
- Positioning the bit at the top of an interval that needs to be cored.
All of these applications, and others, were achieved with drill-bit seismic technology in the 1980s and 1990s.
Even with these proven applications, drill-bit seismic technology is not as widely used today as it was 15 and 20 years ago.
The principal reason for the technology’s demise has been the conversion to poly-diamond-composite (PDC) bits by drilling contractors. PDC bits cut by a scraping action – not by vertical impacts of chisel teeth, as occurs with a rotary-cone bit.
Effective seismic wavefields are difficult to achieve with PDC bits. However, in current drilling practice, if a significant interval of rock is to be drilled with rotary-cone bits, the technique of drill-bit seismic technology is still on the shelf ready to be used.
Alternate technology that allows usable seismic data to be acquired when PDC bits are utilized has now come onto the scene and will be described in next month’s article.