Click to view article in PDF format.
GCSeismic-While-Drilling: Techniques using the Drill Bit as the Seismic Source*
Bob Hardage1
Search and Discovery Article #40411 (2009)
Posted May 6, 2009
*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, March, 2009, Part 1 entitled “Drill-bit Seismic Still Has Teeth”, and April 2009, Part 2 entitled “Bit by Bit: A Good Seismic Strategy”. Editor of Geophysical Corner is Bob A. Hardage. Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director.
1Bureau of Economic Geology, The University of Texas at Austin ([email protected])
General Statement
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.
|
Principal of Drill-bit Seismic
The principle of geologic
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
Examples
Figure 2
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
A second example, comparing a drill-bit image with surface-recorded seismic data across the drilled well is displayed in Figure 3. At this latter well site, the drill-bit image was a good match to the surface seismic data.
Applications
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 · Real-drill-time 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.
PDC Bits
Alternate technology that allows usable seismic data to be acquired when
PDC bits are utilized has now come onto the scene. Polydiamond crystalline (PDC) bits are now replacing rotary-cone
bits in many drilling programs. PDC bits cut rock by a scraping action –
not by an axial chiseling action, as does a rotary-cone bit. And
because of its rock-cutting style, a PDC bit does not generate a seismic wavefield that is adequate for seismic
Technologies are now available that acquire
seismic-while-drilling (SWD) data by embedding seismic sensors in the drill string near the drill bit (Figure 4). With this technology, vertical seismic profile (VSP) data can be acquired while
drilling with any kind of bit, including PDC bits, by using these downhole sensors together with a surface-based
seismic source. At each
|