Last month we looked at the concept of utilizing the axial impacts of the rotating teeth of a rotary-cone drill bit as a downhole seismic source, which allows seismic data to be acquired by surfacepositioned sensors as a well is being drilled.
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 imaging or for other seismic applications, unlike the robust wavefield produced by a rotary-cone bit.
Technologies are now available that acquire seismicwhile- drilling (SWD) data by embedding seismic sensors in the drill string near the drill bit (figure 1). 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 depth where seismic information needs to be obtained, drilling action must cease for several minutes so that the downhole sensors are in a quiet environment as they record the seismic wavefield produced by the source. The responses of the drill-string sensors are stored in a downhole memory unit included in the drill-string system.
The data are retrieved when bit trips are made and the seismicsensor section is returned to the drill floor.
This downhole seismic sensor technology allows numerous seismic applications to be implemented as a well is being drilled, with examples being:
- Predicting overpressure intervals ahead of the bit.
- Imaging below and laterally away from the well bore.
- Defining the relationship between drilling depth and seismic image time in difficult velocity areas in real drilling time.
- Guiding the bit to a target identified on a surface-based seismic image.
- Positioning core barrels at the onset of a seismic reflection interval of interest.
Numerous other applications are possible, and several encouraging proof-of-concept tests have been done.
An example of the data quality that can be achieved with drillstring seismic sensors is illustrated on figure 2. Conventional VSP data acquired in the same well with wirelinedeployed sensors also are shown to aid in evaluating the quality of the SWD data.
VSP data almost always are recorded at regularly spaced depth intervals, as they are in this data display. However, as in this example, SWD data may be recorded at irregularly spaced stations positioned at depth coordinates where well conditions allow drilling to be stopped so a quiet seismic condition can be produced in the borehole.
The reflections noted in these particular SWD data are of sufficient quality for the data to be used in seismic imaging applications.
An example of an image produced from drill-string seismic sensors is displayed as figure 3.
These data were acquired as a deviated well was drilled toward the targeted interval marked by the robust seismic reflection events on the seismic profile. The intent was to ensure that the well penetrated the objective at a structurally high position where there was optimal time thickness of the target interval.
These data are an example of SWD data being used to guide a drill bit to a seismic-defined point of penetration on a target.
Acquiring seismic data while drilling is good strategy in areas where:
- Precise time-vs.-depth relationships are not known.
- There is concern about drilling into an over-pressured interval.
- Where a core needs to be collected starting at the top of a seismic-defined stratigraphic interval.
Contact your well services provider to find out how to implement SWD technology when you are confronted with drilling in any of these challenging situations – plus numerous other applications that have not been illustrated in this short review.