Bit by Bit: A Good Seismic Strategy

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.

Comments (0)

 

Geophysical Corner

The Geophysical Corner is a regular column in the EXPLORER that features geophysical case studies, techniques and application to the petroleum industry.

VIEW COLUMN ARCHIVES

Image Gallery

See Also: Bulletin Article

Interpretation of seismic data from the Sorvestsnaget Basin, southwest Barents Sea, demonstrates gradual middle Eocene basin infilling (from the north) generated by southward-prograding shelf-margin clinoforms. The basin experienced continued accommodation development during the middle Eocene because of differential subsidence caused by the onset of early Eocene sea-floor spreading in the Norwegian-Greenland Sea, faulting, salt movement, and different tectonic activity between the Sorvestsnaget Basin and Veslemoy high. During this time, the margin shows transformation from an initially high-relief margin to a progradation in the final stage. The early stage of progradation is characterized by the establishment of generally oblique clinoform shifts creating a flat shelf-edge trajectory that implies a gentle falling or stable relative sea level and low accommodation-to-sediment supply ratio (lt1) in the topsets. During the early stage of basin development, the high-relief margin, narrow shelf, stable or falling relative sea level, seismicity, and presumably high sedimentation rate caused accumulation of thick and areally extensive deep-water fans. Seismic-scale sandstone injections deform the fans.

A fully prograding margin developed when the shelf-to-basin profile lowered, apparently because of increased subsidence of the northern part. This stage of the basin development is generally characterized by the presence of sigmoid clinoform shifts creating an ascending shelf-edge trajectory that is implying steady or rising relative sea level with an accommodation-to-sediment supply ratio of greater than 1, implying sand accumulation on the shelf. This study suggests that some volume of sand was transported into the deep water during relative sea level rise considering the narrow shelf and inferred high rates of sediment supply.

Desktop /Portals/0/PackFlashItemImages/WebReady/evolution-of-shelf-margin-clinoforms-and.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 7965 Bulletin Article

See Also: CD DVD

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4119 CD-DVD

See Also: DL Abstract

We have an unprecedented ability to realistically depict the spatial distributions of lithofacies in the subsurface thanks to developments in sequence stratigraphy, sedimentology, structural geology, geostatistics, and geophysics. As important as these developments have been, however, they in themselves have a limited ability to accurately predict rock properties–particularly in regions with high thermal exposures and restricted well control. We are developing a next-generation modeling platform that rigorously simulates processes in 3D at the grain scale. This 3D approach has the potential to provide unique predictive models of pore network geometries and grain contact properties for rocks in undrilled areas.

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 11651 DL Abstract

See Also: Field Seminar

The seminar will utilize traverses to examine multiple thrust sheets exposed in Sun River Canyon, the famous Teton Anticline, and an outstanding example of an exposed fractured reservoir along a fault‐propagated fold in Mississippian carbonates as Swift Reservoir. Participants will examine the mechanics of fracturing, folding, and faulting in thrust belt terrains, identify and discuss new ideas regarding the geometry and kinematics of the development of thrust belts, compare seismic interpretation with outcrop examples, and analyze stratigraphic concepts which are essential in the exploration of thrust belt targets.
Desktop /Portals/0/PackFlashItemImages/WebReady/fs-Fractures-Folds-and-Faults-in-Thrusted-Terrains.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 150 Field Seminar

See Also: Online Certificate Course

Unconventional Resources is an online course that enables participants to learn about shale gas, shale oil and coalbed methane.

Desktop /Portals/0/PackFlashItemImages/WebReady/oc-cc-unconventional-resources.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 1473 Online Certificate Course