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.


Image Gallery

See Also: Bulletin Article

Volumetric restoration can provide crucial insights into the structural evolution of three-dimensional (3-D) petroleum systems. A major limitation to its widespread application is the need to include complex architectures and realistic mechanics such as flexural slip. We apply an implicit approach that allows for, including unconformities, thin and/or pinched-out layers in the models but that cannot explicitly localize slip along horizons. To take advantage of this approach while accounting for flexural slip in 3-D restoration, we investigate new geomechanical properties. We consider flexural slip folding as a result of stacked rigid and thin weak layers, which can be modeled using transversely isotropic properties. We compare restorations of an anticline using transversely isotropic properties, isotropic properties, and a stack of rigid isotropic layers with nonfrictional slip between the layers. Our results show that transversely isotropic properties reasonably approximate flexural slip folding. We use these new tools to model the evolution of a complex system located in the Niger Delta toe. The system includes a detachment fold, a fault-bend fold, and a structural wedge formed in series. Growth stratigraphy and erosional surfaces delimit the kinematics of deformation. Regional erosive surfaces, 3-D gradients of fault slip, and vertical variations in mechanical strength motivated the use of our new restoration techniques. Restoring two growth units results not only in reinforcing the interpretation that the area is behaving as a deforming thrust sheet at critical taper, but also in highlighting coeval activity on both the hinterland structures and the toe of the thrust belt.
Desktop /Portals/0/PackFlashItemImages/WebReady/Handling-natural-complexity-in-three-dimensional.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3564 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 4113 CD-DVD

See Also: DL Abstract

When evaluating paleosystems, there will always be a shortage of data constraints and a surplus of plausible geological scenarios for a basin evaluation. Modelling paleosystems with constraints from the modern has been used as a successful approach to better understand petroleum systems.

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3094 DL Abstract

Hydrocarbon exploration beneath the shallow allochthonous salt canopy of the ultra-deepwater central Gulf of Mexico has encountered three thick, sand-rich, submarine fan successions that punctuate an otherwise relatively condensed and fine-grained basin center stratigraphy. These sand-rich fans are Late Paleocene, Early Miocene, and Middle Miocene in age and each coincide with periods of very high sediment flux and basin margin instability. They are the primary exploration targets in most ultra-deepwater fields, recent discoveries, and failed exploration tests.

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 845 DL Abstract

See Also: Online e Symposium

Technical Writing Triage is a condensed course which identifies the key professional and technical writing in today’s workplace, discusses the most common problems/issues, and provides quick, easy-to-implement solutions for producing high-quality, effective communications.

Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-technical-writing-triage.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3811 Online e-Symposium