Article | May 30, 1999

Borehole Seismic Reduces Exploration Risks


  • Types of VSPs
  • During periods of low or fluctuating oil prices, it is crucial that the technology applied in locating and producing hydrocarbons be efficient to reduce uncertainty and E&P risk. Risk has always been a factor in exploration and production, but it is lessened considerably these days by advances in technology that increase confidence in the images of the substrata upon which drilling and completion is based.

    Those advances are in the integration of 3D seismic data acquired on the surface of the land or sea with vertical seismic profiles (VSPs) acquired deep within the borehole of the well itself—a major technological development for the coming decade. Furthermore, those same VSPs can now be merged with well log data in the processing parameter selections to produce images that could be relied upon, thereby decreasing the probability of a dry hole disaster.

    Pinpointing potential reserves (back to top)
    Three-dimensional seismic measurements of the substrata have made a magnitude of difference in the accuracy of pinpointing potential reservoirs and selecting drilling sites. Furthermore, massively parallel computing systems and fully integrated software to capture and reflect the image of reservoirs and their surrounding strata, have taken these decisions even farther in accuracy and dependability by effectively directing the drillbit.

    But surface seismic is limited in that it measures time, not depth, and it is often unable to "see" through or around certain geological structures, thus leaving gaps in the data that can often be critical to the success of the project. Both the source and the receivers of the sound energy used to acquire seismic data are on the surface of either the land or water. Thus, when the energy vibrates from the source into the earth, it propagates in all directions, but only the down-going vibrations provide the information needed, as they strike layer upon layer of strata and are reflected back to the surface and the receivers.

    Traditionally, images are produced from these reflections, which are in the form of wiggling lines that vary with the time, but without measurements of the velocities of the layers encountered, these images do not provide indications of depth.

    New borehole seismic techniques overcome this limitation by acquiring these missing velocities to convert surface seismic sections to depth and to fill the gaps left in these sections by unpenetrated geological phenomena such as salt diapirs. In addition, borehole seismic measurements may be integrated with the surface seismic data to produce more valuable, calibrated images incorporating far greater detail.

    Borehole seismic differs from surface-acquired seismic in that it is obtained by placing the receivers at pre-established intervals within the wellbore, with the source on the surface. The vertical seismic profiles (VSPs) are produced from the energy vibrations propagating downward directly to the receivers and from the reflections of the energy from layers beneath the receivers.

    The commonly done check shot is the least complicated form of borehole survey. In it, the surface source is essentially directly over the receivers, which are specifically spaced in the wellbore at varying depths. When the source is fired, energy traveltimes are recorded by the receivers, and the velocities are determined between them which can be used to convert surface seismic from time to depth.

    Types of VSPs (back to top)
    There are several types of VSPs: zero-offset, with the source near the wellhead; offset, with the source some distance from the wellhead; walkaway, with a series of sources progressively farther from the wellhead in various patterns; walkabove, with the sources places above the receivers in horizontal or extended wellbores; and drillbit, where the drilling bit itself acts as the source and receivers are on the surface. Variations on these types include 3D VSPs, essentially multiline walkaways; shear-wave, acquired with shear-wave sources; and near-salt, with the source above the salt structure and the receivers parallel to it, to mention but a few.

    An offset VSP, courtesy Geofizyka (Turon, Poland).

    Applications of borehole seismic are growing exponentially. Not only is it vital in reducing exploration and development risks by its integration with surface-acquired seismic data to produce superior images and provide depth measurements, but in revealing the substrata in and around structures that obscure it from surface seismic. By "undershooting" salt domes via parallel placement of receivers to the salt structure, shadow zones are illuminated and reservoirs beneath the dome's overhang are spotlighted. By providing noiseless data to be merged with surface shot data, VSPs shot from below can penetrate gas clouds to reveal the hidden strata and reservoirs that were undetectable by conventional methods and provide highly dependable positioning information for wellsites.

    And VSPs can be used to recognize multiples (false reflections) in surface seismic data, eliminating costly misinterpretations.

    A major breakthrough in the use of VSPs is, however, just beginning to be applied as a real-time predictive tool that allows looking ahead and around the drillbit in the surrounding subsurface. To do this, drilling is briefly suspended at certain depths above the target to run 3D VSPs and thereby produce an image of the near-bit area, which is then used to direct the drilling farther and more accurately to its target.

    In addition to these important uses for borehole seismic data, the data derived from borehole seismic technology—amplitude, velocity, phase, and polarization—are ultimately employed to verify the quality of surface-acquired seismic data and provide confidence in drilling targets. But over the next decade, we should see borehole seismic data utilized for converted wave interpretation, which could reveal the nature of the lithology and even the kind of fluid content of a reservoir. Even greater quality and position information will likely be developed from VSPs.

    Beyond these applications, which are already being tested, there is the probability that simultaneous surface and borehole seismic acquisition will be achieved, leading to the pinpointing of sweet spots for immediate drilling.

    By Dev George