Borehole seismic applicationsReservoir geophysics should aggressively take advantage of data from boreholes that are very close to the target itself, not just for correlating seismic data to the well but also using those wells for the collection of novel geophysical data from below the noisy surface or weathered zone. New techniques for acquisition of seismic data from wellbores are available, and should become routine tools in the arsenal of the reservoir geophysicist.

Single-well techniquesSingle-well techniques involve placing seismic sources and receivers in the same well and include:

Sonic logging Single-well imaging

Sonic loggingSonic logging has become routine, and the collection of compressional and shear velocities in fast and slow formations is more-or-less straightforward, particularly with the use of dipole sonic tools and waveform processing. The application of modified sonic-logging tools for imaging near the wellbore is not routine but has been demonstrated in several cases; research and development continues in this area.

Modern sonic logging tools can provide a good measure of compressional and shear velocities, values that are required for calibrating seismic data at wells and for the investigation of lithology and fluid content from seismic data. Of course, the interpreter must be careful to know if the data represent invaded or uninvaded conditions and make appropriate corrections if necessary. Modern sonic logging tools can often provide reliable values for velocities through casing; often, the most-reliable sonic logs in soft shales can only be found behind casing because of the inability to log openhole the depth intervals in which shales are flowing or collapsing.

Compressional sonic log values are used in reservoir geophysics to tie well depths to seismic two-way travel time. First the sonic transit time is integrated to obtain a depth-calibrated time scale, then synthetic seismograms are created through determination of reflection coefficients (including the density log) and convolution with a known or assumed wavelet. This synthetic seismogram is often adjusted to account for:

Borehole effects Absence of data in the shallowest section Other unspecified effects, including velocity dispersion caused by thin-bed layering below seismic

resolution

The shear sonic log values are then added to create synthetic seismograms that demonstrate AVO behavior for comparison with the prestack data near the well. Often, additional work is conducted to model the changes in seismic response when rocks of slightly different lithology or fluid saturation are encountered away from the well. Both the compressional and shear sonic data are required to accomplish fluid-substitution modeling, although some empirical models and other short-cuts are available.[1] The most common fluid substitution models employ Gassmann[2] in clastic rocks; a number of models also exists for fractured rocks.[3]

Single well imagingSingle-well imaging, although not yet widespread, may provide a useful tool for detailed close-up structural studies, such as salt-proximity studies designed to assist in the planning of a development sidetrack from an exploration well, or in determining the location of interfaces with respect to a horizontal well. In general, a sonic-logging tool or a string of VSP receivers (geophones and/or hydrophones), coupled with a downhole seismic source, is lowered into the well, often using tubing-conveyed methods in highly deviated wells. The experiment then becomes similar to a surface reflection-seismic experiment, except that reflections may come from any direction around the well, not just from beneath it. The technique has been shown to be useful to image fractures [4] and to determine proximity to upper and lower interfaces in horizontal wells [5] as demonstrated in Fig. 1.

Fig. 1—Single-well imaging. The use of a string of receivers and a seismic source in a highly deviated well is shown in this example to provide an image showing the proximity of the well to layers of interest above and below it.[5]

Multiple-well techniquesBy placing a seismic source in one well and receivers in another well, a seismic velocity model between the two wells can be constructed using tomographic techniques, and a reflection image can be obtained by processing the reflected arrivals.[6]Although the images are constrained to lie in a plane connecting the two wells, the additional fine-scale information, available from such surveys, [7] can be of significant value to the reservoir engineer (Fig. 2).

Fig. 2—An example of crosswell imaging and the associated surface seismic and log data, showing the relative scales involved (after Harris et al.72).[7]

Well-to-surface techniquesMethods of calibrating seismic data and imaging that involve sources and/or receivers in one well and others at the surface include checkshot surveys, VSP, reverse VSP, and seismic-while-drilling. Checkshots and VSPs were developed primarily to assist in the tie between surface seismic data and well observations, but they have been extended beyond that in many cases. VSPs provide the best data for detailed event identification and wavelet determination, but they can also be used to image the near-wellbore environment, and the image can be improved if a number of offsets and azimuths (for a 3D VSP) are used for the source location. The ability to create a 3D image from borehole methods is greatly enhanced by placing a seismic source[8][9] in one well and deploying surface receivers, which are already around the well, in a reverse VSP configuration. Images from such experiments can be highly detailed[10] (see Fig. 3), and the time required for 3D reverse VSP acquisition is significantly reduced over the 3D VSP case in which the source is moved around the surface.

Fig. 3—VSP and reverse VSP reflection images of different portions of the same reef in Michigan. The left image was obtained using surface sources in a VSP configuration, and the right image was obtained using a downhole source in a reverse VSP configuration. The Earth model is also shown (after Turpening et al.66).[10]

The drill bit can also be used as a seismic source[11] much like an uncontrolled, but monitored, vibrator; it is capable of providing, in at least some instances, information useful for selecting casing or coring points and for estimating proximity to overpressure zones.[12][13] Through the use of receivers in a logging-while-drilling unit near the bit, a surface VSP source can be recorded during pauses in the drilling operation, which occur as a new joint of pipe is being added. [14]