High resolution borehole-to-borehole seismic tomography can provide much of the information needed to identify subtle reservoir heterogeneities thus allowing strategic placement of production wells. High resolution seismic techniques are ideally suited to bard rock applications where formation velocities exceed 10,000 feet per second. However, to date, the acquisition of high frequency tomographic data in gas reservoirs has not been altogether practical because of the large spacing between wells (on the order of 5000 feet). The piezoelectric sources typically used in high resolution cross-well seismics are unsuitable in these applications because they are not efficient below 200 Hz; therefore, to achieve a reasonable correlation result, the source must be operated up to 1600 Hz. However, with large well spacings, this upper frequency requirement becomes increasingly difficult if not impossible to achieve. The planned piezoelectric wall-locked seismic source has been specifically designed for lowfrequency long-range applications. It is expected to exhibit a usable bandwidth of less than 50 Hz up to 1 kHz. As such, it could be reasonably correlated with an upper frequency limit of 400 Hz. The source is intended for operation under extreme temperature and pressure conditions approaching 200øC and 20,000 psi. This project involves the construction and evaluation of a prototype piezoelectric wall-locked seismic source. The Phase I work involves laboratory and field investigations of the bender transducer elements and a fourelement wall-locked transducer assembly. The work will result in specific source properties being defined, including (1) usable bandwidth, (2) source behavior over this band, (3) performance of the wall-lock clamp assembly, and (4) the shear-wave capabilities of the source.Anticipated Results/Potential Commercial Applications as described by the awardee:The low frequency capabilities of the wall-locked piezoelectric source should extend the range limitations previously encountered with fluidcoupled piezoelectric sources. The source is not intended to replace the conventional piezoelectric source but to complement its capabilities by bridging the operating frequency gap between conventional borehole piezoelectric sources and mechanical borehole sources. The source high temperature and pressure capabilities should make it suitable for deep enhanced oil recovery monitoring applications in which seismic techniques can be employed. The source compressions/shear wave capabilities make it useful in a variety of reservoir characterization applications.
