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Abstract Passive borehole seismometry enables mapping the orientation Passive borehole seismometry enables mapping the orientation of induced hydraulic fracture planes by using a triaxial seismometer clamped in the treatment well to map acoustic emissions from the fracture. Presently, the method utilizes only those seismic events attributable to shear failure along the fracture plane which exhibit both compressional and shear wave phases. The plane which exhibit both compressional and shear wave phases. The fracture azimuth is seen to parallel the polarization of the compressional wave phase. Whereas other geophysical techniques used to determine fracture orientation are principally surface-deployed measurements and, therefore, lose their effectiveness as the fracture depth exceeds 1500–2000 meters, the borehole seismic has worked effectively at 3300 meters in past applications. The method, principally, gives only fracture azimuth as is demonstrated by two examples of field test data.
Introduction The use of borehole seismic methods for fracture azimuth determination has been offered as a research program through the U.S. Department of Energy since 1978 and only recently as a commercial service. The importance of azimuth information to the successful and optimal application of hydraulic fracturing to oil and gas field development is well known, particularly in low permeability formations or in fields where well spacing is permeability formations or in fields where well spacing is continually being reduced.
Several authors have reported the occurrence of acoustic emissions observed during and following hydraulic fracturing and have utilized these as a means of mapping the fracture plane in situ. In the context of hydraulic fracturing, "acoustic emissions" may be thought of as extremely small microearthquake-like events caused by brittle fracture of the rock mass due to formation of the fracture plane. It is believed that these shear failure type events are caused by the open, pressurized fracture, the localized high pore pressure zone surrounding the fracture, and the presence of pressure zone surrounding the fracture, and the presence of inhomogeneities (existing fractures, bedding planes, well-cemented zones, etc.) existing in the reservoir rock. The source of seismicity, then, is not restricted to tensile failure at the fracture tip as one might expect but is believed distributed all along the fracture plane as the rock mass adjusts to the sudden departure from equilibrium. Although other classes of seismic events which are clearly of a non-shear failure origin have been reported, these events are of speculative origin (harmonic resonance of the fracture?) and are currently deleted from those signals analyzed for azimuth determination. Given that the shear failure type signals, i.e. those seismic events exhibiting clear compressional ("P") and shear ("S") wave phase, are spatially associated with the fracture plane, we may determine the fracture orientation if we can accurately determine the location of the sources of these seismic events.
A further characteristic of these seismic signals is that they are of very small magnitude. For this reason, only shallow fracturing operations have successfully utilized surface deployed seismometers to detect these signals. This implies that for general application to hydraulic fracturing, the measurements need to be made either in nearby observation wells or in the frac well itself. The ability to make such "close in" measurements, in fact, eliminates the problems encountered with all surface-deployed geophysical techniques such as tiltmeters or magnetometers which, although providing highly diagnostic information for favorable signal-to-noise environments (i.e. shallow), lose their effectiveness as fracturing depths exceed 1500-2000 m (5000–6600 ft). The passive borehole seismic method described in this paper has been shown effective in detecting signals at fracture depths exceeding 3300 m (11000 ft).
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