The goal of advanced fracture detection technologies is to observe subsurface fractures prior to drilling. Detection technologies generally rely on advanced seismic collection and analysis techniques to reveal directional differences (anisotropies) in the reservoir's seismic response that may be related to fracturing.

In general, seismic technologies involve sending carefully designed and controlled pulses of energy into the earth, then measuring the nature of the energy as it is progressively reflected back from deeper and deeper horizons. Seismic information can be collected along a line, providing a 2-dimensional (cross-sectional) view of the structures below the line. 2-dimensional seismic was traditionally used for prospecting in frontier regions. Recently, the acquisition of data in a grid, providing a 3-dimensional view of the subsurface, has become a standard tool in the development of many structurally or stratigraphically complex areas.

Two types of returning seismic waves can be measured. Pressure waves (P-waves; individual particles oscillating in the direction the wave is moving) are the conventional data source. The changing timing, frequency, and amplitude of P-wave arrivals can give accurate representations of the subsurface structure of individual rock layers. Shear waves (S-waves; particles oscillating perpendicular to the direction of wave motion) are much more difficult and expensive to collect, but may provide more information on the varying properties of the rocks through which they travel.

The recently-completed Wind River basin multi-azimuth seismic project investigated cost-effective seismic technologies for characterizing the spatial distribution of fractures. The main purpose of the work was to compare information on the directional differences detectable in a small four-azimuth P- and S-wave seismic data set to that from a larger two-azimuth, P-wave survey. Statistical analyses of the two-azimuth data showed that seismic attributes (wave velocity, frequency, and amplitude) did vary directionally, and that these variances correlated predictably with the ultimate recovery of the wells. Velocity data taken from the presumed fracture-perpendicular direction was determined to be optimal for mapping relative fracture density. Although the two-azimuth method was determined to be robust enough for practical fracture detection, the more costly four-directional approach was found to be capable of detecting smaller magnitude anisotropies of arbitrary orientation.

In the fall of 1999, the Department of Energy initiated new projects that will continue to resolve the technical issues related to direct detection of natural fractures. A project in the San Juan basin is focusing on improved multi-azimuth 3-D seismic data collection and analysis techniques. Work in the northern Appalachian basin is testing innovative methods that may provide fracture detection capabilities to those without 3-D seismic. Also, a new method of analyzing shear wave data based on the initial separation of horizontal and vertical components will be tested.

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