Cracks, Gas and Waves in Underground Imaging

The amplitude and frequency of seismic waves in the subsurface are functions of the density of the rocks. The presence of fractures and fluids change the density, and in most cases make material properties anisotropic. Recent research supported by the Geosciences Research Program on computational modeling of seismic wave propagation demonstrates that not only can crack orientations be imaged in surveys between boreholes, but also that fracture widths and fluid-gas contents within the fractures can be determined.  Fracture widths in the millimeter range can be resolved by seismic waves at common frequencies if gas concentrations inside the fractures become large. Although the wave velocity changes are small across the gas+water filled cracks, the wave amplitudes decreases strongly because of scattering of wave energy.

 The Lawrence Berkeley National Laboratory theoretical study demonstrated its ability to explain established seismic responses measured during previous subsurface investigations on fractures at the Conoco test facility.  Surveys between wells therefore can be used to compute gas concentrations and fracture widths at depth in order to predict preferential flow paths for fluid extraction or for gas injection. This research was a collaborative project among Conoco and Amoco who provided site support, the Office of Fossil Energy, who provided stipend support, and the Office of Basic Energy Sciences who supported the computations and field work through the LBNL Center for Computational Seismology.

[OBES] [Office of Science] [DOE]

Send Comments to SC.Geosciences.science.doe.gov

Disclaimer