Tube Waves in Ultra-deep Waters: Preliminary Results



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The oil and gas industry defines ultra-deep-water regions as areas in which water depths are greater than 1500 m. It is now well established that there are hydrocarbons in these regions. The reservoirs in these areas are generally located below basalt rocks or below salts. The focus of this thesis is to understand reflections, refractions, diffractions and scattering for acoustic lenses located below basalt rocks. The results of this study can potentially be used to understand the effect of tube waves on borehole seismic data in ultra-deep waters.

Finite-difference modeling technique was used for this study. Finite-difference modeling allowed us to model refractions, reflections, diffractions and scattering; actually all events in surface seismic data, as well as borehole seismic data can be modeled. However, because of limited computational resources, this study will be based on a 2D finite difference instead of a 3D finite difference. This limitation implies that laterally infinite lenses were used to describe cylindrical boreholes.

The four main characteristics of the geological constructs used here in simulating the ultra-deep-water regions were the size of the water column, the topography of the sea floor, the interfaces of basalt layers with the surroundings rocks, and the structure of heterogeneities inside the basalt layers.

The average wavelength of wave propagation below the basalt layer is 125 m, which is very large compared to the size of a typical borehole (0.1 m). A lens with a thickness of 2.5 m, which corresponds to a dimension 50 times smaller than the average wavelength, sub-basalt was constructed. Also included were some lateral extensions in the construction of the lens to simulate wash-out zones.

This study investigates the wave propagation below the basalt rocks and the effect of tube waves on borehole seismic data below the basalt layer by using these lenses instead of a cylindrical borehole. As the borehole geometry is different from that of the lens, the results are considered preliminary. Results suggest that tube waves are negligible in ultra-deep waters below basalt rocks because the wavelength of the seismic waves is large in comparison to the wash-out zone (192 times larger).