| dc.description.abstract | Reservoirs in the Arabian Gulf, home to some of the world’s largest oil reserves, consist primarily of heterogeneous carbonate rocks and successfully recovering hydrocarbons from them is a challenge. With existing technologies, most of the oil reserves located in the offshore carbonate fields in the Arabian Gulf will remain unrecoverable. Shear wave information is essential for reservoir characterization in the offshore fields of the Arabian Gulf. Used together with P waves, S waves can help better delineate reservoir structures and discriminate bypassed hydrocarbons for production monitoring. The Arabian Gulf has extremely shallow waters (~ 10 m) and varied hard sea-bottom, which results in highly dispersive waves, water-bottom reverberations, and severe anti-aliasing. Previous studies have also observed strong shear wave energy conversion at the sea-bottom in the Arabian Gulf. In this dissertation, I will analyze the source mechanism in order to understand and utilize the strong shear wave generated in the region.
Classical wave propagation analysis, especially the ray theory, is not adequate to address the
complex interactions between the acoustic source and sea-bottom in the shallow waters. It remains a long-standing challenging problem to explain the S* and SH* waves observed in the field decades ago. In this dissertation, we propose a conceptual body force model that explains the generation of SH* and S* waves from an explosive source in the shallow water with a hard sea-bottom. This model defines an effective source on the seafloor directly under the acoustic source load from mechanical deformation analysis. Instead of a wave propagation model, the mechanical model considers the source and the boundary together as one volume element of elastic deformation. From the mechanical model, the body force components for P, SV, and SH waves are explicitly defined by the water depth and the Poisson ratio difference of the media. Exact analytical solutions are derived for the near-field and far-field wave propagation from the effective source. Analysis is given for three sea-bottom scenarios, soft mud, intermediate, and hard coral reef, to observe the effect of source distance to the sea-bottom and sea-bottom hardness on wave propagation. Shear waves generated from shallow source and hard sea-bottom are more than a thousand times stronger compared to far source and soft sea-bottom. Further, finite-difference wave modeling is utilized to analyze the source mechanism in the shallow marine environment using the full wave-form solutions.
Field 2D 4C seismic data acquired in the Arabian Gulf is processed for confirmation of the
existence of shear waves generated from the explosive source in the shallow waters and for direct shear wave imaging of the subsurface. The processed inline shear wave is not the evanescent PS* waves as other authors proposed. It is the body shear waves which are generated at the impact point by effective source’s SV component. It propagates into the medium as a pure shear wave that we refer to as direct shear (PSS). The SH* waves generated at the impact point by the effective source’s SH component should be detected by processing the 2D 4C crossline data or 3D 4C data. An established model-driven approach is used, based on reliable P and S wave velocity from well log. The proposed workflow enhances reflected energy signals to extract shear wave information. P and S wave extracted seismic sections have similar geological structural features, but the S stacked section has higher frequency and higher resolution on the inline component than P wave sections. This processing workflow greatly enhances the signal-to-noise ratio of the seismic data and directly extracts shear wave information for reservoir characterization. | |
