Li, Jun (LandOcean Energy Service Inc.) | Li, Yan (LandOcean Energy Service Inc.) | Huang, Jingding (LandOcean Energy Service Inc.) | Zheng, Tiancai (LandOcean Energy Service Inc.) | Mo, Yexiang (LandOcean Energy Service Inc.) | Rizer, William (LandOcean Energy Service Inc.)
Following our previous work on Amplitude Tomography that deals with amplitudes alone, we extend our effort to include the compensation of bandwidth and phase of seismic signals that are distorted by seismic attenuation. Our new approach involves utilizing tomographic inversion for estimating the quality factor (Q) from prestack depth migrated common image gathers. By filtering the seismic data into different frequency bands and measuring the effect of attenuation on amplitudes in each band, the frequency dependent effect, which was ignored in our previous work, of attenuation is fully taken into account, allowing Q to be estimated from our tomographic method. By using the estimated Q volume in one of the migration methods that incorporates Q in the traveltime computation, we demonstrate, through examples, that our workflow provides an optimal compensation solution that resolves amplitude and bandwidth distortions due to seismic attenuation.
Garcia, Gorka (Schlumberger Reservoir Seismic Services) | Sanz, Carlos (Schlumberger Reservoir Seismic Services) | Sherratt, Peter (Schlumberger Reservoir Seismic Services) | Assefa, Solomon (Schlumberger Reservoir Seismic Services) | Pallottini, Fabio (Schlumberger Consulting Services) | Bendel, Eduardo (Schlumberger Consulting Services) | Arroyo y, Jose Luis (Pemex E & P.) | de la Rosa, Victor Hugo (Pemex E & P.)
Shear wave anisotropy responds to the presence of fractures in the subsurface. In the present study, S wave splitting parameters, such as amplitude changes and time splitting analysis from the multi-component data has been used to delineate fractures in the reservoir. A new approach to warping has been used to study shear wave anisotropy, which aims to determine the velocity changes between fast and slow shear datasets, accounting simultaneously for time shifts and amplitude changes.
The study has been conducted to characterize the tight gas sandstone reservoirs in Rulison Field, Colorado. The matrix permeability of these sandstones is very low in the millidarcy range and hence natural and artificial fracturing is required for enhanced production. The need for understanding this type of unconventional reservoir are driven by the significant hydrocarbon reserves that exist within this field and other tight gas accumulations worldwide. To have more economic wells it is important to find the high fractured zones.
Shear wave data have been used to find highly fractured zones. The first reason to use shear wave data is that these sandstones have higher shear impedance contrast as compared to acoustic impedance contrast, which leads to better S wave imaging. But the main reason is due to the presence of vertically aligned fracture sets, the shear wave will split into fast and slow shear wave. The fast shear waves (S11) are polarized parallel to the fracture set and travel through the stiff matrix component of the rock and are unaffected by the fracture component. Whereas the slow shear waves (S22) are polarized perpendicular to the fracture set and propagates through both the matrix and fracture component of the rocks hence reducing its velocity. The amount of shear wave splitting tells us about the fracture density in the subsurface.
Conventional S wave splitting methods for estimating the anisotropy due to these fractures are horizon based algorithms that use the travel time differences between the S11 and S22 components (Jansen, 2005). To predict changes in fracture intensity using these conventional methods is complicated by the thickness and lack of consistent reflectors in the reservoir. A better approach is to correct for the time shifts between S11 and S22 and then compute the amplitude changes or impedance changes (Davis, et. al., 2008). But we know that the changes in seismic velocity cause both amplitude and travel time differences, and hence the two effects cannot be easily decoupled. The warping method used in this study doesn''t decouple these two effects and takes into consideration the observed time shifts and the amplitude changes simultaneously.
Rulison Field is located in the Piceance basin, northwest Colorado (Figure 1) and is classified as basin centered gas accumulation. The Piceance basin is a Rocky mountain foreland basin that covers an area of 7255 sq. mi., and produces gas mainly from the Cretaceous sandstones. The reservoir rock is the William Fork formation (Figure 2) and consists of stacked fluvial sandstones and shales, which are 1700-2400 ft thick. The sandstones are discontinuous and typically below seismic resolution.