Detailed Study of Seismic Wave Attenuation in Carbonate Rocks: Application on Abu Dhabi Oil Fields

Abstract:

Seismic wave attenuation is a promising attribute for the petroleum exploration, thanks to its high sensitivity to physical properties of subsurface. It can be used to enhance the seismic imaging and improve the geophysical interpretation which is crucial for reservoir characterization. However getting an accurate attenuation profile is not an easy task, this is due to complex mechanism of this parameter, although that many studies were carried out to understand it. The degree of difficulty increases for the media composed of carbonate rocks, known to be highly heterogeneous and with complex lithology. That is why few attenuation studies were done successfully in carbonate rocks.

The main objectives of this study are,
Getting an accurate and high resolution attenuation profiles from several oil fields. The resolution is very important target for us, because many reservoirs in Abu Dhabi oil fields are tight.
Separation between different modes of wave attenuation (scattering and intrinsic attenuations).
Correlation between the attenuation profiles and other logs (Porosity, resistivity, oil saturation…), in order to establish a relationship which can be used to detect the reservoir properties from the attenuation profiles.
Comparison of attenuation estimated from VSP and sonic waveforms.
Provide spatial distribution of attenuation in Abu Dhabi oil fields.
To reach these objectives we implemented a robust processing flow and new methodology to estimate the attenuation from the downgoing waves of the compressional VSP data and waveforms acquired from several wells drilled in Abu Dhabi. The subsurface geology of this area is primarily composed of carbonate rocks and it is known to be highly fractured which complicates more the situation, then we separated successfully the intrinsic attenuation from the scattering.
The results show that the scattering is significant and cannot be ignored. We found also a very interesting correlation between the attenuation profiles and the other petrophysical logs as the porosity, oil saturation, resistivity and fractures. We deduced that the combination between the scattering and the intrinsic attenuation is a promising and is very useful for the petroleum exploration, especially in the case of Abu Dhabi reservoirs known to be highly fractured.