Application of the seismic quality factor versus offset and azimuth (QVOA) for fractured reservoir characterization

Abstract

Fracture characterization of a reservoir is very important because the presence of fractures determines the flow of hydrocarbons during production. Accurate modeling of the fracture network can help in optimizing the production of the reservoir. Fractures affect the amplitudes of the seismic waves, therefore, seismic attenuation is used to determine their characteristics. Here, I use a new technique called QVOA which involves the evaluation of the seismic attenuation and its variation with offset (O), and azimuth (A). Variation of seismic attenuation from QVOA methodologies can help in determining fracture characteristics where conventional methods fail. The QVOA method is a two step process where seismic attenuation is computed first and then its variation is determined with respect to offset and azimuth. I compute seismic attenuation using four different techniques based on the spectral ratio and frequency-shift methods. The variation with respect to offset and azimuth is determined using approximate method of sectors (ASM) and approximate truncate method (ATM). Orientation and the B-gradient of the fracture characteristics are obtained using this QVOA technique. I apply this QVOA technique to 3D seismic data acquired over the Gulf of Mexico region where the target is a naturally fractured carbonate reservoir. Fracture orientation in the reservoir region obtained using the QVOA technique are verified with well log data. Finally, a comparative analysis of different techniques of seismic attenuation computation is provided, where the frequency-shift methods perform better than the spectral ratio method, and are more stable in the presence of noise. Variation of the B-gradient versus the azimuth suggests the presence of attenuation anisotropy in this reservoir.