Evaluation of Open Fractures-Sonic Velocity Relation in Fractured Carbonate Reservoirs

Rock physics evaluation of heterogeneous fractured reservoirs where such fractures arc the main fluid conduits is necessary for production optimization and field development In this regard, fracture parameters (i.e., aperture, density, etc.) play a significant role in acoustic wave velocities and propagation, which should be investigated. However, the effect of fracture parameters on sonic wave velocities and their propagation is relatively vague in fractured reservoirs. This study aims to evaluate the mentioned relation by integration of velocity deviation logs, image logs, conventional logs, sonic waveforms, core data, and thin sections in the four studied wells. The results of the velocity deviation logs and sonic waveforms indicate that fracture density and fracture aperture directly control their fluctuations. For example, results showed that zones with higher fracture aperture and lower fracture density, mainly in the dolomitic facies, exhibited more deviations (<-500) compared with the zones with higher fracture density and lower fracture aperture. Hence, the negative deviations on the velocity deviation log are not directly controlled by fracture density, and fracture aperture is the most effective factor on the negative deviation of the velocity deviation log. So, it seems that this parameter controls sonic waves scattering and reservoir behavior. It was found that open fractures and zones with higher porosity (>10%) would have a great impact on sonic wave velocities, attenuation, and other rock physics properties. Ultimately, it was confirmed that a simple but robust combination of the velocity deviation log with sonic waveforms can be used to delineate fractured zones with high fracture aperture (>0.1 mm) in the wells where core samples or image logs are not available.