A synchronous measurement system for fully characterizing anisotropic elastic responses of fractured rocks during hydrate evolution

Fracture-filling hydrate reservoirs that have been extensively discovered commonly show anisotropic elastic properties. Comprehensive understanding of anisotropic elastic responses is an important premise to accurately assess the fracture-filling hydrate reservoirs. Rock physics experiment is one of the best means to understand the elastic responses of hydrate-bearing reservoirs. However, a synchronous measurement system for understanding the anisotropic elastic responses in fracture-filling hydrate reservoirs is still missing. In this paper, a novel experimental system that can synchronously measure five anisotropic acoustic velocities of fractured rocks during hydrate evolution to fully characterize their anisotropic elastic responses is reported. A synchronous experiment using a fractured rock with evolving hydrate was conducted to verify the validity of the synchronous measurement system and understand the anisotropic elastic responses in fracture-filling hydrate reservoirs. We find that hydrate formation and dissociation, respectively, increase and decrease the measured velocities, and the increasing gradients among the anisotropic velocities with forming hydrate are different when hydrate saturation is higher. We also find that the elastic anisotropic parameters calculated based on the measured velocities are dependent on hydrate saturation, and the compressional wave anisotropy ε sharply reduces with increasing hydrate as saturation exceeds about 10% and quickly enhances at the initial stage of hydrate dissociation. The results are reasonably analyzed and interpreted by the shaping and decomposition of bridging hydrate morphology in the aligned fractures. The results provide new insights into the anisotropic elastic responses and their influencing mechanism in fracture-filling hydrate reservoirs.