Laboratory Investigations of Acoustic Anisotropy in Artificial Porous Rock With Aligned Fractures During Gas Hydrate Formation and Dissociation

摘要

Fractures can widely exist in hydrate-bearing reservoirs and can give rise to anisotropic acoustic properties that play an important role in the accurate assessment of fractured reservoirs. However, the knowledge of the responses of acoustic anisotropy in fractured hydrate-bearing reservoirs is still poorly understood. To obtain such understanding, we designed and implemented dedicated laboratory experiments to measure the anisotropic acoustic velocities in an artificial sandstone with aligned penny-shaped fractures parallel to the layers during hydrate formation and dissociation at the confining pressures of 20 and 50 MPa, respectively. We showed that although the velocities at higher confining pressure were systematically greater, all the velocities at both confining pressures increased and decreased during hydrate formation and dissociation, respectively. We also showed that the velocities during hydrate formation were significantly higher than those during dissociation at hydrate saturation greater than 10%. It was found that all the velocities smoothly reduced with decreasing saturation during dissociation, while the varying velocities exhibited distinct increasing gradients during formation, where the compressional wave anisotropy reduced dramatically at saturation between 10% and 25%. Analyses of the experimental results suggested that the floating hydrate evolved to bridge the surfaces of fractures when hydrate saturation exceeded 10% during hydrate formation, and the bridging hydrate gradually transformed to floating in the pore system during dissociation. The results provide new insights into hydrate distribution and its effects on acoustic anisotropy in fractured reservoirs during hydrate formation and dissociation and will contribute to improving the accuracy of hydrate assessment in such reservoirs.