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.

中文翻译：

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水合物形成与离解过程中带缝隙排列的人工多孔岩声学各向异性的室内研究

裂缝可能在含水合物的储层中广泛存在，并可能引起各向异性的声学性质，这在准确评估裂缝性储层中起着重要作用。但是，关于裂缝性水合物储层的声各向异性响应的知识仍然知之甚少。为了获得这样的理解，我们设计并实施了专门的实验室实验，以测量人造砂岩中各向异性的声速，这些人造砂岩在水合物形成和分解过程中分别在20和50 MPa的围压下平行于各层平行排列成一字形的裂缝。我们发现，尽管较高围压下的速度在系统上较大，但在水合物形成和分解过程中，两种围压下的所有速度都会增加和减少，分别。我们还表明，在水合物饱和度大于10％时，水合物形成过程中的速度显着高于解离过程中的速度。发现在解离过程中，所有速度随饱和度的降低而平稳降低，而变化的速度在形成过程中表现出明显的梯度增加，其中在饱和度在10％和25％之间时，压缩波各向异性显着降低。对实验结果的分析表明，当水合物形成过程中水合物饱和度超过10％时，漂浮的水合物演化为桥接裂缝表面，而在解离过程中，桥接的水合物逐渐转变为漂浮在孔隙系统中。