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Electrical anisotropy due to gas hydrate-filled fractures

机译:天然气水合物填充裂缝引起的电各向异性

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摘要

In 2006, the Indian National Gas Hydrate Program Expedition 01, or NGHP-01, discovered gas hydrate as fill in near-vertical fractures in unconsolidated sediments at several drilling sites on the Indian continental margins. These gas hydrate-filled fractures were identified on logging-while-drilling resistivity images. The gas hydrate-filled fracture intervals coincide with high measured resistivity at the NGHP-01 sites. High measured resistivity translates into high hydrate saturations via Archie's equation; however, these high saturations contradict lower gas hydrate saturations determined from pressure core and chlorinity measurements. Also, in intervals with near-vertical gas hydrate-filled fractures, there is considerable separation between phase shift and attenuation resistivity logs, with 2-MHz resistivity measurements being significantly higher than 400-kHz resistivity measurements. We modeled the sensitivity of the propagation resistivity measurements in the gas hydrate-filled fracture intervals at NGHP-01 Sites 5 and 10. Near-vertical hydrate-filled fractures can cause the abnormally high resistivity measurements in vertical holes due to electrical anisotropy. The model suggests the gas hydrate saturations in situ are usually significantly lower than those calculated from Archie's equation. In addition, these modeled gas hydrate saturations generally agree with the lower gas hydrate saturations obtained from pressure core and chlorinity measurements at NGHP-01 Sites 5 and 10.
机译:2006年,印度国家天然气水合物计划第01探险队(NGHP-01)在印度大陆边缘的几个钻探地点发现了天然气水合物作为未固结沉积物中近垂直裂缝的填充物。在随钻测井的电阻率图像上可以识别出这些充满天然气水合物的裂缝。天然气水合物充填的裂缝间隔与NGHP-01处测得的高电阻率相吻合。测得的高电阻率通过阿奇方程转化为高水合物饱和度;但是,这些高饱和度与通过压力岩心和氯测量确定的较低的天然气水合物饱和度相矛盾。同样,在接近垂直的天然气水合物充填裂缝的间隔中,相移和衰减电阻率测井曲线之间存在相当大的间隔,其中2MHz电阻率测量值明显高于400kHz电阻率测量值。我们对天然气水合物填充裂缝在NGHP-01站点5和10处的传播电阻率测量值的敏感性进行了建模,由于电各向异性,垂直水合物填充裂缝会导致垂直孔中异常高的电阻率测量。该模型表明,原位的天然气水合物饱和度通常明显低于根据阿奇方程计算的饱和度。此外,这些模拟的天然气水合物饱和度通常与从NGHP-01站点5和10的压力岩心和氯度测量获得的较低天然气水合物饱和度一致。

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