On the resource evaluation of marine gas-hydrate deposits using seafloor compliance methods
On the resource evaluation of marine gas-hydrate deposits using seafloor compliance methods
复制标题
基于海底达标法的海洋天然气水合物矿床资源评价
DOI：
10.1111/j.1365-246x.1997.tb06610.x
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发表时间：
1997
影响因子：
2.8
通讯作者：
R. N. Edwards
中科院分区：
文献类型：
--
作者：
E. Willoughby;R. N. Edwards
总结
甲烷水合物是水和气体甲烷的固体、非化学计量的混合物。它们存在于世界各地的海底沉积物中，估计海底沉积物的总质量超过1 016公斤。由于每一体积的水合物可以产生多达164体积的天然气，近海甲烷水合物被认为是一种非常重要的天然能源。
水合物带的深度范围和稳定性由甲烷和水合物混合物的相图决定，由环境压力和温度决定。在大于约300米的海洋深处，压力足够高，温度足够低，足以在海底形成水合物。水合物在沉积物中的比例通常随深度增加而增加。水合物带的底部是固体水合物与游离气体和水之间的相边界。它的深度主要由地热梯度值决定，在地震剖面上突出为明亮的反射。扩散的上边界没有很好地标记，因此水合物的总质量不能仅通过地震测量来确定。
海洋表面重力波引起低频、水平传播的压力场，使海底变形。海底的位移取决于大洋地壳的密度和弹性参数，特别是剪切性质。海底柔度是海底变形和压力之间的传递函数，是频率的函数。在特定频率下进行的顺应性测量被调谐到特定深度处的结构。甲烷水合物就像永冻层中的冰一样，改变了其所在物质的物理性质，降低了密度，同时增加了压缩速度，特别是剪切速度。我们应用Crawford、Webb和Hildebrand（1991年）的方法，并说明如何增加对剪切速度变化特别敏感的柔度数据，以帮助评估资源。
两个勘探方案进行了研究，通过数值模拟。在第一个例子中，一个非常简单的例子说明了遵约反应的一些基本特征。该部分的大多数特性，包括水合物区的可能区域厚度（200米），都是通过地震勘测和现场钻探而确定的。水合物在有效孔隙空间中的量是唯一的自由参数。以百分比表示的水合氯离子含量可确定为约±2 μ g/g，给出的顺应性测量误差为± 0.0%。该规则适用于预期水合物含量值的整个范围。
在第二种情况下，假设先验信息较少。需要进行补充合规调查，以查明海底以下约200米厚的水合物区的厚度和水合物含量，这些水合物区的可用孔隙空间中水合物含量分别高达20%和40%。线性本征函数分析表明，对于这两个模型，水合物的总质量，水合物含量和厚度的乘积，可以估计到约2.81千分之一和1.83千分之一，分别准确度，给定的遵守测量的准确度为百分之十。
SUMMARY
Methane hydrates are solid, non-stochiometric mixtures of water and the gas methane. They occur worldwide in sediment beneath the seafloor and estimates of the total mass available there exceed 1016 kg. Since each volume of hydrate can yield up to 164 volumes of gas, off-shore methane hydrate is recognized as a very important natural energy resource.
The depth extent and stability of the hydrate zone are governed by the phase diagram for mixtures of methane and hydrate, determined by ambient pressures and temperatures. In sea depths greater than about 300 m, the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor. The fraction of hydrate in the sediment usually increases with depth. The base of the hydrate zone is a phase boundary between solid hydrate and free gas and water. Its depth is determined principally by the value of the geothermal gradient, and stands out on seismic sections as a bright reflection. The diffuse upper boundary is not as well marked so that the total mass of hydrate cannot be determined by seismic measurements alone.
Ocean surface gravity waves induce a low-frequency, horizontally propagating pressure field which deforms the seafloor. The displacement of the seafloor depends on the oceanic crustal density and elastic parameters, particularly the shear properties. Seafloor compliance is the transfer function between seafloor deformation and pressure as a function of frequency. Compliance measurements made at specific frequencies are tuned to structure at specific depths. Methane hydrate, like ice in permafrost, changes the physical properties of the material in which it is found, decreasing the density while increasing the compressional and especially the shear velocities. We apply the method of Crawford, Webb & Hildebrand (1991) and show how the addition of compliance data, which is particularly sensitive to changes in shear velocity, can aid in the evaluation of the resource.
Two exploration scenarios are investigated through numerical modelling. In the first, a very simple example illustrates some of the fundamental characteristics of the compliance response. Most of the properties of the section including the probable regional thickness of the hydrate zone, 200 m, are assumed known from seismic surveys and spot drilling. The amount of hydrate in the available pore space is the only free parameter. Hydrate content expressed as a percentage may be determined to about ±2ɛ given compliance measurements with ɛ per cent error. The rule holds over the complete range of anticipated hydrate-content values.
In the second, less information is assumed available a priori. The complementary compliance survey is required to find both the thickness and the hydrate content in hydrate zones about 200 m thick beneath the seafloor, which contain up to 20 and 40 per cent hydrate in the available pore space, respectively. A linear eigen-function analysis reveals that for these two models the total mass of hydrate, the product of hydrate content and thickness, may be estimated to an accuracy of about 2.81ɛ and 1.83ɛ per cent, respectively, given compliance measurements with an accuracy of ɛ per cent.