天然气水合物（简称水合物）是21世纪理想的清洁替代能源，主要存在于深海沉积物和永久冻土带中。目前已有多个国家开展水合物的试采工作，日本在2013年实现全球首次近海水合物试采，我国在2017年成功地连续试采60天。

在试采过程中，水合物的分解会引起地层孔压增加和有效应力降低，导致土体软化，对海洋结构如井口造成破坏。自然界中，在上覆层渗透率较低的情况下，水合物因相平衡破坏而发生分解后，产生的甲烷气体的渗漏速率远小于气体的聚集速率，导致超孔隙压力的累积，从而挤压周围土体，引起井口周围土体大变形，甚至导致气体喷发现象，因此需要对井口周围土层的稳定性进行研究和预测。主要开展了如下的工作：

首先，根据现场得到的地质数据，结合理论分析得到水合物的分解范围随时间的变化，对开采区域由水合物分解导致井口周围土体局部变形进行数值模拟分析，评估井口安全性。在开采范围为井口周围3m的条件下，模拟得到的最大竖向位移量约为2.0-3.0cm，对井口的安全不会构成威胁。但随着分解范围的增加，储层厚度越厚，土层竖向位移量越大，越容易达到大面积塑性破坏，导致出砂、井口坍塌等灾害。

然后，针对在水平井开采水合物的过程中，水合物的分解和水平井的存在对地层稳定性影响进行了分析。从模拟结果看出，在用水平井开采的过程中，随着水合物分解半径的逐渐变大，竖向位移也逐渐变大，最大竖向位移主要发生在水平井孔的上方，水平位移也逐渐变大，土体呈扩张趋势。土体竖向位移相对水合物储层厚度较小，水平位移的影响可以忽略不计。

最后，对百慕大地区出现的由甲烷气体爆炸而产生的巨坑的现象进行了分析，首先进行了量纲分析，找出主要影响参数。在此基础上，采用商用数值模拟软件进行了破坏机制分析。结果表明，水合物分解区半径与上覆层厚度的值越大，超孔隙压力比越小，越容易发生破坏。

关键词：水合物分解，开采，井口安全性，气体喷发

Natural gas hydrate is an ideal clean alternative energy in the twenty-first Century. It mainly exists in deep-sea sediments and permafrost zones. The trial production of gas hydrate has been carried out inseveral countries. In 2013, Japan successfully carried out the first trial production of offshore hydrate in the world. In 2017, China realized trial exploitation lasting for 60 days.

During the exploitation, the decomposition of hydrates will cause the increase of pore pressure and the decrease of stratum’s effective stress, resulting in softening of soil, damage to offshore structure such as wellbore. At the same time, if the permeability of overlying soil is low, the drainage rate of methane gas produced by the decomposition of phase equilibrium is much less than that the gas accumulation rate, which leads to the accumulation of excess pore pressure. Accordingly the surrounding soil is extruded, resulting in large deformation of the soil around the wellbore and even the phenomenon of gas eruption. Therefore, it is required to study the stability of the sediment surrounding the wellbore and the gas-induced eruption. The main studies are as follows.

Firstly, the decomposition range of the hydrate in a certain time is obtained by the theory according to the geological data obtained from the site. In view of the possible location of the exploitation zone, the local deformation of the sediment caused by hydrate decomposition is numerically simulated, and the wellbore safety is also evaluated. Under the condition of decomposition zone of 3m, the maximum settlement is about 2.0-3.0cm. It is safety of the wellbore in this case. However, with the increase of the decomposition zone and the thickness of gas hydrate layer, the settlement of the sediment increases, and zone with plastic damage becomes larger, which can result in sand production, wellbore collapse and other disasters.

Secondly, we analyzed the influences of hydrate decomposition and the occurrence of horizontal well on the formation stability during the process of hydrate exploitation. It can be seen that the settlement also becomes larger with lager hydrate decomposition radius during exploitation with the horizontal well. The maximum settlement occurs mainly over the horizontal well, and the horizontal displacement increases gradually. The vertical deformation relative to the hydrate is relatively small. The influence of horizontal displacement can be neglected.

Thirdly, the phenomenon of gas eruption is analyzed referencing the large holes caused by gas explosion after gas hydrate decoposition in Bermuda area. The main factors are found based on dimensional analysis. On this basis, the numerical simulation is carried out to analyze the specific conditions. The results show that the value of the radius of the hydrate decomposition zone and the thickness of the overlying soil layer is larger, the excess pore pressure ratio is smaller, the damage occurs more easily.

Key Words: Hydrate decomposition, exploitation, wellhead safety, gas eruption