天然气水合物（简称水合物）是我国的战略能源。勘探表明我国南海具有很大的水合物资源量，并于2017年进行了抽取水合物储层流体降压的试验性开采。但试验性开采过程产气量衰减很快，产气速率不能满足商业化开采的要求。国内外相关研究工作主要考虑气和水两相渗流，分析不同开采方式（注热、降压、CO₂置换等）对产气速率和产气量的影响规律，但尚未研究清楚水合物分解过程相变、传热与多相渗流的相互作用机制。本论文结合我国南海水合物降压试采工程，主要开展室内模型实验，研究天然气水合物降压分解过程中，包含水、天然气甚至细小颗粒多相在多孔介质中的渗流规律，以探讨试采工程中所发现的产气衰减规律。

首先，采用含水合物沉积物合成、分解及力学特性测量实验装置，以粉细砂为土骨架，开展了含水合物沉积物的样品制备和静、动三轴的力学特性测量，获得了高饱和度水合物对强度的影响特征和不同动载荷幅值下非饱和含水合物粉细砂土沉积物强度变化规律。结果表明：水合物饱和度超过55%以后，含水合物沉积物的静强度出现下降的趋势；水合物饱和度越高，水合物分解后的静强度越低；动载荷幅值/静强度超过0.5时，含水合物粉细砂沉积物的力学响应由振动压密转变为破坏液化的临界现象。这部分研究工作为水合物矿层降压过程中渗流实验设计提供基础力学参数。

再者，完善了水合物矿层降压过程中渗流实验模拟技术，制备水合物沉积层样品，进行了不同水合物饱和度条件下的降压（一次降压和分步降压）过程渗流实验，获得了水合物分解过程温度、压力和产气的基本数据，探讨了水合物分解相变阵面的传播规律。结果表明气体渗流前锋的传播距离与时间平方根成正比，沉积物初始温度略高于冰点时，由于水合物分解吸热，引起结冰和水合物二次生成，与前人结果一致；比较了一次降压和逐级降压两种开采方式，说明分步降压能够控制结冰和水合物二次生成，有利于延长快速产气的时间。上述结果与现场试采产气特征相一致，解释了试采产气衰减问题。

最后，为了弄清水合物分解过程土层中细小颗粒的运移特征，进行解耦分析，首先探讨了不含水合物土层中渗流的基本规律。研制了实验装置，探讨岩土中渗流导致出砂的基本现象和特征，分析了细颗粒含量、颗粒级配、渗流压差对出砂的影响，实验说明渗流致出砂现象主要取决于多孔介质中细颗粒含量及孔隙结构大小；出砂量与渗流压差呈现正相关，渗流压差越大，出砂程度越高。

本文开展的研究工作，初步地获得了天然气水合物分解过程土层中渗流的现象和基本规律，建议开展研究与渗流相关的压力、温度等多场的物理机制，进而提出实用的数值计算模型，为水合物试采/开采方案的制定与评价提供基本依据。

Gas hydrate is the strategic energy resource of the 21st century in China, and exists extensively in South China Sea. In 2017, the trial production of gas hydrate was conducted by the depressurization method. Test results show that the gas production rate reduces due to the lack of thermal supply, and can not meet the requirements of the commercial exploitation. Previous research work focuses on water-gas two phase seepage, and evaluates the production rate of different mining methods (thermal injection, depressurization, CO₂ displacement, etc.), while the mechanism of gas hydrate dissociation, heat transfer, and fluid seepage are not well understood. Experiments by depressurization-induced hydrate dissociation are conducted considering different hydrate saturations and depressurization modes to provide a reference for the data explanation of the trial production in South China Sea in this paper.

First, a series of tests on the static and dynamic properties of hydrate-bearing sediments (HBS) were conducted by using silt sand as skeleton. The effects of high saturation hydrate on the failure strength of HBS, and dynamic load amplitudes on the failure strength of unsaturated HBS were obatained. The test results show that when the hydrate saturation exceeds 55%, the failure strength decreases; the failure strength after hydrate dissociation decreases with the increase of hydrate saturation; the critical phenomenon occurs when the relative dynamic load amplitude exceeds 0.5, and the failure and liquification occur. The mechanical properties provide a reference for the experimental set-up of seepage in decompressing process for natural gas hydrate deposits.

Second, the gas hydrate deposits were prepared simulating the physical and mechanical parameters of in-situ gas hydrate stratum, and depressurization method was used to dissociate the hydrate into gas and water in the porous media. Different hydrate saturations were considered, and one-step and multi-step depressurization method were adopted. The temperatures, pressures, produced gas instant and cumulative volume during hydrate dissociation were recorded, and the propagation of the gas seepage length was calculated. The results show that the propagation length of the gas seepage is proportional to the square root of time. The freezing of released water from gas hydrate dissociation and the reformation of gas hydrate occur when the initial temperature is close to the freezing point. Multi-step depressurization can control the dissociation rate of gas hydrate, allow the temperature recovery of the deposits, and high gas production rate can be reached. The simulated results are consistent with the field trial test.

Third, the sand production is found to be a detrimental problem during depressurization gas hydrate production. We regard that the mechanism is seepage-induced fine grain migration in the porous media. Hence, the apparatus simulating the seepage-induced fine grain migration in deposits is developed. Different soils were adopted to prepare the deposits. The effects of grain size distribution, mass percentage of the fine grain, and pressure gradient on the sand production were investigated. The results indicate that under a same seepage pressure gradient, the sand production depends on the percentage of the fine grains and the pore structure of the porous media. Under a same grain size distribution, sand production increases linearly with the pressure difference.

In this paper, the seepage phenomenon during depressurization-induced gas hydrate dissociation in deposits is obtained. Subsequent research on the modeling of the multi-physical effects and the formulation of the seepage with fine grains during gas hydrate dissociation are suggested.