天然气水合物是一种具有巨大全球资源量（约 3000万亿立方米CH4）和高储能能力（约170 v/v）的清洁替代能源，被认为是低碳转型过程的最佳过渡燃料。安全、高效开采水合物资源对于减少进口天然气依赖、满足国内日益增长的使用需求以及构建我国高效安全能源体系具有战略意义。近年来世界各国多次开展试采作业，但现阶段实现水合物商业化开采仍然存在重大挑战。这其中的核心科学问题是对开采过程中水合物沉积物复杂孔隙结构内的多相多组分多场耦合渗流特征的认识尚未明晰。因此，本文从孔隙尺度研究水合物沉积物微观渗流特性演化，应用数值模拟手段揭示多因素影响下单相及气水两相渗流规律。本文开发了一种根据目标粒径分布曲线生成非均质沉积物颗粒模型的算法。基于水合物富集区域的砂质沉积物粒径分布范围，构建了三个具有相同孔隙度和中值粒径，但不同粒径分布的非均质水合物沉积物模型（S1-S3）和一个均质模型（S0），并均设置0-50%系列饱和度的水合物。应用分水岭图像分割算法提取沉积物的复杂孔隙结构并定量化表征参数孔隙半径和吼道宽度。基于OpenFOAM对构建的水合物沉积物模型进行单相渗流模拟，应用达西定律计算有效渗透率和归一化渗透率。模拟结果表明水合物沉积物的非均质性越强，初始渗透率越低（分别为8.45 D、5.06 D和2.76 D，与实际储层贴近），且有效渗透率随水合物饱和度增加的下降率越小。此外，孔隙半径和孔喉宽度的分布范围更窄。提出了考虑粒径分布参数的改进Kozeny-Carman渗透率模型，提高了非均质水合物沉积物的预测准确度。构建了相对渗透率曲线及毛细管力测定的数值模拟方法。水合物饱和度越高，水相相对渗透率越小，而气相相对渗透率差异不明显；随着含水饱和度的提高，水相相对渗透率增加，且气水相互作用对气相流动的阻碍作用更显著。测得残余水饱和度为26%-34%，残余气饱和度为1%-2%。观察并分析了润湿性对渗流路径的影响，在水相对砂粒的接触角较小时，形成的优势通道清晰；接触角较大时，渗流通道相互连通并呈片状流动。综上，本文从孔隙结构演化及模拟流动过程的视角揭示了颗粒非均质分布、水合物饱和度等多因素共同作用下水合物沉积物渗透率的演化特性。为深入探究开采过程中的多场耦合渗流特性提供了孔隙尺度的模拟方法支撑，为解决我国南海泥质粉砂型水合物开发中的渗流难题提供了必要的理论基础。
Natural gas hydrate is a clean alternative energy source with enormous global resource reserves (~ 3000 trillion m3 CH4) and high energy storage capacity (170 CH4 v/v methane hydrate). It is considered the best transition fuel for the low-carbon transition process. The safe and efficient exploitation of hydrate resources is of strategic significance for reducing dependence on imported natural gas, meeting the growing domestic demand, and building an efficient and safe energy system in our country. In recent years, various countries around the world have carried out trial mining operations many times, but there are still major challenges in realizing commercial hydrate mining at this stage. The core scientific issue is that the understanding of the multi-phase, multi-component, and multi-field coupled seepage characteristics within the complex pore structure of hydrate sediments during the mining process has not yet been clearly understood. Therefore, this paper studies the evolution of microscopic seepage characteristics of hydrate sediments from the pore scale and uses numerical simulation methods to reveal the single-phase and gas-water two-phase seepage rules under the influence of multiple factors.This paper develops an algorithm for generating heterogeneous sediment particle models based on target particle size distribution curves. Three heterogeneous hydrate sediment models (S1-S3) with the same porosity and median particle size but different particle size distributions were constructed. A series of hydrate particles with 0-50% saturation is set within those models. The watershed image segmentation algorithm was applied to extract the complex pore structure of sediments and quantitatively characterize the parameters of pore radius and channel width.Using OpenFOAM, a single-phase seepage simulation was performed on the constructed hydrate sediment model, and Darcy‘s law was applied to calculate the effective permeability and normalized permeability. The simulation results show that the stronger the heterogeneity of sediments, the lower the initial permeability (8.45 D, 5.06 D, and 2.76 D respectively, which is close to the actual reservoir), and the effective permeability increases with the hydrate saturation. The lower the rate of decline. In addition, the distribution ranges of pore radius and pore throat width are narrower. An improved Kozeny-Carman permeability model considering particle size distribution parameters is proposed to improve the prediction accuracy of heterogeneous sediments.A numerical simulation method for the relative permeability curve and capillary force determination was constructed. The higher the hydrate saturation, the smaller the water phase relative permeability, while the difference in gas phase relative permeability is not obvious. The relative permeability of water increases with the water saturation, and the gas-water interaction hinders the gas phase flow. more significant. The effect of wettability on the seepage path was observed. When the contact angle between water and sand is small, the dominant channel formed is clear; when the contact angle is large, the seepage channels are connected with each other and flow in a sheet-like manner.In summary, this paper reveals the evolution characteristics of hydrate sediment permeability under the combined action of multiple factors such as particle heterogeneous distribution and hydrate saturation from the perspective of pore structure evolution and simulated flow processes. It provides pore-scale method support for in-depth exploration of multi-field coupled seepage characteristics during the mining process and provides the necessary theoretical basis for solving seepage problems in the development of muddy silt-type hydrates in the South China Sea.
