海洋天然气水合物降压开采过程沉积物变形规律及微观机理研究

In recent years, China has successively discovered large-scale natural gas hydrate resources onshore and offshore, and the safe and efficient hydrate production is a major issue facing the utilization of natural gas hydrate resources in China. Depressurization is the method commonly used for gas hydrate production trial. In the previous three marine natural gas hydrate production trials in the world, the depressurization method was carried out. During the process of hydrate production by depressurization, the effective stress state of the formations and the hydrate saturation change with the decrease of pore pressure, leading to significant mechanical property differences between the hydrate dissociated and undissociated zones, potentially causing uneven deformation and local failure of the formations. This project is aimed at the microscopic mechanism of the deformation of hydrate-bearing sediments in different phases of the hydrate production by depressurization. Based on Computational Topography (CT) triaxial test system, the consolidation deformation tests of the sediments with and without hydrate dissociation will be carried out. The effects of the variations of effective stress state and hydrate dissociation on the sediment skeleton due to depressurization will be analyzed. The microscopic mechanism of sediment deformation during hydrate production by depressurization will be revealed. The critical state model for hydrate-bearing sediments will be upgraded. The upgraded constitutive model will be incorporated into a fully-coupled thermo-flow-mechanical numerical simulation to predict the deformation of marine hydrate-bearing sediments during the process of hydrate production by depressurization, which lays a theoretical fundamental for the stability assessment of the hydrate formations during hydrate production.

我国近年在陆地、海上相继探明大规模天然气水合物资源，对其安全、高效开采是我国天然气水合物资源利用面临的重大课题。降压法被广泛使用于天然气水合物试采中，世界上已成功实施的三次海洋天然气水合物试采均采用了降压法。降压开采水合物过程中，由于孔隙压力降低引起地层有效应力状态及水合物饱和度的变化，地层中水合物分解区域与未分解区域的力学特性具有明显的差异，潜在引起地层的不均匀变形和局部破坏。本项目针对降压开采不同阶段水合物沉积物宏观变形规律及其微观机理，基于CT三轴试验平台，开展水合物稳定与分解过程沉积物降压变形试验研究。分析降压引起的有效应力状态变化和水合物分解对沉积物骨架的影响，揭示水合物降压开采过程沉积物宏观变形的微观机理，修正水合物沉积物临界状态本构模型，结合热-流-力多场耦合数值模拟，实现水合物降压开采过程沉积物变形预测，为水合物开采过程地层稳定性评价奠定理论基础。

我国在南海成功实现了海上水合物生产试验，证明了粘土-粉砂质水合物储层的高产能。本项目按照既定研究目标，获取了水合物降压开采过程沉积物宏观变形以及颗粒运动、颗粒破碎、孔隙结构、水合物分布和胶结形态等微观结构变化规律，探明了降压开采引起的水合物沉积物宏观变形和微观结构变化机理，修正了水合物沉积物临界状态本构模型，建立了水合物降压开采多场耦合模型，实现了水合物降压开采过程地层变形计算和预测，为海洋天然气水合物降压开采过程地层稳定性评价提供理论基础。基于项目成果，发表SCI检索论文6篇，培养研究生4人，1人获得副教授职位。项目成果得到多家单位应用并获得显著成效，完成项目研究目标。