海洋天然气水合物降压开采过程中储层和井筒的变形与破坏机理研究

Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. There are tremendous amounts of natural gas hydrates distributed in the deep marine sediments and the cold permafrosts around the globe. Natural gas hydrate is expected to become one of the important alternative energy resources in the future. Because of the high cost of exploitation, the immature technology, and the high environmental risk, the gas hydrate exploitation needs to be studied deeply and fully technical reserve. This proposed project is going to carry out a basic research on the important issues related to the exploitation of marine gas hydrate, which is directly related to the energy supply, the energy security, and the sustainable development of economy and society for our country, and has an important strategic significance..During natural gas hydrate exploitation, the cementing property of the reservoir decreases and its mechanical strength reduces due to hydrate decomposition. This may lead to the destruction of the reservoir and wellbore and thus endanger the safety of exploitation. The aim of this research project is to ensure the safety of the reservoir and wellbore for gas production from marine gas hydrate deposits by depressurization. In order to achive this goal, the studies on the stress-strain relationship and shear strength of gas-hydrate-bearing sediments, the multiphysics coupling of thermo-hydro-mechanical-chemical (THMC) processes within the reservoir, and the deformation and failure of the reservoir and wellbore during gas production by depressurization would be carried out by use of the theoretical analyses and numerical simulations. Through the systematic studies on the above problems, it is expected that (1) the three-dimensional multimechanism bounding surface model considering the dynamic influence of hydrate cementation will be developed for anisotropic gas-hydrate-bearing sediments; (2) the numerical simulation technology of fully coupled THMC accounting for the effect of sand production will be constructed for the gas hydrate exploitation; (3) the mechanism of multiphysics coupling interaction, the dynamic variation rule of physical and mechanical parameters, and the gas productivity of the reservoir will be revealed during gas production; (4) the characteristics of deformation and damage evolution for the reservoir and wellbore will be determined during gas production; and (5) the scientific assessment methods in the stability of the reservoir and wellbore by the depressurized exploitation will be presented, which can provide the theoretical basis and technical support for the safe exploitation of the marine gas hydrate reservoir for our country in the future.

天然气水合物广泛分布于深水海底沉积物或寒冷永久冻土中，有望成为本世纪重要的替代能源之一。由于天然气水合物开采成本大、技术不成熟、环境风险高，需要对其进行深入的研究和充分的技术储备。本项目拟对海洋天然气水合物开采的相关重要问题进行基础研究，它直接关系到将来我国的能源供给安全，具有重要的战略意义。项目拟采用理论分析和数值模拟相结合的方法，开展水合物沉积物的变形与强度特性、降压开采过程中储层的热-水力-力学-化学（相变）多场耦合、储层和井筒的变形与破坏等问题的研究，建立各向异性水合物沉积物三维多重机构边界面模型，构建考虑出砂影响的固液气三相多场全耦合天然气水合物开采数值模拟技术，揭示降压开采过程中储层的多场耦合作用机理、储层物性参数和产能的动态变化规律、储层和井筒的变形与损伤演化特征，提出降压开采储层和井筒稳定性的科学评价方法，为将来我国海洋天然气水合物的安全开采提供理论基础和技术支撑。

天然气水合物广泛分布于深水海底沉积物或寒冷永久冻土中，有望成为本世纪重要的替代能源之一。由于天然气水合物开采成本大、技术不成熟、环境风险高，因此有必要进行深入研究和充分的技术储备。本项目的研究重点是揭示海洋天然气水合物降压开采过程中储层的多场耦合机理以及储层和井筒的变形与损伤演化规律。运用理论分析、数值模拟等方法开展了系统研究。主要研究成果如下：.（1）考虑水合物胶结的动态变化，建立了与天然气水合物沉积物状态相关的三维多重剪切边界面模型和下加载面统一硬化本构模型。模拟了人工合成和原位采样天然气水合物沉积物在不同条件下的室内试验，结果表明这两个模型可以较好地描述水合物沉积物的各向同性压缩、应变硬化和软化、剪胀和剪缩、水合物分解引起的坍塌。.（2）建立了粒状土-结构接触面的状态相关三维多重剪切边界面模型。模拟了不同类型接触面的单调和循环剪切试验，结果表明该模型能够反映法向和剪切方向的耦合效应，较准确地模拟接触面的非线性力学行为。.（3）构建了固-液-气三相热-水力-力学-化学（相变）多场全耦合天然气水合物开采模拟的数学物理模型和数值分析方法，开发了天然气水合物开采固-液-气三相多场全耦合数值模拟软件。天然气水合物沉积物降压分解室内试验的数值模拟结果表明，该软件具有良好的多场预测精度。.（4）揭示了海洋天然气水合物降压开采过程中储层的多场耦合作用机理、储层和井筒的变形与损伤演化规律，构建了储层和井筒稳定性的科学评价方法，并提出了确保储层和井筒稳定的生产条件和完井方法。