胶结型水合物对黏土质沉积物力学特性影响规律

Influence laws of cemented hydrate on the mechanical behaviors of clayey sediments

研究低有效应力状态下胶结型水合物对黏土质沉积物力学特性的影响规律,可以为预测我国南海浅层天然气水合物开采风险和保证储层安全提供理论支持。为此,基于水合物三轴试验平台,制备了不同饱和度的胶结型水合物,开展了固结排水剪切试验,研究了低有效围压状态下含胶结型水合物黏土质沉积物强度和变形特性。研究结果表明:①胶结型水合物的存在会降低黏土质沉积物的初始固结程度,并有效提高其弹性模量和破坏强度,在低有效围压状态下沉积物的黏聚力与内摩擦角均会有不同幅度的增长;②含高饱和度水合物沉积物初始表现为较明显的剪胀现象,而随着有效围压升高,所有含水合物沉积物均表现为持续剪缩现象;③含水合物黏土质沉积物剪切变形主要受初始固结程度、水合物胶结作用和有效应力共同影响。结论认为,水合物开采过程中胶结弱化导致的强度损失和水合物相变导致的固结变形是黏土质储层面临的重要风险之一。

The commonly used exploitation methods of natural gas hydrate(hereinafter referred to as hydrate)are mainly the individual depressurization method or other assisted depressurization methods,which decompose hydrate by reducing reservoir pore pressure below the hydrate phase equilibrium pressure,so as to realize the goal of natural gas production.Researching the influence laws of cemented hydrate on the mechanical behaviors of clayey sediment under the state of low effective stress can provide a theoretical support for predicting the risks of shallow hydrate exploitation in the South China Sea and ensuring the reservoir safety.Based on the hydrate triaxial test platform,this paper prepares cemented hydrate with different saturation,carries out consolidated drainage shear tests,and studies the strength and deformation behaviors of cemented hydrate-bearing clayey sediments under the state of low effective confining pressure.And the following research results are obtained.First,the presence of cemented hydrate can decrease the initial consolidation degree of clayey sediments and increase the elastic modulus and failure strength effectively.And under low effective confining pressure,the cohesion and internal friction angle of sediments show different increase amplitudes.Second,the sediments with high-saturation hydrate initially exhibit more pronounced shearing-dilation behavior,while all hydrate-bearing sediments performs continuous shearing shrinkage with the increase of effective confining pressure.Third,the shearing deformation of hydrate-bearing clayey sediments is primarily affected by the initial consolidation degree,the hydrate cementation effect and the effective stress.In conclusion,it is confirmed by the independently developed hydrate triaxial instrument that the strength loss caused by cementation weakening and the consolidation deformation caused by hydrate phase change in the process of hydrate exploitation are the important risks faced by clayey reservoirs.