天然气水合物降压分解诱发储层变形破坏正交数值模拟实验研究

Experimental Study with Orthogonal Numerical Simulation on Reservoir Deformation and Failure Induced by Depressurization and Decomposition of Natural Gas Hydrate

使用降压法进行天然气水合物开采时,天然气水合物沉积层的失稳破坏是制约其有效开发的关键因素。考虑水合物饱和度和有效应力同时变化对水合物沉积层主要力学参数的影响,建立天然气水合物降压分解诱发储层变形破坏的流固耦合弹塑性模型,编制USDFLD子程序,采用正交数值模拟实验方法,研究初始水合物饱和度、井底压力和有效主应力差对水合物沉积层近井储层变形破坏影响的敏感程度。结果表明:井底压力和有效主应力差是影响近井储层变形破坏的2个显著因素;对近井储层等效塑性应变最大值的影响程度由大到小依次为井底压力、有效主应力差、初始水合物饱和度;对塑性区范围的影响程度由大到小依次为有效主应力差、井底压力、初始水合物饱和度;建议根据水合物藏的地质条件,对井底压力进行优化设计。研究成果对采用降压法进行天然气水合物开采的安全性、可控性具有重要意义。

In the exploitation of natural gas hydrate with depressurization method,the instability and failure of hydrate bearing sediment is a key factor restricting its effective development.Taking into account the effect of the simultaneous changes in hydrate saturation and effective stress on the main mechanical parameters of hydrate bearing sediment,a fluid-solid coupled elastoplastic model was established for the reservoir deformation and failure induced by depressurization and decomposition of natural gas hydrate,USDFLD subroutine was programmed,and a experimental method with orthogonal numerical simulation was adopted to study the sensitivity of initial hydrate saturation,downhole pressure and effective principal stress difference to the deformation and failure of hydrate bearing sediment near the well.The results demonstrated that the downhole pressure and the effective principal stress difference were two significant factors affecting the deformation and failure of the near-well reservoir,the effect on the maximum equivalent plastic strain of the near-well reservoir was descending in order of downhole pressure,effective principal stress difference and initial hydrate saturation,and the effect on the plastic range was descending in order of effective principal stress difference,downhole pressure and initial hydrate saturation.It is recommended to optimize the downhole pressure design according to the geological conditions of hydrate reservoir.The study results are of great significance to the safety and controllability of natural gas hydrate exploitation with depressurization method.