海域天然气水合物降压开采诱发储层力学性质劣化及沉降规律建模研究

A modeling analysis of depressurization-induced mechanical property deterioration and subsidence in marine natural gas hydrate-bearing reservoirs

摘要:

降压法是海域天然气水合物储层开采的一种常见方法。降压开采会导致近井储层出现复杂的多物理场耦合响应，诱发压力变化、温度变化、水合物分解、储层力学性质劣化及地层沉降。本研究通过一种全耦合流固热化数值模型分析水平井筒降压导致的海域天然气水合物储层力学性质劣化及沉降特征，表征水平井筒及井周储层的多场耦合响应规律，明确储层力学性质劣化区域及沉降的影响因素。模拟结果显示：储层压力和温度变化的波及区域远大于水合物分解前缘，有效正应力的分布在不同方向差异明显，降压开采诱发的内聚力劣化区域与塑性区和水合物分解区关联程度高，水平井筒以浅区域和以深区域的沉降呈现出不同特征，沉降程度可相差5 mm以上。模拟结果对水平井降压开采海域天然气水合物的储层稳定性分析具有参考意义。

Abstract:

The depressurization method is a common approach for the exploitation of natural gas hydrate reservoirs in marine areas. This method can induce complex multi-physical coupling responses in the near-wellbore reservoir， leading to pressure changes， temperature variations， hydrate decomposition， deterioration of reservoir mechanical properties， and formation subsidence. This study utilizes a fully-coupled hydro-thermo-mechanical numerical model to analyze the mechanical property deterioration and subsidence characteristics of marine natural gas hydrate reservoirs caused by depressurization in horizontal wellbores， characterizing the multi-field coupling response laws of the horizontal wellbore and surrounding reservoir， and identifying the influencing factors of reservoir mechanical property deterioration and subsidence. The simulation results indicate that the affected area of reservoir pressure and temperature changes is much larger than the decomposition front of hydrates， and the distribution of effective normal stress varies significantly in different directions. The deterioration area of cohesion induced by depressurization is highly correlated with the plastic zone and hydrate decomposition zone. The subsidence characteristics in the shallow and deep areas relative to the horizontal wellbore exhibites distinct features. The simulation results provide a reference for the stability analysis of marine natural gas hydrate reservoirs during depressurization exploitation through horizontal wellbores.