循环应力对储气库岩石微观结构及力学性质影响数值模拟

Numerical simulation for the effect of cyclic stress on microstructure and mechanical properties of reservoir rocks during operations of gas storage

关键词:  循环应力  储气库  微观结构  力学性质  离散元

Key words:  cyclic stress  gas storage  microstructure  mechanical properties  discrete element

摘要:

针对储气库循环注采过程中循环应力对储气库岩石产生疲劳损伤的问题。利用颗粒离散元法建立岩心真三轴数值模型,根据岩心室内试验标定数值模型参数,然后对数值模型进行多周期等幅值轴向循环应力加载,研究循环应力对储气库岩石微观结构及力学性质的影响。结果表明:循环应力加载过程中,模型内生成的无黏结接触数量和微裂缝数量逐渐增多,而生成位置受原始裂缝控制明显,模型孔隙度逐渐减小,且模型内无黏结接触数量、微裂缝数量和孔隙度的变化速率均呈递减趋势;相同应力水平下模型侧向应变和轴向应变均增大,但侧向应变占主导地位;模型弹性模量逐渐降低,而模型泊松比和损伤量逐渐增加,且模型弹性模量、泊松比和损伤量的变化速率均呈递减趋势。

Abstract:

In order to investigate the fatigue damage of reservoir rocks caused by cyclic stress in the process of frequent injection and production for gas storage, a true triaxial numerical model of rock core was established based on a particle discrete element method in this paper. The parameters of the core model were calibrated according to the results of laboratory experiments,and then a multi-period axial cyclic stress loading with equal-amplitude was applied to the core model to study the effect of cyclic stress on the microstructure and mechanical properties of the rocks during the operation of gas storage. The results show that during the cyclic stress loading, the number of non-bonded contacts and micro-cracks in the core model increases gradually, and the initiation position of those cracks is obviously controlled by the original cracks in the core. The porosity of the core model decreases gradually, and the variation rates of the non-bonded contacts number, micro-cracks number and porosity in the model also show a decreasing trend. Under a same cyclic stress, both the lateral and axial strains of the core model increase, while the lateral strain dominates. The elastic modulus of the model decreases gradually, while the Poisson 's ratio and the failure number of the core model increase gradually, and the variation rate of the elastic modulus, Poisson 's ratio and failure of the model decreases gradually.