Abstract:

In oil and gas development engineering, shale may serve either as a reservoir subjected to repeated hydraulic fracturing or as a caprock subjected to long-term gas injection and production in underground gas storage. In both cases, it is exposed to the coupled effects of cyclic loading and fluid invasion. Therefore, clarifying the deformation behavior, damage accumulation, and instability mechanisms of shale under such coupled conditions is of great significance for understanding its mechanical response and evaluating its engineering stability. In this study, Fuling shale was selected as the research object. Uniaxial monotonic loading tests and graded cyclic loading tests were conducted under three conditions, namely dry, oil-saturated, and water-saturated states. In addition, cyclic loading tests within a high-stress range were carried out under the water-saturated condition. Through these tests, the evolution laws of shale strength, deformation, residual strain, and energy dissipation under different fluid conditions were systematically analyzed, and the effects of fluid state on the cyclic mechanical behavior of shale were further compared. The results show that dry shale exhibits the highest overall strength, whereas water saturation significantly weakens both the strength and stiffness of shale and markedly increases the proportion of energy dissipation during deformation and failure. Oil saturation has only a limited influence on the elastic modulus, but it enhances the residual deformation and energy dissipation during cyclic loading. Under all three fluid conditions, cyclic instability occurs when the stress level is still lower than the peak stress under monotonic loading, indicating that shale may become unstable before reaching its monotonic peak strength under repeated loading. Meanwhile, the corresponding peak total strain at instability is close to that at monotonic failure. Before the onset of instability, the growth rate of residual strain shows a pronounced increasing trend, reflecting the accelerated accumulation of irreversible deformation. Under all three conditions, the evolution of cyclic residual strain exhibits clear stage characteristics: the deformation in the first cycle is relatively large, then decreases and tends to stabilize, and finally increases rapidly again as failure approaches, showing an obvious stage-dependent pattern. These findings indicate that, compared with traditional evaluation methods based only on peak stress or uniaxial compressive strength, the criteria based on total strain, the growth rate of residual strain, and the energy dissipation ratio are more suitable for evaluating wellbore stability during shale oil and gas development and during long-term gas injection and production in underground gas storage. Therefore, these parameters can provide more appropriate indicators for stability assessment under coupled cyclic loading and fluid invasion conditions.

Key words: shale, oil/water saturation, cyclic loading test, residual strain, energy dissipation, rock mechanics

摘要：

在油气开发工程中，页岩既可作为储层经历重复压裂，亦可作为盖层承受储气库长期注采，二者均面临循环加载与流体入侵的耦合作用。因此，揭示页岩在此耦合条件下的变形、损伤累积与失稳机制具有重要意义。本文以涪陵页岩为对象，开展了干燥、饱和油和饱和水3种状态下的单轴单调加载、梯度循环加载，以及饱和水条件下高应力区间循环加载实验，系统分析了不同流体状态下页岩的强度、变形、残余应变与能量耗散演化规律。结果表明：干燥页岩的整体强度最高，水饱和则显著削弱页岩强度与刚度，并显著提高了能量耗散占比；油饱和对弹性模量影响有限，但会增强循环残余变形和能量耗散。不同饱和流体下，页岩均在应力低于单调峰值应力时即发生循环失稳，且失稳时对应的峰值总应变与单调破坏条件相近。临近失稳前，残余应变的增长速率呈明显升高趋势。3种状态下，循环残余应变的演化均表现出阶段性特征：首个循环变形较大，随后减小并趋于稳定，临近破坏前再次快速升高。由此可见，相比传统仅依据峰值应力或单轴抗压强度的评价方法，基于总应变、残余应变增长率及能量耗散比的判别指标，更适用于页岩油气开发及储气库长期注采过程中的井壁稳定性评价。

关键词: 页岩, 饱和油/水, 循环加载实验, 残余应变, 能量耗散, 岩石力学

CLC Number: 

• TU458.3

• TE3