Abstract During methane production in CBM reservoirs, the influence of proppant embedment and permeability damage cannot be neglected – especially where the wall-rock is soft. Effective stresses are elevated during methane recovery, increasing both normal loading stress and confinement and simultaneously overprinting sorption-induced volumetric strains. Experiments and analytic modeling are conducted to define key mechanisms controlling these competitive effects. We independently measure overall sample compaction (external LVDT) and local strain (strain gauge) in the matrix to deconvolve proppant embedment in a propped fracture for different conditions of confining stress. The results show symptomatic behaviors of elastic (shale) and elastoplastic (coal) responses of embedment. Different from shale, the evolution of embedment is convex upwards with increased stress where indented depth increases more rapidly as loading stress increases under constant confinement. In addition, a stress-hardening effect is found to play a pivotal role in determining the characteristics of indentation, which are examined in terms of evolution profiles, deformation regimes, embedment slopes, curvatures, yield points and irreversible indentations. Based on the experimental observations a semianalytical model predicts indentation and the evolution of propped permeability under recreated in-situ stress conditions. A simplified case study is conducted to further illustrate the evolution of aperture and permeability of a propped fracture in CBM reservoirs. The modeling results suggest that proppant embedment is significantly overestimated if the variable stress-hardening (VSH) effect is neglected, especially when effective stress is large. Moreover, a decrease in indentation depth possibly occurs during late stage methane production, resulting in a reversal/recovery in fracture closure. This is because desorption-induced shrinkage becomes the predominant effect, causing an increase in aperture and a reduction in the indented volume of proppant. The resulting recovery in permeability implies that the propped coal fracture has the potential to optimally facilitate methane production as a pathway, even at high closure stresses generated by methane drainage.

中文翻译：

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支撑剂嵌入煤和页岩：应力硬化和吸附的影响

摘要 煤层气储层产气过程中，支撑剂嵌入和渗透破坏的影响不容忽视，尤其是围岩较软的地方。有效应力在甲烷回收过程中升高，增加了正常加载应力和约束，同时叠加了吸附引起的体积应变。进行实验和分析建模来定义控制这些竞争效应的关键机制。我们独立测量基质中的整体样品压实（外部 LVDT）和局部应变（应变仪），以针对不同的围压条件对支撑裂缝中的支撑剂嵌入进行解卷积。结果显示了嵌入的弹性（页岩）和弹塑性（煤）响应的症状行为。与页岩不同，嵌入的演变随着应力的增加而向上凸出，其中在恒定约束下，随着加载应力的增加，压痕深度增加得更快。此外，发现应力硬化效应在确定压痕特征方面起着关键作用，这些特征在演化剖面、变形机制、嵌入斜率、曲率、屈服点和不可逆压痕方面进行了检查。基于实验观察，半分析模型预测了在重建的原位应力条件下压痕和支撑渗透率的演变。进行了一个简化的案例研究，以进一步说明煤层气储层中支撑裂缝的孔径和渗透率的演变。建模结果表明，如果忽略可变应力硬化 (VSH) 效应，尤其是当有效应力很大时，支撑剂嵌入会被显着高估。此外，在后期甲烷生产过程中，压痕深度可能会减小，导致裂缝闭合逆转/恢复。这是因为解吸引起的收缩成为主要影响，导致孔径增加和支撑剂缩进体积减少。由此产生的渗透率恢复意味着，即使在甲烷排放产生的高闭合应力下，支撑煤裂缝也有可能以最佳方式促进甲烷生产作为一条途径。压痕深度的减少可能发生在后期甲烷生产过程中，导致裂缝闭合逆转/恢复。这是因为解吸引起的收缩成为主要影响，导致孔径增加和支撑剂缩进体积减少。由此产生的渗透率恢复意味着，即使在甲烷排放产生的高闭合应力下，支撑煤裂缝也有可能以最佳方式促进甲烷生产作为一条途径。压痕深度的减少可能发生在后期甲烷生产过程中，导致裂缝闭合逆转/恢复。这是因为解吸引起的收缩成为主要影响，导致孔径增加和支撑剂缩进体积减少。由此产生的渗透率恢复意味着，即使在甲烷排放产生的高闭合应力下，支撑煤裂缝也有可能以最佳方式促进甲烷生产作为一条途径。