Abstract Coal matrix is subjected to shrinkage as a result of gas desorption. This can contribute to the stress distribution changes around the wellbore during the exploitation of Coal Seam Gas (CSG) resources. The knowledge of these stresses is essential for coal shear failure prediction and assessing coal fines production risks, particularly in horizontal wells which nowadays are becoming common in CSG fields. This study introduces an analytical model to estimate the stress distribution around the wellbore by coupling the effects of depletion, matrix shrinkage, and wellbore trajectory. The model is applied for coal failure analysis in both vertical and horizontal wells in Moranbah Coal Measures, in the Bowen Basin eastern Australia. The results reveal that the stress path value in CSG reservoirs, is not constant during production and it can even be more than one due to the matrix shrinkage. The shrinkage effect can significantly alter the effective horizontal stresses which accordingly can cause a considerable change to the near-wellbore stress distribution. It is shown that the stress differential may increase or decrease, depending on shrinkage/swelling magnitude and wellbore trajectory. In a normal fault stress regime, the matrix shrinkage decreases stress differential on the wellbore wall in vertical wells and consequently, reduces the coal failure potentials. However, in highly deviated and horizontal wells, the matrix shrinkage causes an extra increase of the tangential stress and reduction of radial stress on the wellbore wall (i.e., greater stress differentials on the wellbore wall). This increases the coal failure risks with depletion. The model was verified versus the observed coal failure pressure in the San Juan Basin, USA.

中文翻译：

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基质收缩对煤层气井筒应力的影响：以东澳大利亚 Bowen 盆地为例

摘要 煤基体因气体解吸而发生收缩。在煤层气 (CSG) 资源的开采过程中，这会导致井眼周围的应力分布发生变化。了解这些应力对于煤剪切破坏预测和评估煤粉生产风险至关重要，尤其是在如今在 CSG 油田中变得普遍的水平井中。本研究引入了一种分析模型，通过耦合损耗、基质收缩和井眼轨迹的影响来估计井眼周围的应力分布。该模型应用于澳大利亚东部 Bowen 盆地 Moranbah Coal Measures 的垂直井和水平井的煤层失效分析。结果表明，CSG 储层的应力路径值，在生产过程中不是恒定的，由于基体收缩，它甚至可能不止一个。收缩效应会显着改变有效水平应力，从而导致近井眼应力分布发生相当大的变化。结果表明，应力差可能会增加或减少，具体取决于收缩/膨胀幅度和井眼轨迹。在正断层应力状态下，基质收缩降低了直井井壁上的应力差，从而降低了煤层破坏的可能性。然而，在大斜度井和水平井中，基质收缩会导致井壁切向应力的额外增加和径向应力的减小（即井壁上的应力差更大）。这增加了煤炭枯竭的风险。该模型与在美国圣胡安盆地观察到的煤破坏压力进行了验证。