Abstract Rock mechanical properties are of key importance in coal mining exploration, coal bed methane production and CO2 storage in deep unmineable coal seams; accurate data is required so that geohazards (e.g. layer collapse or methane/CO2 leakage) can be avoided. In this context it is well established that coal matrix swelling due to water adsorption significantly changes the coal microstructure. However, how water adsorption and the associated with microstructural changes affect the mechanical properties is only poorly understood, despite the fact that micro-scale mechanical properties determine the overall geo-mechanical response as failure initiates at the weakest point. Thus, we measured nanoscale rock mechanical properties via nanoindentation tests and compared the results with traditional acoustic methods on heterogeneous medium rank coal samples in both dry and brine saturated conditions. The microscale heterogeneity of the rock mechanical properties was mapped and compared with the morphology of the sample (measured by SEM and microCT). While the nanoindentation tests measured decreasing indentation moduli after water adsorption (−60% to −66%), the traditional acoustic tests measured an increase (+17%). We concluded that acoustic tests failed to capture the accurate rock mechanical properties changes for the heterogeneous coal during water adsorption. It is thus necessary to measure the coal rock mechanical properties at the microscale to obtain more accurate data and reduce the risk of geohazards.

中文翻译：

---

非均质煤吸水后纳米级岩石力学性质变化

摘要 岩石力学特性对于深部不可采煤层的煤矿勘探、煤层气生产和二氧化碳封存具有重要意义；需要准确的数据，以避免地质灾害（例如地层坍塌或甲烷/二氧​​化碳泄漏）。在这种情况下，众所周知，由于吸水引起的煤基质膨胀显着改变了煤的微观结构。然而，尽管微观力学性能决定了整体地质力学响应，但水吸附及其与微观结构变化相关的力学性能如何影响力学性能却知之甚少，因为失效始于最薄弱点。因此，我们通过纳米压痕测试测量了纳米级岩石力学特性，并将结果与​​传统声学方法在干燥和盐水饱和条件下的非均质中等煤样上进行了比较。绘制了岩石力学特性的微观非均质性，并与样品的形态（通过 SEM 和 microCT 测量）进行了比较。纳米压痕测试测量的是吸水后压痕模量降低（-60% 至 -66%），而传统声学测试测量的是增加（+17%）。我们得出结论，声学测试未能捕捉到非均质煤在吸水过程中的准确岩石力学特性变化。因此，有必要在微观尺度上测量煤岩力学特性，以获得更准确的数据，降低地质灾害风险。