Creep in Shale Across Space and Time

页岩是一种细粒沉积岩，由粘土和有机物等较软的物质以及石英、长石、黄铁矿和碳酸盐等较硬的矿物组成。它是地球上最常见的沉积岩，估计占所有沉积岩的44%到56%。与变形时容易破裂的结晶岩不同，页岩可以起到密封的作用，因为它们更柔韧，可以经历显著的变形而不会破裂。然而，众所周知，它们也表现出显著的时间相关的变形行为，或蠕变，这是在空间和时间尺度上观察到的。页岩的蠕变倾向与这种岩石中较软和较硬的成分如何分担施加的载荷密切相关。此外，蠕变过程中样品体积的减少表明，这种现象是通过固体颗粒之间的孔隙空间压实来调节的。后一过程可能导致重大地面下沉，这往往会损害民用基础设施的完整性和可持续性。利用实验室和数值模拟技术，该项目将研究页岩中跨越空间和时间的多尺度蠕变行为。实验室实验包括在纳米尺度上探索蠕变行为的压痕试验，以及在毫米尺度上研究蠕变的圆柱形岩石试件的三轴试验。该项目的实验室实验和数值模拟相结合的活动将包括本科生通过暑期研究参与。将为招收研究生和户外活动制作实验室测试和数值模拟结果的宣传片。该项目的目标是在空间和时间上以多个尺度捕捉页岩中的蠕变过程。将开发出将纳米、微米和毫米尺度的蠕变过程联系起来的尺度桥接技术。页岩将被模拟为较软物质(粘土、有机物)和较硬物质(石英、长石、黄铁矿、碳酸盐)的混合物，其蠕变行为由粘塑性理论描述。根据美国国家科学基金会先前项目开发的一个名为“亚微米级页岩蠕变”的框架将被用来量化来自纳米压痕测试的较软物质的蠕变和来自微压痕测试的页岩基质的蠕变。将对纳米和微压痕过程中的保持时间进行计时，以充分描述蠕变压痕的预期指数衰减。模型的验证将通过毫米尺度的三轴蠕变试验进行。压痕蠕变试验通常持续不超过几分钟，而三轴蠕变试验持续几个小时甚至几天。这项研究将调和这两个实验在时间尺度上的巨大差异。在整个测试过程中，将使用透射X射线显微镜(TXM)、微型计算机断层扫描(MCT)和常规计算机断层扫描(CT)对材料的微观结构进行成像，以描绘不同尺度上页岩的空间非均质性。页岩的蠕变是岩石工程中最有趣和最具挑战性的现象之一，因为这种材料具有高度的非均质性。实验-数值相结合的研究将适应异质性，并将导致更好地理解页岩中的蠕变过程，并在多个尺度上建立更现实的模型。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Shale is a fine-grained sedimentary rock composed of softer materials such as clay and organics, as well as stiffer minerals such as quartz, feldspar, pyrite, and carbonates. It is the most common sedimentary rock on Earth, estimated to represent between 44 and 56 percent of all sedimentary rocks. Unlike crystalline rocks that tend to fracture under deformation, shales can serve as a seal because they are more pliable in the sense that they can undergo significant deformation without breaking. However, they are also known to exhibit significant time-dependent deformation behavior, or creep, that is observed across spatial and temporal scales. The tendency of shale to creep is well correlated with how the softer and stiffer components of this rock share an imposed load. In addition, the reduction in sample volume during creep suggests that this phenomenon is accommodated by compaction of the pore spaces between the solid grains. The latter process can lead to significant ground subsidence that often compromises the integrity and sustainability of civil infrastructures. Using laboratory and numerical modeling techniques, this project will investigate the multiscale creep behavior in shale across space and time. Laboratory experiments include indentation tests to probe the creep behavior at the nanometer scale, and triaxial tests on cylindrical specimens of rock to investigate creep at the millimeter scale. The combined laboratory experimentation-numerical simulation activities of the project will involve the participation of undergraduate students through summer research. A promotional video of the laboratory tests and numerical simulation results will be produced for recruiting graduate students as well as for outreach.The objective of this project is to capture the creep processes in shale at multiple scales in both space and time. Scale-bridging techniques will be developed linking creep processes at the nanometer, micrometer, and millimeter scales. Shale will be modeled as a mixture of softer matter (clay, organics) and stiffer matter (quartz, feldspar, pyrite, carbonates) whose creep behavior is described by Viscoplasticity Theory. A framework developed from a previous NSF project entitled "Creep in Shale at Submicron Scale" will be employed to quantify creep of the softer matter from nano-indentation tests and creep of the shale matrix from micro-indentation tests. Hold periods during nano- and micro-indentation will be timed to fully delineate the expected exponential decay of creep indentation. Validation of the model will be conducted from triaxial creep tests at the millimeter scale. Indentation creep tests typically last no more than a few minutes, whereas triaxial creep tests last for several hours and even days. This research will reconcile this very large discrepancy in the time scales for these two experiments. Throughout the course of testing, the microstructure of the material will be imaged using Transmission X-ray Microscopy (TXM), micro-Computed Tomography (mCT), and conventional Computed Tomography (CT) to delineate the spatial heterogeneity of shale at different scales. Creep of shale is one of the most intriguing and challenging phenomena in rock engineering because of the highly heterogeneous nature of this material. The combined experimental-numerical investigation will accommodate heterogeneity and will lead to better understanding and more realistic modeling of creep processes in shale at multiple scales.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.