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%。与结晶岩石在变形下容易破裂不同，页岩可以起到密封的作用，因为它们更柔韧，在某种意义上说，它们可以经历明显的变形而不会破裂。然而，它们也表现出明显的随时间变化的变形行为或蠕变，这是在空间和时间尺度上观察到的。页岩的蠕变趋势与岩石中较软和较硬的部分如何分担施加的载荷密切相关。此外，蠕变过程中试样体积的减小表明，这种现象是由固体颗粒之间孔隙空间的压实所调节的。后一种过程可能导致严重的地面沉降，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，该项目将研究页岩在空间和时间上的多尺度蠕变行为。室内试验包括纳米尺度下的压痕试验和毫米尺度下的三轴试验。该项目的实验室实验与数值模拟相结合的活动将涉及本科生通过暑期研究参与。将制作实验室测试和数值模拟结果的宣传片，以招收研究生和外联。该项目的目标是在空间和时间的多个尺度上捕捉页岩的蠕变过程。尺度桥接技术将开发连接蠕变过程在纳米，微米和毫米尺度。页岩将被建模为较软物质（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。从之前的NSF项目“亚微米尺度页岩蠕变”发展而来的框架将被用于量化纳米压痕试验中较软物质的蠕变和微压痕试验中页岩基质的蠕变。在纳米和微压痕期间的保持时间将被定时，以充分描述蠕变压痕的预期指数衰减。该模型的验证将在毫米尺度上进行三轴蠕变试验。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数小时甚至数天。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，以描绘不同尺度下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，因为这种材料的高度非均质性。结合实验-数值研究将适应非均质性，并将更好地理解和更真实地模拟页岩在多个尺度上的蠕变过程。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.