Creep in Shale Across Space and Time

Stanford University, Stanford CA

Investigators

Abstract

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.