This project will investigate fluid flow within rock masses in the shallow regions of Earth's crust, from the surface to ~10 km. Our work will focus on how dynamic stresses, for example caused by seismic waves, change flow properties of Earth's crust. Previous work shows that fluid permeability can change dramatically when rocks are shaken during earthquakes. The effects of strong shaking can be estimated, but the effects of weak shaking, for example due to a distant earthquake, are less well understood. We will perform laboratory experiments to investigate the processes and mechanisms that cause transient and permanent permeability changes due to dynamic stressing. The lab work will be coupled with theory and numerical methods to develop conceptual and quantitative models for permeability changes.
Elastic waves produced during earthquakes can trigger a range of phenomena including seismicity, volcanic eruptions, and geyser activity. Dynamic stressing via the passage of seismic waves (or from other sources of transient loads) can also increase spring discharge, fluid flow in streams, and oil production, in some cases tripling the effective permeability of the natural system. These observations have been attributed to shaking-induced changes in permeability of shallow aquifers. However, the underlying mechanisms and the affect of dynamic stresses on poromechanical properties of rocks are poorly understood. Here we propose to investigate permeability enhancement by dynamic stressing using a multidisciplinary approach. Our preliminary work shows clear evidence of permeability enhancement in fractured rock subject to fluid pressure oscillations. The proposed work will expand the laboratory data while developing the theory and focusing on the underlying mechanisms. We will use knowledge of the processes and mechanisms operative in the laboratory to address the problem of upscaling our results to field conditions. We propose a series of experiments and models informed by observations of natural systems to (1) establish clear relationships between the controlling variables and the resulting changes in permeability, (2) analyze the physics of the enhancement and identify the underlying processes and (3) build appropriate numerical models of the results that can be applied at the laboratory and field scales. Results of the proposed experiments are expected to have significant impact on understanding fluid flow in the Earth's crust and seismic hazard. Understanding the physical basis for transient changes in permeability will lead to improved engineering approaches for oil reservoir and hydrological use.
This project focused on processes that influence fluid flow within rock masses in the shallow regions of Earth’s crust, from the surface to ~ 10 km. We investigated how dynamic stresses, for example caused by seismic waves, change the way fluids flow within Earth’s crust and how those fluids influence rock properties. Our work was motivated by observations showing that elastic waves that travel through Earth’s crust can trigger a range of phenomena including seismicity, volcanic eruptions, and geyser activity. The largest of these elastic waves produce the shaking that damages building and causes destruction in the vicinity of a large earthquake, but seismometers record small elastic waves also. Most of these can’t be felt but still have an effect on rock properties near Earth’s surface. Previous work had shown that these elastic waves can sometimes cause increased spring discharge, ?uid ?ow in streams, and changes oil production, in some cases tripling the effective permeability of the natural system. We wanted to know how this worked, and specifically how the small changes in local stress caused by elastic waves could alter fluid flow and permeability near Earth’s surface. We performed laboratory experiments to investigate the processes and mechanisms that cause transient and permanent permeability changes due to dynamic stressing. The lab work was coupled with theory and numerical methods to develop conceptual and quantitative models for permeability changes. We found clear evidence of permeability enhancement in fractured rock subject to fluid pressure oscillations. Pore pressure oscillations, simulating dynamic stresses, were applied to intact and fractured Berea sandstone samples under confining stresses of tens of MPa. We found that dynamic stressing produces an immediate permeability enhancement ranging from 1-60%, which scales with the amplitude of the dynamic strain followed by a gradual permeability recovery. We investigated the mechanism by: (1) recording deformation of samples both before and after fracturing during the experiment, (2) varying the chemistry of the water and therefore particle mobility, (3) evaluating the dependence of permeability enhancement and recovery on dynamic stress amplitude, and (4) examining micro-scale pore textures of the rock samples before and after experiments. We found that dynamic stressing does not produce permanent deformation in our samples. Water chemistry has a pronounced effect on the sensitivity to dynamic stressing, with higher ionic strengths favoring particle mobilization. The magnitude of permeability enhancement and the rate of permeability recovery scales with ionic strength of the pore fluid. Permeability recovery rates generally correlate with the permeability enhancement sensitivity. Microstructural observations of our samples show clearing of clay particulates from fracture surfaces during the experiment. From these four lines of evidence, we conclude that a flow-dependent mechanism associated with mobilization of fines controls both the magnitude of the permeability enhancement and the recovery rate in our experiments. We also find that permeability sensitivity to dynamic stressing increases after fracturing, which is a process that generates abundant particulate matter in situ. Our results suggest that areas of the Earth’s crust where pore fluids are in disequilibrium should be more sensitive to dynamic stressing.
