Laboratory observations of permeability enhancement by fluid pressure oscillation of in situ fractured rock

We report on laboratory experiments designed to investigate the influence of pore pressure oscillations on the effective permeability of fractured rock. Berea sandstone samples were fractured in situ under triaxial stresses of tens of megapascals, and deionized water was forced through the incipient fracture under conditions of steady and oscillating pore pressure. We find that short-term pore pressure oscillations induce long-term transient increases in effective permeability of the fractured samples. The magnitude of the effective permeability enhancements scales with the amplitude of pore pressure oscillations, and changes persist well after the stress perturbation. The maximum value of effective permeability enhancement is 5 × 10^(−16) m^2 with a background permeability of 1 × 10^(−15) m^2; that is, the maximum enhanced permeability is 1.5 × 10^(−15) m^2. We evaluate poroelastic effects and show that hydraulic storage release does not explain our observations. Effective permeability recovery following dynamic oscillations occurs as the inverse square root of time. The recovery indicates that a reversible mechanism, such as clogging/unclogging of fractures, as opposed to an irreversible one, like microfracturing, is responsible for the transient effective permeability increase. Our work suggests the feasibility of dynamically controlling the effective permeability of fractured systems. The result has consequences for models of earthquake triggering and permeability enhancement in fault zones due to dynamic shaking from near and distant earthquakes

Average crack-front velocity during subcritical fracture propagation in a heterogeneous medium

We study the average velocity of crack fronts during stable interfacial fracture experiments in a heterogeneous quasibrittle material under constant loading rates and during long relaxation tests. The transparency of the material (polymethylmethacrylate) allows continuous tracking of the front position and relation of its evolution to the energy release rate. Despite significant velocity fluctuations at local scales, we show that a model of independent thermally activated sites successfully reproduces the large-scale behavior of the crack front for several loading conditions

Downscaling of fracture energy during brittle creep experiments

We present mode 1 brittle creep fracture experiments along fracture surfaces that contain strength heterogeneities. Our observations provide a link between smooth macroscopic time-dependent failure and intermittent microscopic stress-dependent processes. We find the large-scale response of slow-propagating subcritical cracks to be well described by an Arrhenius law that relates the fracture speed to the energy release rate. At the microscopic scale, high-resolution optical imaging of the transparent material used (PMMA) allows detailed description of the fracture front. This reveals a local competition between subcritical and critical propagation (pseudo stick-slip front advances) independently of loading rates. Moreover, we show that the local geometry of the crack front is self-affine and the local crack front velocity is power law distributed. We estimate the local fracture energy distribution by combining high-resolution measurements of the crack front geometry and an elastic line fracture model. We show that the average local fracture energy is significantly larger than the value derived from a macroscopic energy balance. This suggests that homogenization of the fracture energy is not straightforward and should be taken cautiously. Finally, we discuss the implications of our results in the context of fault mechanics. Copyright © 2011 by the American Geophysical Union

Average crack-front velocity during subcritical fracture propagation in a heterogeneous medium.

We study the average velocity of crack fronts during stable interfacial fracture experiments in a heterogeneous quasibrittle material under constant loading rates and during long relaxation tests. The transparency of the material (polymethylmethacrylate) allows continuous tracking of the front position and relation of its evolution to the energy release rate. Despite significant velocity fluctuations at local scales, we show that a model of independent thermally activated sites successfully reproduces the large-scale behavior of the crack front for several loading conditions

Seismic waves increase permeability

Photo of Moonshine Rapid on the Green River in Split Mountain Canyon, Dinosaur National Monument, during Aaron B. Ross's river trip down the Green in July of 196

Can a fractured caprock self-heal?

The ability of geologic seals to prevent leakage of fluids injected into the deep subsurface is critical for mitigating risks associated with greenhouse-gas sequestration and natural-gas production. Fractures caused by tectonic or injection-induced stresses create potential leakage pathways that may be further enhanced by mineral dissolution. We present results from reactive-flow experiments in fractured caprock (dolomitic anhydrite), where additional dissolution occurs in the rock matrix adjacent to the fracture surfaces. Preferential dissolution of anhydrite left a compacted layer of dolomite in the fractures. At lower flow rate, rock-fluid reactions proceeded to near equilibrium within the fracture with preferential flow paths persisting over the 6-month duration of the experiment and a negligible change in permeability. At higher flow rate, permeability decreased by a dramatic two orders of magnitude. This laboratory-scale observation of self-healing argues against the likelihood of runaway permeability growth in fractured porous caprock composed of minerals with different solubilities and reaction kinetics. However, scaling arguments suggest that at larger length scales this self-healing process may be offset by the formation of dissolution channels. Our results have relevance beyond the greenhouse-gas sequestration problem. Chemical disequilibrium at waste injection sites and in hydrothermal reservoirs will lead to reactive flows that may also significantly alter formation permeability

Fracture and relaxation in dense cornstarch suspensions

Dense suspensions exhibit the remarkable ability to switch dynamically and reversibly from a fluid-like to a solid-like, shear-jammed (SJ) state. Here, we show how this transition has important implications for the propensity for forming fractures. We inject air into bulk dense cornstarch suspensions and visualize the air invasion into the opaque material using time-resolved X-ray radiography. For suspensions with cornstarch mass fractions high enough to exhibit discontinuous shear thickening and shear jamming, we show that air injection leads to fractures in the material. For high mass fractions, these fractures grow quasistatically as rough cavities with fractured interfaces. For lower mass fractions, remarkably, the fractures can relax to smooth bubbles that then rise under buoyancy. We show that the onset of the relaxation occurs as the shear rate induced by the air cavity growth decreases below the critical shear rate denoting the onset of discontinuous shear thickening, which reveals a structural signature of the SJ state.</p

Dataset in support of the journal article &#39;Fracture and relaxation in dense cornstarch suspensions&#39;

This dataset contains three sections: Analyzed Data, Code and X-Ray Data. Analyzed Data: data plotted in figures 1 to 5 - data_figure_1b.xlsx: volume and time for the three experiments shown in Figure 1b - data_figure_1c.xlsx: gauge pressure, hydrostatic pressure, mass fraction, volume rate and confidence intervals as shown in Figure 1c - data_figure_2c.xlsx: equivalent radius R and centroid height Z for the experiments shown in Figure 2c - data_figure_2d.xlsx: mass fraction, gauge pressure, effective viscosity, equivalent radius, centroid height, vertical velocity, growth velocity, Vb/Vg and dZ/dR as shown in Figure 2d - data_figure_3c.xlsx: gradient intensity and probability (fraction) of gradient intensity for the experiments shown in Figure 3c - data_figure_3c_inset.xlsx: mass fraction and metric for local fracture characterization as shown in the inset of Figure 3c - data_figure_4b_4c.xlsx: time, volume, equivalent radius, shear rate from growth and metric for local fracture characterization for the experiment shown in Figure 4b and c - data_figure_4d.xlsx: mass fraction, gauge pressure, critical shear rate from rheology, time for relaxation and error, time for critical shear rate and error as shown in Figure 4d - data_figure_5.xlsx: stress, shear rate and viscosity for the 57% and 58% mass fraction rheology data shown in Figure 5 Code: functions to calculate air thickness and gradient from X-ray data - intensityToThickness.m: MATLAB function to calculate the air thickness in mm from the X-ray intensity recorded in the X-Ray Data section - calculateGradient.m: MATLAB function to calculate the air thickness gradient intensity from the air thickness data X-Ray data: Full X-ray data for experiments shown in figures 1 to 4 - figure_XX_phimYYpc_PgZZkPa: XX figure number, YY cornstarch mass fraction in percent, ZZ gauge pressure in kPa Scale is 0.112 mm/px for the 1000x1000 data and 0.056 mm/px for the 2000x2000 data. Load the data and use intensityToThickness.m to convert to air thickness data.</span