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DOE OSTI2015

Rock fracture processes in chemically reactive environments

Rock fracture is traditionally viewed as a brittle process involving damage nucleation and growth in a zone ahead of a larger fracture, resulting in fracture propagation once a threshold loading stress is exceeded. It is now increasingly recognized that coupled chemical-mechanical processes influence fracture growth in wide range of subsurface conditions that include igneous, metamorphic, and geothermal systems, and diagenetically reactive sedimentary systems with possible applications to hydrocarbon extraction and CO 2 sequestration. Fracture processes aided or driven by chemical change can affect the onset of fracture, fracture shape and branching characteristics, and fracture network geometry, thus influencing mechanical strength and flow properties of rock systems. We are investigating two fundamental modes of chemical-mechanical interactions associated with fracture growth: 1. Fracture propagation may be aided by chemical dissolution or hydration reactions at the fracture tip allowing fracture propagation under subcritical stress loading conditions. We are evaluating effects of environmental conditions on critical (fracture toughness KIc) and subcritical (subcritical index) fracture properties using double torsion fracture mechanics tests on shale and sandstone. Depending on rock composition, the presence of reactive aqueous fluids can increase or decrease KIc and/or subcritical index. 2. Fracture may be concurrent with distributed dissolution-precipitation reactions in the hostrock beyond the immediate vicinity of the fracture tip. Reconstructing the fracture opening history recorded in crack-seal fracture cement of deeply buried sandstone we find that fracture length growth and fracture opening can be decoupled, with a phase of initial length growth followed by a phase of dominant fracture opening. This suggests that mechanical crack-tip failure processes, possibly aided by chemical crack-tip weakening, and distributed solution-precipitation creep in the hostrock can independently affect fracture opening displacement and thus fracture aperture profiles and aperture distribution.

58 GEOSCIENCES↗

DOE OSTI2015

Influence of vanadium incorporation on the microstructure, mechanical and tribological properties of Nb–V–Si–N films deposited by reactive magnetron sputtering

Composite Nb–V–Si–N films with various V contents (3.7–13.2 at.%) were deposited by reactive magnetron sputtering and the effects of V content on the microstructure, mechanical and tribological properties of Nb–V–Si–N films were investigated. The results revealed that a three-phase structure, consisting of face-centered cubic (fcc) Nb–V–Si–N, hexagonal close-packed (hcp) Nb–V–Si–N and amorphous Si{sub 3}N{sub 4}, co-exists in the Nb–V–Si–N films and the cubic phase is dominant. The hardness and critical load (L{sub c}) of Nb–V–Si–N films initially increased gradually and reached a summit, then decreased with the increasing V content in the films and the maximum values were 35 GPa and 9.8 N, respectively, at 6.4 at.% V. The combination of V into Nb–Si–N film led to the fracture toughness linearly increasing from 1.11 MPa·m{sup 1/2} at 3.7 at.% V to 1.67 MPa·m{sup 1/2} at 13.2 at.% V. At room temperature (RT), the average friction coefficient decreased from 0.80 at 3.7 at.% V to 0.55 at 13.2 at.% V for the Nb–V–Si–N films. The wear rate of Nb–V–Si–N films initially decreased and then increased after reaching a minimum value of about 6.35 × 10{sup −} {sup 7} mm{sup 3}/N·mm at 6.4 at.% V. As the rise of testing temperature from 200 °C to 600 °C, the average friction coefficient of Nb–V–Si–N films decreased with the increase of the testing temperature regardless of V content. However, the wear rate gradually increased for all films. The average friction coefficient and wear rate at RT and elevated temperatures were mainly influenced by the vanadium oxides with weakly bonded lattice planes. - Highlight: • Fcc-Nb–V–Si–N, hcp-Nb–V–Si–N and amorphous Si{sub 3}N{sub 4} co-existed in the films. • The amount of Si{sub 3}N{sub 4} decreased with increasing V content in the films. • Hardness of Nb–V–Si–N film (6.4 at.%) reached a maximum value of 35 GPa. • Addition of V led to the increase of fracture toughness. • Tribological properties were influenced by lubricant wear debris.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Efficient Monte Carlo With Graph-Based Subsurface Flow and Transport Models

Simulating flow and transport in fractured porous media frequently involves solving numerical discretizations of partial differential equations with a large number of degrees of freedom using discrete fracture network (DFN) models. Uncertainty in the properties of the fracture network that controls flow and transport requires a large number of DFN simulations to statistically describe quantities of interest. However, the computational cost of solving more than a few realizations of a large DFN can be intractable. As a means of circumventing this problem, we utilize both a high-fidelity DFN model and a graphbased model of flow and transport in combination with a multifidelity Monte Carlo (MC) method to reduce the number of high-fidelity simulations that are needed to obtain an accurate estimate of the quantity of interest. We demonstrate the approach by estimating quantiles of the breakthrough time for a conservative tracer in an ensemble of fractured porous media. Our results demonstrate that a multifidelity MC estimate, whose computational cost is equal to the cost of 10 DFN simulations, can be as accurate as a standard MC estimate that utilizes 1,000 DFN simulations. Thus the combination of our graphbased model with multifidelity MC estimates effectively reduces the computational cost of the problem by a factor of approximately 100.

58 GEOSCIENCES↗

NASA NTRS1995

The Effects of Fiber Surface Modification and Thermal Aging on Composite Toughness and Its Measurement

A detailed experimental study was conducted to establish the structure-property relationships between elevated temperature aging and fiber-matrix bonding, Mode 2 interlaminar fracture toughness, and failure modes of carbon fiber/PMR-15 composites. The fiber-matrix adhesion was varied by using carbon fibers with different surface treatments. Short beam shear tests were used to quantify the interfacial shear strength afforded by the use of the different fiber surface treatments. The results of the short beam shear tests showed that, for times up to 1000 hr, the aging process caused no changes in the bulk of the three composite materials that would degrade the shear properties of the material. Comparisons between the interlaminar shear strengths (ILSS) measured by the short beam shear tests and the GIIC test results, as measured by the ENF test, indicated that the differences in the surface treatments significantly affected the fracture properties while the effect of the aging process was probably limited to changes at the starter crack tip. The fracture properties changed due to a shift in the fracture from an interfacial failure to a failure within the matrix when the fiber was changed from AU-4 to AS-4 or AS-4G. There appears to be an effect of the fiber/matrix bonding on the thermo-oxidative stability of the composites that were tested. The low bonding afforded by the AU 1 fiber resulted in weight losses about twice those experienced by the AS 1 reinforced composites, the ones with the best TOS.

Bowles, Kenneth J.↗

DOE OSTI2021

Hydro-mechanical measurements and x-ray imaging of sheared crystalline rock fractures from EGS Collab experiments 1 & 2

Measuring hydro-mechanical properties of natural fractures is a prerequisite for optimizing hydraulic stimulation design and well placement. We completed experiments to characterize shear on natural fractures in schist, amphibolite, and rhyolite specimens drilled from EGS Collab Project’s field sites at the Sanford Underground Research Facility (SURF) in South Dakota. A triaxial direct shear method and coupled x-ray imaging were used to perform hydroshearing and mechanical shearing at the site’s in-situ stress conditions. This produced simultaneous measurements of fracture and matrix strength, permeability, stress-dependent aperture, dilation, and friction strength. Our results identified that only a subset of the natural fractures was weak enough for hydroshearing. Generally, hydroshearing increases fracture permeability by a factor of 10 or more and the enhancement is retainable over time. However, the shear slip does not always result in permeability enhancement. High content of phyllosilicates was found to associate with exceptionally weak fractures that also exhibited poor or even negative enhancement after stimulation. Combining our measurements with site data, we can predict that most observable fractures at the two EGS Collab sites do not meet the criteria for hydroshearing before tensile opening. In some cases, the visible fractures are low permeability and as strong as the adjacent rock. To induce hydroshearing before tensile opening, injection must target known weak and favorably oriented fractures with confirmed pre-existing permeability.

58 GEOSCIENCES↗

DOE OSTI2019

Effect of Annealing on the Impact Resistance and Fracture Mechanism of PNC-60 Sinters After Cold Plastic Deformation

Compaction of sintered metal powders by free upsetting at room temperature dangerously reduces their plastic properties, particularly the fracture toughness. The reason for the occurrence of two unfavorable phenomena accompanying this deformation route is sought. The first phenomenon depends on the initial porosity of the preform and the amount of cold work resulting in the destruction of bonds and violation of continuity between sintered powder particles. The second phenomenon is the deformation of the metal matrix, which reduces the plasticity of the sintered product. To correctly assess the effect of changes occurring in the matrix as a result of deformation, it is necessary to use the properly defined sintered matrix hardening parameter discussed in this study. The adverse effects of cold forming can be eliminated by annealing the deformed sinters at a temperature below the sintering point and above the recrystallization temperature. The annealing temperature causes favorable changes in the structure and increases the impact resistance of the deformed sinters depending on the degree of deformation of the sintered matrix and the initial porosity of the preform. Annealing at a properly selected temperature restores the ductility of the metal matrix and improves the integrity of the sintered material impaired by the effect of deformation. Annealing also causes favorable changes in the porosity morphology, removing defects in the form of gaps and resulting in the spheroidization of voids. With an appropriately selected initial porosity and degree of deformation, subsequent annealing of sinters compacted by free cold upsetting provides products with strength properties comparable to sinters with the same density and subjected to single compaction and sintering but with considerably higher fracture toughness.

36 MATERIALS SCIENCE↗

NASA NTRS1997

The Effects of Fiber Surface Modification and Thermal Aging on Composite Toughness And its Measurement

A detailed experimental study was conducted to establish the structure-property relationships between elevated temperature aging and (I) fiber-matrix bonding, (2) Mode II interlaminar fracture toughness, and (3) failure modes of carbon fiber/PMR-15 composites. The fiber-matrix adhesion was varied by using carbon fibers with different surface treatments. Short beam shear tests were used to quantify the interfacial shear strength afforded by the use of the different fiber surface treatments. The results of the short beam shear tests definitely showed that, for aging times up to 1000 hr, the aging process caused no observable changes in the bulk of the three composite materials that---would degrade the shear properties of the material. Comparisons between the interlaminar shear strength (ILSS) measured by the short beam shear tests and the GII c test results, as measured by the ENF test, indicated that the differences in the surface treatments significantly affected the fracture properties while the effect of the aging process was probably limited to changes at the starter crack tip. The fracture properties changed due to a shift in the fracture from an interfacial failure to a failure within the matrix when the fiber was changed from AU-4 to AS-4 or AS-4G. There appears to be an effect of the fiber/matrix bonding on the thermo-oxidative stability of the composites that were tested. The low bonding afforded by the AU-4 fiber resulted in weight losses about twice those experienced by the AS-4 reinforced composites, the ones with the best TOS.

Bowles, Kenneth J.↗

DOE OSTI2017

Laboratory system for studying cryogenic thermal rock fracturing for well stimulation

The concept of cryogenic fracturing is that a sharp thermal gradient developed by applying a cryogenic fluid on a rock surface causes a strong local tensile stress that initiates fractures. Prior field tests suggest that field application with special equipment rated for cryogenic temperatures may deliver potential benefits. The tests did not, however, identify the fracture mechanisms at work in downhole conditions. We present our laboratory designs and procedures developed for studying cryogenic fracturing mechanisms in a well environment, and examine typical data indicative of the performance of the system. The experimental apparatus and procedures were specifically designed to conduct cryogenic fracturing tests in specimens under confining stress, with integrated cryogen transport, measurements, and fracture characterization. A true-triaxial loading system was built to simulate reservoir stress levels and anisotropic stress application, and was designed to avoid thermal stresses in tests involving cryogen by arranging discrete components in an open chamber. To maximize thermal shock on the wellbore surface, tubing and wellhead are configured so that liquid nitrogen enters the borehole and flow out after contributing to thermal shock. The temperature at boreholes and along flow lines, borehole pressure, and liquid nitrogen consumption are monitored throughout treatments. Acoustic transmission and pressure-decay measurements are used to characterize fractures before and after the experiments. Breakdown tests performed on un-stimulated specimens and stimulated specimens compare breakdown pressures of the two groups and thus evaluate the performance of thermal fracturing. The laboratory design was able to effectively apply cryogenic stimulations to laboratory rock specimens. Finally, the characterization methods were able to capture fracture creation and rock property changes due to cryogenic fracturing.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2023

Comparative Analysis of Imaging and Measurements of Micrometer-Scale Fracture Aperture Fields Within a Heterogeneous Rock Using PET and X-ray CT

Knowledge of the spatial distribution of fracture apertures is essential for reliable characterization of flow and transport processes in fractured systems and for better understanding of physicochemical matrix–fracture interactions. Here, we propose and test two image-based methods, thereby extending the current experimental capabilities to characterize aperture size distribution in structurally heterogeneous geologic porous media noninvasively. The first approach utilizes an inversion method based on the dataset acquired from positron emission tomography (PET) and the second approach considers an extension of the classic missing attenuation technique that relies on clinical X-ray computed tomography (X-ray CT). Independent sets of imaging experiments are conducted on a fractured basalt core with heterogeneous matrix properties and aperture distributions to compare the two methodologies. A repeat of each experiment is conducted to verify the proposed workflows. The performance of these two imaging techniques is systematically evaluated through the analysis of signal-to-noise ratio, minimum fracture size detectability, and measurement errors. While both approaches provide a reliable estimation of fracture aperture distributions, PET yields a signal-to-noise ratio that is substantially higher than the corresponding X-ray CT measurements. Furthermore, uncertainties of the aperture values for PET are considerably lower ($\bar {\sigma}_{\text {d}} = 15\%$) compared to those obtained from X-ray CT ($\bar {\sigma}_{\text {d}} = 29\%$), allowing for the detection of minimum aperture sizes of 20 $\mu$m with 70% confidence level. Finally, these approaches provide key experimental tools for better understanding dynamic hydromechanical fracture properties in geologic systems.

58 GEOSCIENCES↗

DOE OSTI2019

Hydromechanical reactivation of natural discontinuities: mesoscale experimental observations and DEM modeling

Fracture interaction mechanisms and reactivation of natural discontinuities under fluid pressurization conditions can represent critical issues in risk assessment of caprock integrity. A field injection test, carried out in a damage fault zone at the decameter scale, i.e., mesoscale, has been studied using a distinct element model. Given the complex structural nature of the damage fault zone hydraulically loaded, the contribution of fracture sets on the bulk permeability has been investigated. It has been shown that their orientation for a given in situ stress field plays a major role. Based on these results, a simpler model with a fluid-driven fracture intersecting a second fracture has been set up to perform a sensitivity analysis. It is in presence of a minimum differential stress value with a minimum angle with the maximum principal stress that the second fracture could be both, hydraulically and mechanically reactivated. Results also showed that in the vicinity of the fluid-driven fracture, a natural fracture will offer contrasted hydromechanical responses on each side of the intersection depending on the stress conditions and its orientation with respect to the stress field. In this case, we show that a hydromechanical decoupling can occur along the same plane. These results provide insights into fracture-controlled permeability of fault zones depending on the properties of the fractures and their hydromechanical interactions for a given in situ stress field.

58 GEOSCIENCES↗

DOE OSTI2016

Development of an empirical model relating permeability and specific stiffness for rough fractures from numerical deformation experiments: Fracture Permeability and Stiffness

The coupling between hydraulic and mechanical properties of porous and fractured geologic media are critical for many geophysical processes and practical applications. Thus, the prediction of this linkage between these properties is broadly important. Here we present a potentially predictive model that links fracture permeability and specific stiffness with empirical coefficients dependent on fracture roughness, correlation length of aperture field, and rock mechanical properties. The equation was developed empirically from results of modeling the deformation and flow through synthetic fractures with aperture fields that follow a normal distribution. The fractures were subjected to increasing normal stress and deformed following an elastic model. Specific stiffness was directly quantified from these numerical deformation experiments. Permeability was estimated through calculation of the local flow field while considering effects of local fracture roughness and tortuosity. The empirical model roughly captures the transition from effective medium to percolation flow regimes with increasing specific stiffness.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2018

Evaluation of statistical variation of microstructural properties and temperature effects on creep fracture of Grade 91

This report describes three improvements made to a physically-based model for creep and creep rupture in Grade 91 steel developed as part of the Advanced Reactor Technologies program: 1. Progress on transitioning the model framework to the MOOSE finite element package to improve parallel scalability, increase the physical size of the simulations, and incorporate multiphysics effects into the model. 2. The development of a physically-based method of scaling the model parameters to accurately capture creep in Grade 91 for temperatures in the range of 450° to 500° C. 3. Extending the model to account for microstructural statistical variations, specifically capturing the effect of grain boundary energy on the physical parameters underling the model for grain boundary void nucleation and cavitation. Put together, these improvements result in an accurate, predictive model for the physical response of Grade 91 steel over the expected use temperature range for the material in future liquid metal cooled fast reactors. The model can be used to more accurately predict engineering properties of the material for very long service lives, which could lead to safer, more economical future advanced reactors.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

DOE OSTI2015

Influence of injection mode on transport properties in kilometer-scale three-dimensional discrete fracture networks

We investigate how the choice of injection mode impacts transport properties in kilometer-scale three-dimensional discrete fracture networks (DFN). The choice of injection mode, resident and flux-weighted, is designed to mimic different physical phenomena. It has been hypothesized that solute plumes injected under resident conditions evolve to behave similarly to solutes injected under flux-weighted conditions. Previously, computational limitations have prohibited the large-scale simulations required to investigate this hypothesis. We investigate this hypothesis by using a high-performance DFN suite, dfnWorks, to simulate flow in kilometer-scale three-dimensional DFNs based on fractured granite at the Forsmark site in Sweden, and adopt a Lagrangian approach to simulate transport therein. Results show that after traveling through a pre-equilibrium region, both injection methods exhibit linear scaling of the first moment of travel time and power law scaling of the breakthrough curve with similar exponents, slightly larger than 2. Lastly, the physical mechanisms behind this evolution appear to be the combination of in-network channeling of mass into larger fractures, which offer reduced resistance to flow, and in-fracture channeling, which results from the topology of the DFN.

42 ENGINEERING↗

DOE OSTI2021

Experimental study on rheological and settling properties of shape memory polymer for fracture sealing in geothermal formations

This article studies the rheology and annular flow of a smart lost circulation material (LCM) at 160 °C, using a high-temperature flow loop. The smart LCM is a shape memory polymer (SMP) that activates at high temperatures, and its particle size increases to seal fractures in geothermal formations. SMP was mixed with the base fluid in two different concentrations, 1.0 and 3.0 wt%, to study rheology, wellbore hydraulics, activation process, and settling behavior under different testing conditions. The results of this study showed that the SMP could be activated at high temperatures. An increase of 80–100% in the particle size was observed at 160 °C. The mud samples showed a high shear-thinning behavior at the two concentrations with a Power-law flow index (n) ranging between 0.025 and 0.101. No additional frictional pressure losses were observed when SMP was added to the base fluid. SMP particles showed an excellent suspension at 1.0 wt% while, at 3.0 wt%, a bed was formed at a low flow rate and without pipe rotation. Increasing the drill pipe rotational speed or flow rate effectively removed the bed and homogeneously dispersed the SMP particles, ensuring a better sealing efficiency. SMP particle dispersion in inclined wells was better than in horizontal wells. Moreover, the findings of this study help optimize the lost circulation treatment by considering a wide range of operating parameters that can further be extended to different systems and geometries.

02 PETROLEUM↗

DOE OSTI2018

Computational Modeling of Hydraulic Properties of a Sheared Single Rock Fracture

Flow through a single mechanically sheared Marcellus shale fracture was investigated computationally and experimentally. To provide a better understanding of the variation of hydraulic and geometrical characteristics of a fracture subjected to shearing, coupled shear flow tests on the fracture for four shearing displacement steps under constant normal stress were performed. At the end of each shearing step, computed tomography (CT) scans with resolution 26.8 μm were obtained and the corresponding fracture geometries were evaluated. The CT images were used to generate full aperture maps of the fracture configuration. In addition, average aperture maps were also created by averaging the full-resolution data over 10 × 10 pixels, smoothing out fine structural details. Computational modeling of water flow through the fractures at different shearing steps was performed using a modified local cubic law approach and the 3D full Navier–Stokes equations with the use of the ANSYS-Fluent software. Both the average aperture maps and full maps were used in these simulations. The experimental pressure drops of the fracture at shearing step 1, which has very small apertures, poorly matched the numerical results, quite likely because the fracture structure was inadequately captured by the scanning resolution. Shearing typically increased the aperture height of the fracture, whose features were then better captured by the CT scan. Good agreement between the experimental data and the numerical results of the full map for shearing step 2 was observed. The simulations were performed for both full and average aperture maps, and the effects of scan resolution and surface roughness on the accuracy of the results were studied. The modified local cubic law and full Navier–Stokes simulations of the averaged map fracture were found to be in good agreement. It was conjectured that this was because the nonlinear losses were insignificant for the smoothed out averaged map fracture. Similar comparisons with those of the full map showed agreement in trends, but there were some quantitative differences. The averaged fracture map simulations also predicted lower pressure drops compared to the full map, particularly for high flow rates. These differences were due to the fine-scale geometrical complexity (surface roughness) of fracture geometry that affects the fluid flow in the fracture. In conclusion, an improved cubic law model was also proposed, and its accuracy was verified by comparing its predictions with those of the Navier–Stokes simulations.

58 GEOSCIENCES↗

DOE OSTI2021

Insights into the Physical-Chemical Properties of a CO2-Responsive Fracturing Fluid

Here we determine the phase behavior of StimuFrac, a CO2 responsive fracturing fluid, under geothermal wellbore conditions. StimuFrac is an aqueous poly(allylamine) fluid that crosslinks in the presence of CO2. StimuFrac significantly reduces the net pressure required to induce fractures, relative to other fracture fluids, and has potential to reduce water use. However, the phase behavior and equations of state to describe StimuFrac’s phase behavior remain unavailable. Here we determine the density and molar volume of the fluid as a function of geothermal relevant temperatures, pressures, and weight fractions of StimuFrac added. In general, experiments find that StimuFrac’s density decreases as temperature and pressure increase. Using these results, equations of saturated state and phase diagrams for different polymer concentrations are reported. These results are critical inputs for planned numerical simulation efforts.

Pease, Leonard F.↗

NASA NTRS2006

Cryogenic Fracture Toughness Evaluation of an Investment Cast Al-Be Alloy for Structural Applications

Aluminum-Beryllium metal matrix composite materials are useful due to their desirable performance characteristics for aerospace applications. Desirable characteristics of this material includes light-weight, dimensional stability, stiffness, good vibration damping characteristics, low coefficient of thermal expansion, and workability, This material is 3.5 times stiffer and 22% lighter than conventional aluminum alloys. electro-optical systems, advanced sensor and guidance components for flight and satellite systems, components for light-weight high-performance aircraft engines, and structural components for helicopters. Aluminum-beryllium materials are now available in the form of near net shape investment castings. In this materials properties characterization study, the cryogenic tensile and fracture properties of an investment casting alloy, Beralcast 363, were determined. Tensile testing was performed at 21 C (70 F), -73.3 C (-100 F), -195.5 C (-320 F) and -252.8 C (-423 F), and fracture (K(sub lc) and da/dN) testing was performed at -73.3 C (-100 F), -195.5 C (-320 F) and -252.8 C (-423 F). Their use is attractive for weight critical structural applications such as advanced

Gamwell, W. R.↗

DOE OSTI2023

Laboratory Experiments Examining the Effect of Thermal and Mechanical Processes on Hydraulic Transmissivity Evolution

Using laboratory slide-hold-slide experiments, at temperatures from 22 to 200 degrees C, to examine effects of fracture reactivation and quasi-static loading on the evolution of fluid transport properties of simulated fractures in Westerly granite. At all temperatures, the in-plane hydraulic transmissivity consistently decays during hold periods resulting in an overall reduction in transmissivity. During the first three to fifteen hours of an experiment, transmissivity decreases rapidly due to the generation of wear products, development of a sliding surface, and compaction of the resulting gouge. Once the sliding surface has developed, the long-term transmissivity decay rate at 22 and 100 degrees C is significantly lower than the transmissivity decay rate during the initial 3-15 hours of the experiment. However, at 200 degrees C, the decay of hydraulic transmissivity remains high throughout the experiment. The long-term decay of hydraulic transmissivity can be fitted with a power law model with more rapid reduction of hydraulic transmissivity at higher temperature. Periods of sliding on the fracture surface result in transient increases in the transmissivity, due to shear dilation, as is expected for Coulomb materials. These transients are superimposed on the long-term decay. When sliding ceases and a new hold period commences, there is a rapid reduction in transmissivity and return to the long-term rate of transmissivity decay. The rate of decay of the transmissivity transients is inversely correlated with temperature, in contrast to the long-term decay and the expected behavior for processes like subcritical crack growth and indentation creep. The higher decay rates that are observed during the initial 3-15 hours of the tests and following sliding, are associated with times that the porosity of the gouge is expected to be high. The difference in decay rates suggests that when the gouge is driven far from equilibrium by active shearing, densification may be dominated by a different mechanism from long-term compaction.

15 GEOTHERMAL ENERGY↗