150 research outputs found
A low-cost X-ray-transparent experimental cell for synchrotron-based X-ray microtomography studies under geological reservoir conditions
A new modular X-ray-transparent experimental cell enables tomographic investigations of fluid rock interaction under natural reservoir conditions (confining pressure up to 20â
MPa, pore fluid pressure up to 15â
MPa, temperature ranging from 296 to 473â
K). The portable cell can be used at synchrotron radiation sources that deliver a minimum X-ray flux density of 109â
photons mmâ2sâ1in the energy range 30â100â
keV to acquire tomographic datasets in less than 60â
s. It has been successfully used in three experiments at the bending-magnet beamline 2BM at the Advanced Photon Source. The cell can be easily machined and assembled from off-the-shelf components at relatively low costs, and its modular design allows it to be adapted to a wide range of experiments and lower-energy X-ray sources.</jats:p
Biotite supports long-range diffusive transport in dissolutionâprecipitation creep in halite through small porosity fluctuations
Mjölnir: a miniature triaxial rock deformation apparatus for 4D synchrotron X-ray microtomography
Dynamic Evolution of Permeability in Response to ChemoâMechanical Compaction
Pressureâsolution creep is an important fluidâmediated deformation mechanism, causing chemoâmechanical transformations and porosity and permeability changes in rocks. The presence of phyllosilicates, in particular, has previously been hypothesized to further reduce porosity and pore connectivity. Nevertheless, a full characterization of the spatioâtemporal evolution of permeability during this process has yet to be reported. A pure NaCl aggregate and a mixture of NaCl and biotite were deformed through pressureâsolution creep while monitoring their microstructural evolution through computed Xâray microâtomography. The evolution of permeability and fluid velocity of the samples were computed by using the pore geometries from the Xâray microâtomography as input for the LatticeâBoltzmann modeling. The results indicate that, as deformation proceeds, porosity and permeability decrease in both samples. In the salt âbiotite sample pressure solution creep causes the formation of a compaction band perpendicular to the direction of loading, forming a barrier for permeability. Along the other two directions, pore connectivity and permeability are retained in the marginal salt layers, making the sample strongly anisotropic. The presence of biotite controls the way pore coordination number evolves and hence, the connectivity of the pathways. Biotite flakes create an enhanced porosity decrease leading to compaction and reduction of pore connectivity. This reduction in porosity affects local stresses and local contact areas, reducing over time the driving force. According to a textureâporosity process, the reduction in porosity causes salt ions to dissolve in the marginal salt and precipitate within the biotiteâbearing layer, where the bulk volume of salt grains increases over time
Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing
Establishing models for the formation of well-mixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemo-mechanical feedbacks can obscure the transient phenomena that may be responsible for domain production. We use scanning electron microscopy and nanotomography to offer critical insights into how the microstructure of a highly deformed quartzo-feldspathic ultramylonite evolved. The dispersal of monomineralic quartz domains in the ultramylonite is interpreted to be the result of the emergence of synkinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener-Stroh cracking and viscous grain-boundary sliding. In initially thick and coherent quartz ribbons deforming by grain-size-insensitive creep, cavities were generated by the Zener-Stroh mechanism on grain boundaries aligned with the YZ plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain boundaries and promoted viscous grain-boundary sliding. With the increased contribution of viscous grain-boundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain-size-insensitive to a grain-size-sensitive rheology
Strain shadow âmegaporesâ in mid-crustal ultramylonites
Mylonitic shear zones are important fluid conduits in the Earth's crust. They host transient and permeable porosity that facilitates fluid transfer and controls fluid-rock interaction. Here we present microstructural observations from a mid-crustal ultramylonite with very large pores that occupy the strain shadows of albite porphyroclasts. Our non-invasive three-dimensional X-ray microtomographic data show that the largest of these strain shadow megapores have substantial volumes of as much as âŒ1.7 Ă 105 ”m3. Given that the sample shows no signs of retrogressive overprint or weathering, these pores must be synkinematic. Importantly, the close proximity of the pores to creep cavities in dynamically recrystallized quartz ribbon grains suggests a potential hydraulic link between fluid in the strain shadow megapores and fluid in the creeping rock matrix. The evolving megapores constitute very large syndeformational local fluid reservoirs in mylonites that likely fed into the granular fluid pump established by the dynamically evolving creep cavities. Our findings add to an emerging picture of the dynamic transport properties of ultramylonitic shear zones, where the formation and destruction of porosity are intrinsically linked to microscale deformation processes. They also suggest that despite many studies on porphyroclast systems, open questions remain, especially concerning the interaction of clasts with their matrix
Dynamic evolution of porosity in lower-crustal faults during the earthquake cycle
Earthquake-induced fracturing of the dry and strong lower crust can transiently increase permeability for fluids to flow and trigger metamorphic and rheological transformations. However, little is known about the porosity that facilitates these transformations. We analyzed microstructures that have recorded the mechanisms generating porosity in the lower crust from a pristine pseudotachylyte (solidified earthquake-derived frictional melt) and a mylonitized pseudotachylyte from Lofoten, Norway to understand the evolution of fluid pathways from the coseismic to the post- and interseismic stages of the earthquake cycle. Porosity is dispersed and poorly interconnected within the pseudotachylyte vein (0.14 vol%), with a noticeably increased amount along garnet grain boundaries (0.25â0.41 vol%). This porosity formed due to a net negative volume change at the grain boundary when garnet overgrows the pseudotachylyte matrix. Efficient healing of the damage zone by fluid-assisted growth of feldspar neoblasts resulted in the preservation of only a few but relatively large interconnected pores along coseismic fractures (0.03 vol% porosity). In contrast, porosity in the mylonitized pseudotachylyte is dramatically reduced (0.02 vol% overall), because of the efficient precipitation of phases (amphibole, biotite and feldspars) into transient pores during grain-size sensitive creep. Porosity reduction on the order of >85% may be a contributing factor in shear zone hardening, potentially leading to the development of new pseudotachylytes overprinting the mylonites. Our results show that earthquake-induced rheological weakening of the lower crust is intermittent and occurs when a fluid can infiltrate a transiently permeable shear zone, thereby facilitating diffusive mass transfer and creep
Time-resolved grain-scale 3D imaging of hydrofracturing in halite layers induced by gypsum dehydration and pore fluid pressure buildup
- âŠ