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 20 MPa, pore fluid pressure up to 15 MPa, temperature ranging from 296 to 473 K). The portable cell can be used at synchrotron radiation sources that deliver a minimum X-ray flux density of 109 photons mm−2s−1in the energy range 30–100 keV to acquire tomographic datasets in less than 60 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

Dynamic Evolution of Permeability in Response to Chemo‐Mechanical Compaction

Pressure‐solution creep is an important fluid‐mediated deformation mechanism, causing chemo‐mechanical 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 spatio‐temporal 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 pressure‐solution creep while monitoring their microstructural evolution through computed X‐ray micro‐tomography. The evolution of permeability and fluid velocity of the samples were computed by using the pore geometries from the X‐ray micro‐tomography as input for the Lattice‐Boltzmann modeling. The results indicate that, as deformation proceeds, porosity and permeability decrease in both samples. In the salt ‐biotite 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 texture‐porosity process, the reduction in porosity causes salt ions to dissolve in the marginal salt and precipitate within the biotite‐bearing layer, where the bulk volume of salt grains increases over time

Strain shadow “megapores” 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 ∼1.7 × 105 µm3. 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 (0.25–0.41 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 &gt;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

4D porosity evolution during pressure-solution of NaCl in the presence of phyllosilicates

Pressure-solution creep is one of the most common crustal deformation mechanisms, inducing changes in the porosity and permeability of rocks. For a variety of rock types undergoing pressure solution, it has been shown that the presence of phyllosilicates may significantly enhance the rate of the pressure-solution process. In this experimental investigation, we present 4-dimensional (three dimensions + time) X-ray microtomographic data that contrast deformation by pressure-solution of a pure NaCl aggregate with that of a mixture of NaCl and biotite. The results show that for mixed samples (NaCl+biotite), phyllosilicates induce a marked reduction in porosity and pore connectivity and contribute to an increase in the local strain rates by an order of magnitude over pure NaCl samples. At the same time, phyllosilicates do not induce strain localization in the sample. We discuss various possible explanations for these observations including a possible positive feedback between the porosity distribution and pressure solution. Our study yields novel insights into the local effects of phyllosilicates during pressure-solution creep and provides full 4-dimensional imaging and characterization of the coupled evolution of porosity and pore connectivity over previously unprecedented experimental time scales

Time Resolved in situ X-Ray Tomographic Microscopy Unraveling Dynamic Processes in Geologic Systems

X-ray tomographic microscopy is a well-established analysis technique in different fields of the Earth Sciences to access volumetric information of the internal microstructure of a large variety of opaque materials with high-spatial resolution and in a non-destructive manner. Synchrotron radiation, with its coherence and high flux, is required for pushing the temporal resolution into the second and sub-second regime and beyond, and therefore moving from the investigation of static samples to the study of fast dynamic processes as they happen in 3D. Over the past few years, several hardware and software developments at the TOMCAT beamline at the Swiss Light Source contributed to establishing its highly flexible and user-friendly fast tomography endstation, making a large variety of new dynamic in situ and operando investigations possible. Here we present an overview of the different devices, including an in-house developed detector, a new highly efficient macroscope and a programmable fast rotation stage. Their tight interplay and synchronization are key for lifting experimental design compromises and follow dynamic processes with high spatial and temporal resolution unfolding over prolonged periods of time, as often required by many applications. We showcase these new capabilities for the Earth Sciences community by presenting three different geological studies, which make use of different sample environments. With a tri-axial deformation rig, chemo-mechanical-hydraulic feedbacks between gypsum dehydration and halite deformation have been studied, while the spatio-temporal evolution of a solute plume has been investigated for the first time in 3D with a flow cell. A laser-based heating system available at the beamline provides access to the high temperatures required to address bubble growth and collapse as well as bubble-bubble interaction and coalescence in volcanological material. With the integration of a rheometer, information on bubble deformation could also be gained. In the near future, upgrades of most large-scale synchrotron radiation facilities to diffraction-limited storage rings will create new opportunities, for instance through sub-second tomographic imaging capabilities at sub-micron length scales