Many engineering and natural materials, as well as earth systems, exhibit a combination of elastic-plastic and viscous behaviors, but precisely evaluating their rheology, and the micromechanics and ...chemical-mechanical feedbacks governing such rheology, can be a challenge. The cross-linked polymer Carbopol has long been used to explore fundamental rheological behaviors, most recently including fracturing during viscous creep in nature. Here, through rheometer experiments we establish that Carbopol 940 is, to a first order, a Herschel-Bulkley material. However, we further establish that the yield stress and viscosity are affected by chemically-sensitive micromechanical controls, namely pH and concentration of the polymer mixture. We explore these effects via the novel use of cryogenic scanning electron microscopy (SEM). Through the SEM imaging we show that there is a semi-quantitative relationship between pH, porosity at the >10-μm scale, and yield stress, a result of the ionic repulsion between polymer links at the molecular scale. We appeal to a model wherein the yield stress is a direct function of jamming expressed at the SEM scale, similar to those described in granular systems. As the pH of polymer dispersion increases, and the porosity decreases, the yield strength increases as a result of the increasingly jammed system. The initial viscosity is thus controlled by the yield stress, but after failure evolves with increasing shear rate due to characteristic unjammed flow of the material. The different controls on the yield stress versus viscous flow rates has implications for borehole engineering efforts (carbon capture and storage) employing Carbopol, and could prove instructive for modeling of natural viscoplastic deformation.
Natural faults have many characteristics in common with granular systems, including granular fault rocks, shear localization, and stick‐slip dynamics. We present experimental results which provide ...insight into granular behavior in natural faults. The experiments allow us to directly image force chains within a deforming granular media through the use of photoelastic particles. The experimental apparatus consists of a spring‐pulled slider block which deforms the photoelastic granular aggregate at a constant velocity. Particles that carry more of the load appear brighter when viewed through crossed polarizers, making the internal stresses optically accessible. The resulting pattern is a branched, anisotropic force chain network inclined to the shear zone boundaries. Under both constant volume and dilational boundary conditions, deformation occurs predominantly through stick‐slip displacements and corresponding force drops. The particle motion and force chain changes associated with the deformation can either be localized to the central slip zone or span the system. The sizes of the experimental slip events are observed to have power law (Gutenberg‐Richter‐like) distributions; the minimum dimensions of events and the behavior of force chains suggest that a particle scale controls the lower limits of the power law distributions. For large drops in pulling force with slip, the shape of the size distributions is strongly affected by the choice of boundary condition, while for small to moderate drops the probability distributions are approximately independent of boundary condition. These size‐dependent variations in stick‐slip behavior are associated with different spatial patterns: on average, small events typically correspond to localized force chain or particle rearrangements, whereas large events correspond to system‐spanning changes. Such force chain behavior may be responsible for similar size‐dependent behaviors of natural faults.
Fracture capture of organic pores in shales Daigle, Hugh; Hayman, Nicholas W.; Kelly, Eric D. ...
Geophysical research letters,
16 March 2017, Letnik:
44, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Shales are heterogeneous media with porosity at many scales and in many microtextural positions, including within organic matter and clay aggregates. Because these materials have contrasting ...mechanical properties, it remains unclear how induced fractures manage to connect with this porosity whether during hydrocarbon production, wastewater injection, or carbon‐capture‐and‐storage efforts. To explore porosity changes related to fracturing, we experimentally failed shale samples in a triaxial load apparatus and observed changes in microstructure through scanning electron microscopy, low‐pressure nitrogen sorption, and nuclear magnetic resonance. We observed a system of microcracks, many of which were likely experimentally induced and localized on grain boundaries. In some cases these fractures propagated into regions of natural porosity in organic matter. In the subsurface this “fracture capture” likely enhances pore connectivity, but only selectively depending upon mechanical conditions. Fracture capture is one possible mechanism by which multiscale compositional heterogeneity in shales may affect rheological heterogeneity.
Key Points
We compared the microstructure in shale before and after failure in a triaxial load apparatus
Failure‐induced microcracks localized on grain boundaries and in some cases propagated into porous regions within organic matter
Fracture capture in organic matter may enhance pore connectivity during hydraulic fracturing, wastewater disposal, or carbon dioxide injection
Onshore and offshore geological and geophysical observations and numerical modeling have greatly improved the conceptual understanding of magma‐poor rifted margins. However, critical questions remain ...concerning the thermal evolution of the prerift to synrift phases of thinning ending with the formation of hyperextended crust and mantle exhumation. In the western Pyrenees, the Mauléon Basin preserves the structural and stratigraphic record of Cretaceous extension, exhumation, and sedimentation of the proximal‐to‐distal margin development. Pyrenean shortening uplifted basement and overlying sedimentary basins without pervasive shortening or reheating, making the Mauléon Basin an ideal locality to study the temporal and thermal evolution of magma‐poor hyperextended rift systems through coupling bedrock and detrital zircon (U‐Th)/He thermochronometric data from transects characterizing different structural rifting domains. These new data indicate that the basin was heated during early rifting to >180°C with geothermal gradients of ~80–100°C/km. The proximal margin recorded rift‐related exhumation/cooling at circa 98 Ma, whereas the distal margin remained >180°C until the onset of Paleocene Pyrenean shortening. Lithospheric‐scale numerical modeling shows that high geothermal gradients, >80°C/km, and synrift sediments >180°C, can be reached early in rift evolution via heat advection by lithospheric depth‐dependent thinning and blanketing caused by the lower thermal conductivity of synrift sediments. Mauléon Basin thermochronometric data and numerical modeling illustrate that reheating of basement and synrift strata might play an important role and should be considered in the future development of conceptual and numerical models for hyperextended magma‐poor continental rifted margins.
Key Points
Recorded synrift exhumation and basin heating of the proximal rift to >180°C with geothermal gradients of ~80°C/km
Synrift heating of distal rift reached >180°C and persisted until exhumation at circa 50 Ma due to Pyrenean inversion
Thermal evolution of magma‐poor hyperextended continental rifts is not predicted by classic pure‐shear model due to depth‐dependent thinning
Fractures that propagate off of weak slip planes are known as wing cracks and often play important roles in both tectonic deformation and fluid flow across reservoir seals. Previous numerical models ...have produced the basic kinematics of wing crack openings but generally have not been able to capture fracture geometries seen in nature. Here we present both a phase‐field modeling approach and a physical experiment using gelatin for a wing crack formation. By treating the fracture surfaces as diffusive zones instead of as discontinuities, the phase‐field model does not require consideration of unpredictable rock properties or stress inhomogeneities around crack tips. It is shown by benchmarking the models with physical experiments that the numerical assumptions in the phase‐field approach do not affect the final model predictions of wing crack nucleation and growth. With this study, we demonstrate that it is feasible to implement the formation of wing cracks in large scale phase‐field reservoir models.
Key Points
Using a phase‐field approach to simulate wing crack formation capturing fracture rotation
Feasibility study to implement wing cracks in large scale reservoir simulations
Successful experimental benchmark of phase‐field model by means of physical experiments
A long‐standing question surrounding rifted margins concerns how the observed fault‐restored extension in the upper crust is usually less than that calculated from subsidence models or from crustal ...thickness estimates, the so‐called “extension discrepancy.” Here we revisit this issue drawing on recently completed numerical results. We extract thinning profiles from four end‐member geodynamic model rifts with varying width and asymmetry and propose tectonic models that best explain those results. We then relate the spatial and temporal evolution of upper to lower crustal thinning, or crustal depth‐dependent thinning (DDT), and crustal thinning to mantle thinning, or lithospheric DDT, which are difficult to achieve in natural systems due to the lack of observations that constrain thinning at different stages between prerift extension and lithospheric breakup. Our results support the hypothesis that crustal DDT cannot be the main cause of the extension discrepancy, which may be overestimated because of the difficulty in recognizing distributed deformation, and polyphase and detachment faulting in seismic data. More importantly, the results support that lithospheric DDT is likely to dominate at specific stages of rift evolution because crustal and mantle thinning distributions are not always spatially coincident and at times are not even balanced by an equal magnitude of thinning in two dimensions. Moreover, either pure or simple shear models can apply at various points of time and space depending on the type of rift. Both DDT and pure/simple shear variations across space and time can result in observed complex fault geometries, uplift/subsidence, and thermal histories.
Plain Language Summary
The areas of the Earth where continental crust gives way to oceanic crust, the continental margins, are one of the most defining qualities of the tectonic plates. As continents extended ‐ as North America is doing in the US Basin and Range today ‐ and then rifted, sedimentary basins developed that hold enormous hydrocarbon resources. In some places the underlying mantle of the Earth approached the surface, and in others volcanoes erupted, and thus rifting plays an enormous role global geochemical cycles, such as control atmospheric CO2 over geologic time. Similarly, these margins preserve the record of sealevel change and climactic changes. Scientists mostly rely on seismic reflection data to understand this process of continental rifting, because most rifts are underwater and sediment. Thus, computational modeling has become a powerful tool with which to understand this process. Here, we use such computational tools to produce an analytical visualization commonly used to interpret seismic data. The result rules out many models for continental rifting, and rules in others, and points the way to research questions bearing on the Earth's crust and mantle.
Key Points
We show thinning profiles for modeled extensional margins
Extension discrepancy is overestimated, and depth‐dependent thinning varies over space and time
Crustal depth‐dependent thinning cannot solve the extension discrepancy, though detachment shear zones are required to accommodate thinning
It is a long-standing question whether granular fault material such as gouge plays a major role in controlling fault dynamics such as seismicity and slip-periodicity. In both natural and experimental ...faults, granular materials resist shear and accommodate strain via interparticle friction, fracture toughness, fluid pressure, dilation, and interparticle rearrangements. Here, we isolate the effects of particle rearrangements on granular deformation through laboratory experiments. Within a sheared photoelastic granular aggregate at constant volume, we simultaneously visualize both particle-scale kinematics and interparticle forces, the latter taking the form of force-chains. We observe stick-slip deformation and associated force drops during an overall strengthening of the shear zone. This strengthening regime provides insight into granular rheology and conditions of stick-slip periodicity, and may be qualitatively analogous to slip that accompanies longer term interseismic strengthening of natural faults. Of particular note is the observation that increasing the packing density increases the stiffness of the granular aggregate and decreases the damping (increases time-scales) during slip events. At relatively loose packing density, the slip displacements during the events follow an approximately power-law distribution, as opposed to an exponential distribution at higher packing density. The system exhibits switching between quasi-periodic and aperiodic slip behavior at all packing densities. Higher packing densities favor quasi-periodic behavior, with a longer time interval between aperiodic events than between quasi-periodic events. This difference in the time-scale of aperiodic stick-slip deformation is reflected in both the kinematics of interparticle slip and the force-chain dynamics: all major force-chain reorganizations are associated with aperiodic events. Our experiments conceptually link observations of natural fault dynamics with current models for granular stick-slip dynamics. We find that the stick-slip dynamics are consistent with a driven harmonic oscillator model with damping provided by an effective viscosity, and that shear-transformation-zone, jamming, and crackling noise theories provide insight into the effective stiffness and patterns of shear localization during deformation.
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