216 research outputs found
Using acoustic velocities and microimaging to probe microstructural changes caused by thermal shocking of tight rocks
Introduction: Large scale, Earth processes and bulk rock properties are influenced by underpinning, dynamic, microstructures within rocks and geomaterials. Traditionally, the amount of porosity has been considered the primary control on important bulk rock properties like seismic wave velocities (Vp and Vs) and permeability. However, in tight rocks, velocity and permeability (k) can change substantially despite small changes in the amount of porosity during cracking. Therefore, other microstructural features inherent to given lithologies, such as heterogeneity and anisotropy in mineral properties are considered as factors controlling these bulk rock properties. Understanding which microstructural features control Vp, Vs, and permeability in tight rocks is useful in applications like enhanced geothermal systems (EGS), where thermal shocking is used to increase permeability. Thermal shocking involves injecting surface water into the subsurface to cool mineral crystals, induce contraction of crystals, and cause thermal cracking.Methods: We tested three tight lithologies with unique microstructures; granodiorite (SWG), basalt (PTB), and carbonate (MSA). We simulated thermal shocking by slowly heating samples to 350°C and then quenching them. We chose a temperature of 350°C because thermal shocking at this temperature is not well documented in literature, and this temperature is relevant to EGS. Using time-lapse microimaging, we assessed how thermal cracking occurs in each lithology and explored how thermal cracks influence connected porosity, Vp, Vs, and k.Results: Microimaging shows extensive cracking in the SWG and MSA lithologies, and little to no cracking in PTB with thermal shocking treatment. Vp and Vs became more pressure sensitive, and elastic moduli decreased with treatment for all lithologies. This may be caused by reduced stiffness between mineral crystal boundaries with treatment.Discussion: Lithologies with minerals that have anisotropy of or a wide range in thermal conductivity and/or thermal expansion coefficients can have increased thermal cracking. In thermally shocked SWG and MSA, Vp and Vs are good indicators of thermal cracking and k increases, but less so in PTB. Lithologies like PTB may require multiple thermal shock stimulations to increase permeability. Our results highlight how micro-scale changes influence bulk rock properties and when we can monitor permeability increases and microscale thermal cracking with Vp and Vs
A rock physics and seismic tomography study to characterize the structure of the Campi Flegrei caldera
The Campi Flegrei (CF) caldera experiences dramatic ground deformations
unsurpassed anywhere in the world. The source responsible for this
phenomenon is still debated. With the aim of exploring the structure of the caldera
as well as the role of hydrothermal fluids on velocity changes, a multidisciplinary
approach dealing with 3-D delay-time tomography and rock physics
characterization has been followed. Selected seismic data were modeled by
using a tomographic method based on an accurate finite-difference travel-time
computation which simultaneously inverts P-wave and S-wave first-arrival times
for both velocity model parameters and hypocenter locations.
The retrieved P-wave and S-wave velocity images as well as the deduced
Vp/Vs images were interpreted by using experimental measurements of rock
physical properties on CF samples, to take into account steam/water phase
transition mechanisms affecting P-wave and S-wave velocities. Also, modelling
of petrophysical properties for site-relevant rocks constrains the role of overpressured
fluids on velocity. A flat and low Vp/Vs anomaly lies at 4 km depth
under the city of Pozzuoli. Earthquakes are located at the top of this anomaly.
This anomaly implies the presence of fractured over-pressured gas-bearing
formations and excludes the presence of melted rocks. At shallow depth, a
high Vp/Vs anomaly located at 1 km suggests the presence of rocks containing
fluids in the liquid phase. Finally, maps of the Vp*Vs product show a high
Vp*Vs horse-shoe shaped anomaly located at 2 km depth. It is consistent with
gravity data and well data and might constitute the on-land remainder of the
caldera rim, detected below sea level by tomography using active source seismic
data. For a more exhaustive description of the utilized methodologies, of
synthetic tests for spatial resolution and uncertainty assessment and, the interpretation
of results, the reader may refer to the paper Vanorio et al. (2005)
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
Three-dimensional tomography and rock properties of the Larderello-Travale
In a geothermal area, a detailed knowledge of the three-dimensional velocity structures aids the managementof
the field and the further development of the geothermal source. Here,we present a high-resolution
study of the three-dimensional S-wave velocity structures from microearthquake travel times for the
Larderello-Travale geothermal field, Italy.We have also deduced the Vp/Vs and Vp ×Vs parameters for this
area toemphasize the deep variations in the physical rock properties due to fluid content and porosity. Furthermore,
effective porousmedium modelling has been performed for site-relevant lithologies, to improve
our interpretation of the results in terms of rock physics signatures. This has allowed us to estimate the
variation range of the seismological parameters investigated, as well as their sensitivity for suitable rock
under specific physical conditions. LowVp/Vs anomalies, arising froma lower Vp compared to Vs, dominate
the geothermal field of Larderello-Travale. These have been interpreted as due to steam-bearing formations.
On the contrary, analysis of Vp ×Vs images provides information on the relative changes in rock
porosity at depth. Comparison of tomographic section images with previously interpreted seismic lines
suggests that the reflective ‘K-horizon’ delineates a transition between zones that have different porosities
or crack gatherings. The ‘K-horizon’ also lies on low Vp/Vs anomalies, which suggests a steam saturation
zone, despite the reduced porosity at this depth
A 3D velocity model for earthquake location in Campi Flegrei area: application to the 1982-84 uplift event
The uplift crisis of the 1982-1984 in the Campi Flegrei area underlined
the importance of seismic surveillance for this volcanic caldera. One of the
key elements for an effective seismic network is to make use of a reliable velocity
model for earthquake location.
In the present work we will discuss criteria for the construction and validation
of a new 3D P-wave velocity model for earthquake location in the Campi
Flegrei area built from the integration of two high-resolution 3D tomographic
images of the region.
The model is used for locating a group of earthquakes from the uplift event of
the 1982-1984
Time-lapse sonic logs reveal patchy CO2 saturation in-situ
Based on time-lapse sonic and neutron porosity logs from the Nagaoka CO2 sequestration experiment, a P-wave velocity-saturation relation at reservoir depth is retrieved. It does not coincide with either of the end-member models of uniform and patchy saturation but falls in between even if realistic error estimates for the host rock properties are considered. Assuming a random distribution of CO2 patches it is shown that the mechanism of wave-induced flow can be evoked to explain this velocity-saturation relation. Characteristic CO2 patch size estimates range from 1 to 5 mm. Such mesoscopic heterogeneity can be responsible for attenuation and dispersion in the well logging frequency band
Hydrothermal formation of fibrous mineral structures: The role on strength and mode of failure
Studying the mechanisms that control the rheology of rocks and geomaterials is crucial as much for predicting geological processes as for functionalizing geomaterials. That requires the understanding of how structural arrangements at the micro and nano scale control the physical and mechanical properties at the macroscopic scale. This is an area of rock physics still in its infancy. In this paper, we focus the attention on the formation of cementitious phases made of micro- and nano-scale fibrous structures, and the controls of the arrangement of these phases on mechanical properties. We use hydrothermal synthesis, and the properties of hydrothermal water, to promote the growth of fibrous mineral phases having nano-size diameter and length of a few microns, creating disordered and entangled mats of fibrous bundles as those found in natural samples. We draw inferences from structural microscopy to inform a statistical model that establishes an interdependence between structural parameters of fibrous structures and bulk mechanical response. Structural parameters include number and length of fibers, spatial orientation, and fraction of fibrous threads bearing the load. Mechanical properties include strength and mode of failure. Results show that as the fibrous microstructure evolves from ordered and aligned to disordered and entangled, the mechanical response of the fibrous composite transitions from a brittle to ductile behavior. Furthermore, the disordered and entangled microstructure exhibits lower strength at failure though strength increases as the number of fibers within the microstructure increases. Finally, the longer the entangled fiber, the larger the strain that the matrix can accommodate. The value of this study lies in further understanding fault healing through hydrothermal fluids and how the physical properties of fibrous microstructures resulting from it control brittle-ductile transitions, and possibly, slow slip events along subduction zones
Using small-angle X-ray scattering to investigate the compaction behaviour of a granulated clay
The compaction behaviour of a commercial granulated clay (magnesium aluminium smectite, gMgSm) was investigated using macroscopic pressure-density measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray microtomography (XμT) and small-angle X-ray scattering (SAXS). This material was studied as a potential compaction excipient for pharmaceutical tabletting, but also as a model system demonstrating the capabilities of SAXS for investigating compaction in other situations.
Bulk compaction measurements showed that the gMgSm was more difficult to compact than polymeric pharmaceutical excipients such as spheronised microcrystalline cellulose (sMCC), corresponding to harder granules. Moreover, in spite of using lubrication (magnesium stearate) on the tooling surfaces, rather high ejection forces were observed, which may cause problems during commercial tabletting, requiring further amelioration. Although the compacted gMgSm specimens were more porous, however, they still exhibited acceptable cohesive strengths, comparable to sMCC. Hence, there may be scope for using granular clay as one component of a tabletting formulation.
Following principles established in previous work, SAXS revealed information concerning the intragranular structure of the gMgSm and its response to compaction. The results showed that little compression of the intragranular morphology occurred below a relative density of 0 · 6, suggesting that granule rearrangements or fragmentation were the dominant mechanisms during this stage. By contrast, granule deformation became considerably more important at higher relative density, which also coincided with a significant increase in the cohesive strength of compacted specimens.
Spatially-resolved SAXS data was also used to investigate local variations in compaction behaviour within specimens of different shape. The results revealed the expected patterns of density variations within flat-faced cylindrical specimens. Significant variations in density, the magnitude of compressive strain and principal strain direction were also revealed in the vicinity of a debossed feature (a diametral notch) and within bi-convex specimens. The variations in compaction around the debossed notch, with a small region of high density below and low density along the flanks, appeared to be responsible for extensive cracking, which could also cause problems in commercial tabletting
- …