Elli Maria Charalampidou | Heriot-Watt University (original) (raw)

Papers by Elli Maria Charalampidou

2nd EAGE International Conference on Fault and Top Seals - From Pore to Basin Scale 2009, 2009

Shear bands and compaction bands have been obtained in Vosges Sandstone cylinders tested in the l... more Shear bands and compaction bands have been obtained in Vosges Sandstone cylinders tested in the laboratory under confining pressures of 30 to160MPa. The specimens have been heavily instrumented under load and subjected to pre-and post-deformation ultrasonic and Xray tomography. The primary purpose of this study is to add a detailed knowledge of the small-scale development of permanent dilational and compactional strains to our understanding of the mechanisms operative in shear/compaction band development and associated mechanical behaviours. This will inform predictions of changed fault zone permeability and lead to better prediction of fault seal or fault leakage under subsurface conditions. Samples were strain gauged and acoustic emissions were also recorded for some samples. Most were subjected to pre-and post-deformation ultrasonic and Xray tomography. Provisional assessment of the shear bands shows that material has densified but the velocities have also decreased. Open fractures are also seen on the processed Xray images. The samples show bulk compaction. The compaction band studies are less advanced, with acoustic emissions showing development of separated discrete bands that may have a shear component. This is an ongoing study and more robust conclusions, including thin section work, are expected by the Meeting.

Compaction bands in porous sandstones are planar zones of finite thickness that form almost norma... more Compaction bands in porous sandstones are planar zones of finite thickness that form almost normal to the direction of maximum compression. Grain-scale mechanisms indicate porosity reduction by grain crushing and movement of fragments into pores, along with grain movements. Such bands can have important impacts on fluid flow in sandstone reservoirs. We investigate by experiments the grain-scale processes occurring during compaction band formation in two porous (~22%) sandstones with similar grain-sizes (~300μm). Vosges sandstone is 93% quartz, 5% microcline, 1% kaolinite and 1% micas; it is moderately sorted with sub-angular to rounded shapes, and cement occurs mostly as quartz overgrowths. The texture of Vosges is more heterogeneous than the Bentheim, which has rounded grains, and is composed of 95% quartz, 3% feldspar and 2% kaolinite. Cylindrical samples of 40mm/80mm (diameter/length) for Vosges, and 50mm/100 mm for Bentheim, were loaded under triaxial compression with confining ...

$Research paper thumbnail of Reactivation of pre-existing fractures by fluid injection experiments at the laboratory scale$

This report contains a Synopsis and Two papers: Coupled continuum modelling of fracture reactivat... more This report contains a Synopsis and Two papers: Coupled continuum modelling of fracture reactivation and induced seismicity during ehnanced geothermal operation, by Wassing, van Wees and Fokker, draft version submitted to Geothermics Self-induced seismicity due to fluid circulation, by Aochi, Poisson, Trusiani, Rachez, and Schmittbul, draft version submitted to Geophysical Journal International. In the synopsis it is clarified that the main aim of the study is to understand the geomechanical causes and processes of induced seismicity in various contexts and at various scales.

Reservoir rocks, such as carbonates, are rapidly becoming key elements for the energy transition.... more Reservoir rocks, such as carbonates, are rapidly becoming key elements for the energy transition. The damage of these reservoir rocks when placed under a stress field must be characterized, to better predict storage capacity distribution. In the shallow subsurface, carbonate rocks accommodate the stress by developing structures at the mesoscale, such as fractures, deformation bands or stylolites, depending on porosity or fluid content. Those are localized, showing a finite damaged area, outside which the relative host rock can accommodate the applied stress in a different way, usually overlooked in low temperature and pressure conditions.In this study, we highlight a new marker of accommodation of shortening, characterized by heterogeneous quartz grain rotation in non-porous carbonate matrix. The studied rock is an upper Cretaceous bioclastic calcarenite from the Cotiella Massif (Spain). This rock is composed of 85% carbonate (micrite and recrystallized microsparite), 10% quartz, an...

Deformation bands, or tabular zones of localised strain, are a common manifestation of deformatio... more Deformation bands, or tabular zones of localised strain, are a common manifestation of deformation in upper crustal sedimentary rocks. Any mining or energy-related engineering applications must consider the possibility of reactivating these pre-existing failure planes because doing so can cause seismicity and compartmentalise the reservoir. However, there has only been a small amount of research done on laboratory-induced deformation in rocks with natural deformation features.On a low porosity bioclastic calcarenite from the Cotiella Basin, Spanish Pyrenees, our current experimental work aims to capture, for the first time to our knowledge, the dominant failure mechanisms during the reactivation of natural deformation bands oriented at different angles to the principal stress direction. At the I12-JEEP beamline at the synchrotron facility of Diamond Light Source, UK, we carried out triaxial compression experiments using a modified version of the Mjolnir cell used by Cartwright-Taylo...

&lt;p&gt;We report for the first time deformation features functionally described... more &lt;p&gt;We report for the first time deformation features functionally described as deformation bands developed in low porosity rocks. This observation contradicts existing knowledge that deformation bands develop only in highly porous rocks. The studied formation is a bioclastic calcarenite of the Upper Cretaceous Maci&amp;#241;os Unit in the Cotiella Massif. It is part of a megaflap adjacent to a salt diapir that has experienced extensional tectonics before the Pyrenean contraction. The bands present thickness variations, and in places they imitate the appearance of stylolites. This observation raises the question: how do deformation bands form in low porosity rocks?&lt;/p&gt;&lt;p&gt;To answer the question, we combine field observations with microstructural analysis to identify the occurring processes for the formation of deformation bands within low porosity rocks. After using optical microscopy and cathodoluminescence spectroscopy to conduct a petrographic study, we observe that the rock underwent multiple diagenetic cycles before the deformation stage, confirming that its porosity was significantly reduced before the deformation stage. Subsequently, we characterized the quartz grains inside the host rock and the dissolution-enabled deformation bands, using non-destructive imaging techniques. Three-dimensional image analysis from X-ray microtomography scans shows low grain size variations between the quartz grains in the host rock and in the bands, suggesting minor grain fracturing along the bands. However, grain reorientation has been reported for the quartz grains inside the bands, in relation to those in the host rock. Strain analysis was performed from Electron Backscattered Diffraction measurements, revealing higher strain along the quartz grain contacts inside the deformation band, compared to those in the host rock and stylolites. Our current data suggest that new porosity was created from local dissolution of the matrix, so the less soluble quartz grains were placed in contact. By wrapping-up the above observations, we propose a conceptual model that demonstrates the genesis and evolution of dissolution-enabled deformation bands in low porosity rocks, through local dissolution of the micritic matrix and transient porosity increase.&lt;/p&gt;

The aim of the development of a geothermal site by hydraulic and/or chemical stimulation is to ir... more The aim of the development of a geothermal site by hydraulic and/or chemical stimulation is to irreversibly increase the injectivity/productivity of a well by a local increase of the transmissivity of the fractures and/or faults intersecting the well and an improvement of the connectivity of the well to fracture and/or fault network. The generation of microseismicity is inherent in these methods of development of wells. However, if a large number of microearthquakes is inevitable, major events (magnitude between 2 and 3) must be studied carefully. Understanding these events may lead to propose scenarios for the stimulation reducing or eliminating them while maintaining the effectiveness of the stimulation relative to the injectivity. The WP4 work package is dedicated to understanding the geomechanical causes and processes of induced seismicity in various contexts and at various scales. This report corresponds to the deliverable D4.6 for the GEISER project. It recalls the results obt...

Etude expérimentale sur la localisation des déformations dans les grès poreux

2nd EAGE International Conference on Fault and Top Seals - From Pore to Basin Scale 2009, 2009

$Research paper thumbnail of Reactivation of pre-existing fractures by fluid injection experiments at the laboratory scale$

Etude expérimentale sur la localisation des déformations dans les grès poreux

In: Alshibli, K. A., Reed, A. H., (eds), Advances in Computed Tomography for Geomaterials: GeoX 2010, 2010

In this work we employ x-ray tomography, 3D digital image analysis and 3D-volumetric digital imag... more In this work we employ x-ray tomography, 3D digital image analysis and 3D-volumetric digital image correlation techniques to characterize localized deformation phenomena in sandstone. The specimens considered have been deformed in triaxial compression under a range of confining pressures (20-190MPa). Shear or compaction bands were observed, at low and higher confinement respectively. X-ray tomography images have been acquired before and after loading (unconfined) at different spatial resolutions (30 and 90 μm voxel size). The combination of both x-ray tomography and 3D DIC provides insights on the geometry and the mechanisms of the localized features.

In: Nakagawa, M., Luding, S., (eds), Powders and Grains: Proceedings of the 6th international conference on micromechanics of granular media, American Institute of Physics, 2009

This paper presents the results of experimental compression tests on a 2D granular material compo... more This paper presents the results of experimental compression tests on a 2D granular material composed of a mixture of circular and rectangular rods. Numerical simulations of the tests, using a Discrete Element Method (Molecular Dynamics),
successfully reproduce the experimental results. The results of the comparison between experimental and numerical results highlights the important role of the contact laws (especially the rolling resistance). A statistical study of the rotations of rectangular particles during compression (experimental and numerical) is carried out. The results of such study are finally compared to the predictions of continuum mechanics.

This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain... more This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain response of the Vosges sandstone (porous rock) tested at a range of confining pressures (i.e., 20-190 MPa) and different axial strain levels. Localised deformation was captured at different scales by a combination of full-field experimental methods, including Ultrasonic Tomography (2D), Acoustic Emissions (3D), X-ray Tomography (3D), and 3D volumetric Digital Image Correlation, plus thin section and Scanning Electron Microscope observations (2D). These experimental methods were performed before, during and after a number of triaxial compression tests. The combined use of the experimental techniques, which have different sensitivity and resolution, described the processes of shear band and shear-enhanced compaction band generation, which formed at low to intermediate and relatively high confining pressures, respectively. Pure compaction bands were not identified. The deformation bands were characterised as zones of localised shear and/or volumetric strain and were captured by the experimental methods as features of low ultrasonic velocities, places of inter- and intra-granular cracking and structures of higher density material. The two main grain-scale mechanisms: grain breakage (damage) and porosity reduction (compaction) were identified in both shear band and shear-enhanced compaction band formation, which presented differences in the proportions of the mechanism and their order of occurrence in time.

Η παρούσα διπλωματική εργασία έγινε στα πλαίσια του προγράμματος Erasmus, σε συνεργασία του Αριστ... more Η παρούσα διπλωματική εργασία έγινε στα πλαίσια του προγράμματος Erasmus, σε συνεργασία του Αριστοτέλειου Πανεπιστημίου Θεσσαλονίκης ( Ε .Παπαμίχος ) και του Université Joseph Fourier στη Grenoble ( G. Viggiani ). Στην εργασία παρουσιά-ζονται τα αποτελέσματα των πειραμάτων που έγιναν στο εργαστήριο 3S ( Laboratoire Sols, Solides, Structures ) της Grenoble. Τα πειράματα πραγματοποιήθηκαν με τη μη-χανή διάτμησης 1γ2ε του εργαστηρίου. Έπειτα από την επεξεργασία των αποτελε-σμάτων σε μακροσκοπική κλίμακα ακολουθούσε η επεξεργασία τους σε μικροσκο-πική κλίμακα. Για το λόγο αυτό, κατά τη διάρκεια των πειραμάτων γίνονταν λήψη φωτογραφιών οι οποίες σκιαγραφούσαν τη συμπεριφορά του δοκιμίου στη μικρο-σκοπική ανάλυση. Ως υλικό του δοκιμίου χρησιμοποιήθηκε ένα προσομοίωμα εδάφους απότελούμενο από ξύλινες βέργες κυλινδρικής και ορθογωνικής διατομής.
Η δομή της εργασίας είναι η εξής:
Στο κεφάλαιο 1 παρουσιάζεται μια σύντομη εισαγωγή. Ακολουθεί στο κεφάλαιο 2 η περιγραφή της συσκευής. Στο επόμενο κεφάλαιο παρουσιάζεται το υλικό που χρησι-μοποιήθηκε στα πειράματα. Στη συνέχεια, στο κεφάλαιο 4, παρουσιάζεται η πειρα-ματική διαδικασία. Στο κεφάλαιο 5 γίνεται ο σχολιασμός ορισμένων πειραμάτων χρη-σιμοποιώντας τις βέργες κυλινδρικής και κυλινδρικής και ορθογωνικής διατομής αντίστοιχα. Ακολουθεί ένα κεφάλαιο με την στατιστική επεξεργασία των αποτελε-σμάτων. Τέλος στο 7 κεφάλαιο παρουσιάζονται τα συμπεράσματα των πειραμάτων και στο παράρτημα 1 που ακολουθεί παρουσιάζονται οι δοκιμές που πραγματοποι-ήθηκαν εκτός εκείνων που έχουν ήδη σχολιάστει στο κεφάλαιο5.

$Research paper thumbnail of Brittle failure and fracture reactivation in sandstone by fluid injection$

European Journal of Environmental and Civil Engineering, 2014

We performed laboratory experiments on sandstone specimens to study brittle failure and the react... more We performed laboratory experiments on sandstone specimens to study brittle failure and the reactivation of an experimentally produced failure plane induced by pore pressure perturbations using constant force control in high compressive stress states. Here, we focus on the shear failure of a dry sample and the later on induced fracture plane reactivation due to water injection. Acoustic Emission (AE) monitoring has been used during both experiments. We also used ultrasonic wave velocities to monitor pore fluid migration through the initially dry specimen. To characterise AE source mechanisms, we analysed first motion polarities and performed full moment tensor inversion at all stages of the experiments. For the case of water injection on the dry specimen that previously failed in shear, AE activity during formation of new fractures is dominated by tensile and shear sources as opposed to the fracture plane reactivation, when compressive and shear sources are most frequent. Furthermore, during the reactivation of the latter, compressive sources involve higher compressive components compared to the shear failure case. The polarity method and the moment tensor inversion reveal similar source mechanisms but the latter provides more information on the source components.

International Journal of Rock Mechanics and Mining Science, 2014

This paper presents results from the analysis of shear-enhanced compaction bands that developed i... more This paper presents results from the analysis of shear-enhanced compaction bands that developed in a porous sandstone during triaxial compression tests at high confining pressures. The analysis uses non-destructive full-field experimental methods: X-ray tomography, 3D-volumetric digital image correlation (DIC) and acoustic emission (AE) monitoring including source mechanisms analysis. The 3D-volumetric DIC measurements reveal that these bands are zones with: a small component of band-parallel slip; a larger component of vertical shortening; compactant volumetric strains; and high maximum shear strains. Low X-ray tomography gray-scale standard deviation values within the bands indicate regions of grain size reduction and grain fragmentation. AE hypocenters detected during loading were concentrated inside these narrow bands and showed predominantly pure and hybrid collapse mechanisms; the latter implies some shear strain and is consistent with the oblique geometry of these bands. The experimental results in general support the hypothesis that laboratory developed shear-enhanced compaction bands, at least those studied here, share more characteristics with compaction bands than with compactant shear-bands; these latter deformation features differ from shear-enhanced compaction bands not only in the mechanical behavior, but also in the kinematics and the grain-scale deformation mechanisms.

International Journal of Rock Mechanics and Mining Science, 2014

International Journal of Rock Mechanics and Mining Science, 2013

We apply a method for retrieving accurate moment tensors in the acoustic emission (AE) experiment... more We apply a method for retrieving accurate moment tensors in the acoustic emission (AE) experiments based on the joint inversion of a family of AE events for their moment tensors and for the sensor amplifications including coupling effects between sensors and a specimen. The accuracy and efficiency of the method is tested on AEs recorded during three different temporal stages of a triaxial compression experiment on a cylindrical Bentheim sandstone specimen. The results show improved quality of the moment tensors indicated by significantly lower root-mean-square residuals between observed and predicted amplitudes. The approach is particularly suitable for detailed studies of the source parameters of AE events, to obtain accurate focal mechanisms and seismic moment tensors and for detecting fracturing regime of microcracks.

Tectonophysics, 2011

The study of localized deformation in porous sandstones at the laboratory scale can yield valuabl... more The study of localized deformation in porous sandstones at the laboratory scale can yield valuable insights into the internal structures and mechanisms of shear zones and compaction bands that might impact on flow at a reservoir scale. Herein, we report results of a laboratory study of shear and compaction band formation in a porous sandstone using a range of full-field experimental techniques: acoustic emissions, ultrasonic tomography, X-ray tomography, and 3D volumetric digital image correlation, plus thin section and Scanning Electron Microscope observations. The two main mechanisms involved in shear and compaction band formation, grain breakage (damage) and porosity reduction (compaction), are both well captured by the combination of all these laboratory techniques. The combined use of these techniques demonstrated the processes of shear and compaction band generation and the associated strain components that developed in the laboratory, and potentially also increased understanding of the naturally developed equivalents. The physical mechanisms of shear and compaction involved seem to be similar, but at the laboratory scale they show differences in the proportions and the order of occurrence in time.