Elli-Maria Christodoulos Charalampidou

Brittle failure and fracture reactivation in sandstone by fluid injection

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.

Triggering processes in rock fracture

We study triggering processes in triaxial compression experiments under a constant displacement rate on sandstone and granite samples using spatially located acoustic emission events and their focal mechanisms. We present strong evidence that event-event triggering plays an important role in the presence of large-scale or macrocopic imperfections, while such triggering is basically absent if no significant imperfections are present. In the former case, we recover all established empirical relations of aftershock seismicity including the Gutenberg-Richter relation, a modified version of the Omori-Utsu relation and the productivity relation-despite the fact that the activity is dominated by compaction-type events and triggering cascades have a swarmlike topology. For the Gutenberg-Richter relations, we find that the b value is smaller for triggered events compared to background events. Moreover, we show that triggered acoustic emission events have a focal mechanism much more similar to their associated trigger than expected by chance.

Mechanical behavior of mixtures of circular and rectangular 2D particles

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.

Compaction Bands in a Porous Sandstone Sample with Pre-induced Shear Bands

Understanding how different modes of deformation bands may interact at the same rock mass is of crucial importance, since such interaction may affect the flow properties within the reservoir rock. In this work we focus on the compaction band nucleation and evolution within a porous sandstone, in which a shear-band has been previously developed. For such a purpose, we performed at the laboratory scale triaxial compression experiments under 20 and 185 MPa confining pressures on a single Bentheim sandstone specimen. Acoustic Emissions (AE) were recorded throughout all experimental stages. AE hypocentre locations and AE source mechanisms were used to describe the spatiotemporal evolution of the developed deformation bands. The shear band evolution was AE controlled. Shear type sources were the prevailing mechanisms up to the peak stress, whereas, the shear band growth was mainly dominated by compressive type sources. The compaction band nucleated at the tip of the pre-existing shear band and evolved towards the circumference of the specimen. A second compaction band nucleated with increasing axial strain at the top part of the specimen and not far from the already developed shear and compaction bands. The dominant mechanisms during the compaction band initiation and formation were compressive type sources.

Shale hydration damage captured by nuclear magnetic resonance

Abstract Wellbore instability could restrict the efficient development of unconventional petroleum resources in shales as drilling fluids may influence the integrity of shales by causing hydration damage. In this work we study the impact of hydration on shale samples from Xi Feng formations using Nuclear Magnetic Resonance. Samples were saturated in distilled water for different durations and then samples’ T2 relaxation time and the spectrum area were analyzed using NMR. Damage variable, which can not only interpret the hydration damage in shales quantitatively but has also a key importance to build the damage theory, was defined with NMR T2 spectrum area, according to a relationship between the T2 relaxation time and the size of damage. Accuracy of the defined damage variable was verified by NMR images.GRAPHICAL ABSTRACT

Localised Deformation of Weakly Cemented Sands: A Case Study

Deformation band occurrence in cemented granular materials is intimately related to grain and cement properties. In this work we focus on a Cretaceous sand deposit (France), in which numerous deformation bands have been locally observed. Material from this deposit is used to fabricate artificially cemented samples at the laboratory scale. We principally use two experimental techniques to visualise the inner structure of the natural and artificial samples and comment on the deformation processes: the Scanning Electron Microscopy (SEM) and the X-Ray Computed Tomography (XRCT). So far, microstructural observations on the material far from the deformation bands have revealed that the medium-sized sand grains are held together by weak menisci bonds made of clays. Inside one deformation band we have discovered, instead, sand grains cemented with clays and syntaxial quartz overgrowths. We argue that grain fragmentation in the deformation band is the main silica source for quartz precipitation. The syntaxial overgrowths formation, however, is still weakly justified by the geological reconstructed pressure and temperature conditions within the sand depositional area. Furthermore, in the clay cement we have identified quartz micro-fragments, derived probably from the mentioned grain breakage, and micro-quartz, which may have generated from the interaction between free silica and clay in the pore space.

How well do we really understand subsurface fractures

Open fractures are notoriously difficult to locate and characterise in the subsurface but are very easy to anticipate in a general way with only a moderate understanding of their structural geology setting and rock types and very little understanding of their mechanical evolution. But a general interpretation is not adequate. How can we become more educated in our prediction of fracture location, character and effect on reservoir performance? Part of this problem involves a number of individual beliefs that may be incomplete or outdated; part is because even apparently supported beliefs, when taken together, are inconsistent. What do we actually know about fracture formation and development and how does it deviate from what we think we know? If we are clearer in our understanding of what we really do know then our thinking about fractures and their character will be clearer, and hopefully more useful. We outline some common beliefs about fractures, primarily from outcrop observations and theory, and assess them individually. We then look at them together assessing how they support or negate each other and place them in a geomechanical context. Finally we consider how experimental results can help consolidate and progress our understanding.

Full-field Laboratory imaging of localised deformation in sandstone

Rocks are neither truly homogeneous nor are laboratory test boundary conditions perfect plus at some stage of loading deformation in a specimen generally localises into shear/compaction bands, tensile/shear cracks or fractures. In the presence of such localised strains and heterogeneity point-wise measurements at a boundary do not well characterise the mechanics of the system. Therefore “Full-field” measurements are needed to better study the behaviour. In this paper full-field measurements, including x-ray tomography, 3D-volumetric digital image correlation and ultrasonic tomography, are used to characterise localised deformation phenomena in a sandstone deformed under triaxial compression. X-ray tomography allows 3D characterisation of the internal structure of laboratory specimens and the consequences of deformation are visible where there has been sufficient change in material density. Complementing x-ray tomography by 3D-volumetric digital image correlation (DIC) techniques allows quantification of 3D displacement and strain fields between different stages of loading by comparison of the corresponding x-ray tomography images. In addition ultrasonic tomography provides full-field measurement of ultrasonic velocities, and thus elastic properties, in laboratory specimens. DIC and x-ray tomography can provide information on structural changes due to deformation and the associated kinematics and strains, whilst ultrasonic tomography gives insight into damage (degradation of elastic properties), which are to be expected, e.g., due to compaction and related porosity reduction or grain crushing.

Laboratory investigation of shear and compaction bands: compaction and dilation identification

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.