Estimation of anisotropy parameters using the P-wave velocities on a cylindrical shale sample

In this paper we present a new approach to the estimation of the Thomsen anisotropy parameters and symmetry axis coordinates from the P-wave traveltime measurements on cylindrical shale samples. Using the tomography-style array of transducers, we measure the ultrasonic P-wave ray velocities to estimate the Thomsen anisotropy parameters for a transversely isotropic shale sample. This approach can be used for core samples cut in any direction with regard to the bedding plane, since we make no assumption about the symmetry axis directions and will estimate it simultaneously with the anisotropy parameters. We use the very fast simulated re-annealing to search for the best possible estimate of the model parameters. The methodology was applied to a synthetic model and an anisotropic shale sample

Microseismicity Induced in the Opalinus Clay by a Gallery Excavation in the Mont Terri Underground Rock Laboratory

International audienc

Laboratory micro-seismic signature of shear faulting and fault slip in shale

This article reports the results of a triaxial deformation experiment conducted on a transversely isotropic shale specimen. This specimen was instrumented with ultrasonic transducers to monitor the evolution of the micro-seismic activity induced by shear faulting (triaxial failure) and subsequent fault slip at two different rates. The strain data demonstrate the anisotropy of the mechanical (quasi-static) compliance of the shale; the P-wave velocity data demonstrate the anisotropy of the elastic (dynamic) compliance of the shale. The spatio-temporal evolution of the micro-seismic activity suggests the development of two distinct but overlapping shear faults, a feature similar to relay ramps observed in large-scale structural geology. The shear faulting of the shale specimen appears quasi-aseismic, at least in the 0.5 MHz range of sensitivity of the ultrasonic transducers used in the experiment. Concomitantly, the rate of micro-seismic activity is strongly correlated with the imposed slip rate and the evolution of the axial stress. The moment tensor inversion of the focal mechanism of the high quality micro-seismic events recorded suggests a transition from a non-shear dominated to a shear dominated micro-seismic activity when the rock evolves from initial failure to larger and faster slip along the fault. The frictional behaviour of the shear faults highlights the possible interactions between small asperities and slow slip of a velocity-strengthening fault, which could be considered as a realistic experimental analogue of natural observations of non-volcanic tremors and (very) low-frequency earthquakes triggered by slow slip events

Design and validation of an innovative 3D printer containing a co-rotating twin screw extrusion unit

This paper presents the design and validation of an innovative 3D printer containing a co-rotating twin screw extrusion unit (Co-TSE). Single screw print heads were developed in the mid-2000s as an alternative to filament-based 3D printers, but they have limited process flexibility and mixing capacity. The new design accepts material in powder or micro-pellet form, and its dispersive and distributive mixing capacity can be fine tuned by setting output and screw rotation speed independently. The design combines a miniaturized modular Co-TSE operated under starve-fed conditions with a benchtop Cartesian platform. Numerical calculations were performed to ascertain whether the appropriate thermomechanical environment for polymer processing could be created by the proposed design. A prototype was built and extrusion tests were performed under different operating conditions, using polypropylene and a 90/10 wt% polypropylene/polystyrene blend. Two screw configurations were used, with and without kneading discs, to assess the response of the extrusion unit in terms of flow characteristics and mixing performance. The restriction to flow created by the mixing elements determines the starting melt position, and the average residence times, while their shearing and extensional action enhances homogenization effectiveness. The screw configuration and rotation speed do not affect the output, which depends only on the feed rate. Preliminary deposition tests were conducted to determine the feasible printing parameters. A standard tensile test specimen, a square scaffold and a multicolored rectangular box were successfully printed, validating the innovative design. The mechanical properties of printed test specimens were within the expected values.This work was supported by the National Council for Scientific and Technological Development (CNPq), grants 2016-4/442109 and 142348/2018-0, and by the Coordination for the Improvement of Higher Education Personnel (CAPES), finance code 001

KG2B, a collaborative benchmarking exercise for estimating the permeability of the Grimsel granodiorite - Part 1: Measurements, pressure dependence and pore-fluid effects

Measuring the permeability of tight rocks remains a challenging task. In addition to the traditional sources of errors that affect more permeable formations (e.g. sample selection, non-representative specimens, disturbance introduced during sample acquisition and preparation), tight rocks can be particularly prone to solid–fluid interactions and thus more sensitive to the methods, procedures and techniques used to measure permeability. To address this problem, it is desirable to collect, for a single material, measurements obtained by different methods and pore-fluids. For that purpose a collaborative benchmarking exercise involving 24 laboratories was organized for measuring the permeability of a single low permeability material, the Grimsel granodiorite, at a common effective confining pressure (5 MPa). The objectives of the benchmark were: (i) to compare the results for a given method, (ii) to compare the results between different methods, (iii) to analyze the accuracy of each method, (iv) to study the influence of experimental conditions (especially the nature of pore fluid), (v) to discuss the relevance of indirect methods and models and finally (vi) to suggest good practice for low permeability measurements. In total 39 measurements were collected that allowed us to discuss the influence of (i) pore-fluid, (ii) measurement method, (iii) sample size and (iv) pressure sensitivity. Discarding some outliers from the bulk data set (4 out of 39) an average permeability of 1.11 × 10−18 m² with a standard deviation of 0.57 × 10−18 m² was obtained. The most striking result was the large difference in permeability for gas measurements compared to liquid measurements. Regardless of the method used, gas permeability was higher than liquid permeability by a factor approximately 2 (kgas = 1.28 × 10−18 m² compared to kliquid = 0.65 × 10−18 m²). Possible explanations are that (i) liquid permeability was underestimated due to fluid-rock interactions (ii) gas permeability was overestimated due to insufficient correction for gas slippage and/or (iii) gases and liquids do not probe exactly the same porous networks. The analysis of Knudsen numbers shows that the gas permeability measurements were performed in conditions for which the Klinkenberg correction is sufficient. Smaller samples had a larger scatter of permeability values, suggesting that their volume were below the Representative Elementary Volume. The pressure dependence of permeability was studied by some of the participating teams in the range 1–30 MPa and could be fitted to an exponential law k = ko.exp(–γPeff) with γ = 0.093 MPa−1. Good practice rules for measuring permeability in tight materials are also provided

Anisotropic modeling of elastic wave velocities evolution in deformed shales

see Abstract VolumeIstituto Nazionale di Geofisica e Vulcanologia, Italy (INGV) Centre National de la Recherche Scientifique (CNRS) ExxonMobil Upstream Research CompanyUnpublishedErice, Italyope

On the interpretation of ultrasonic laboratory measurements in anisotropic media

There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yield phase or group velocities. The main problem here is that the size of the transducers used in the laboratory for ultrasonic measurements is comparable with the size of the rock sample or propagation distance. As a result, it is not clear how to interpret the measured traveltimes of ultrasonic waves. The main question is how to define an appropriate effective travel path, its length, and orientation. To answer this question, we look in detail at the full wavefront generated by a finite-size transducer using a specifically designed experimental setup, and a synthetic material (phenolic grade CE). We devise an algorithm for the interpretation of traveltimes measured in the laboratory with finite-size transducers arbitrarily oriented with respect to the symmetry planes of the material. To illustrate/validate the proposed algorithm, we designed a special testing program. The results of the experiments are presented and turn out to be in very good agreement with the analytical predictions on which we base our interpretation algorithm

Seismic and Microseismic Signatures of Fluids in Rocks: Bridging the Scale Gap

The aim of this collection is to record a snapshot of our current state of understanding of the impact of fluids on the evolution and monitorability of subsurface rock formations during anthropogenic fluid injection/withdrawal operations, accounting for scale and frequency effects. In this introduction we provide a summary of the key challenges and findings reported in these 23 articles. This suite of articles addresses a variety of issues related to the impact of fluids on subsurface rocks, which can be grouped in three themes: (i) impact of fluids on wave velocity and attenuation/dispersion (five articles); (ii) fluids, fractures, and seismicity (nine articles); and (iii) dynamic fluid injection and substitution (nine articles)