Analysis of Eshelby-Cheng's model in anisotropic porous cracked medium: An ultrasonic physical modeling approach.

Many effective medium theories are designed to describe the macroscopic properties of a medium (the rock, or reservoir in this case) in terms of the properties of its constituents (the background matrix of the rock and the inclusions, for our scenario). A very well known effective medium theory is the Eshelby-Cheng model, which was studied by us in previous work, being tested for the case where the background medium was weakly-anisotropic and porous. The analysis was done testing elastic velocities and Thomsen parameters - as a function of crack density for fixed values of aspect ratio - predicted by the model with data acquired from synthetic rock samples. In this work, we aim to complete the analysis of the Eshelby-Cheng model capabilities when applied to rocks with porous and vertical transversely isotropic (VTI) backgrounds, testing the model for the elastic velocities as functions of aspect ratio - for fixed values of crack density - against experimental data. The data used to test the model were obtained from 17 synthetic rock samples, one uncracked and 16 cracked, the latter divided into four groups of four samples each, each group with cracks having the same aspect ratio, but with the samples having different crack densities. In these samples, ultrasonic pulse transmission measurements were performed to obtain the experimental velocities used to test the model. As was not possible to acquire data for velocity as a function of aspect ratio for fixed values of crack density, we performed interpolations of the experimental data to estimate these velocities. Eshelby-Cheng model effective velocities and Thomsen parameters were calculated using three formulations proposed for the crack porosity: one proposed by Thomsen, the second one proposed in our previous work (which depends only on the crack density) and the third one proposed in this work (which depends on the crack porosity and the aspect ratio, just like Thomsen's proposal). The comparisons between elastic velocities and Thomsen parameters - as function of crack aspect ratio, for fixed values of crack density - predicted by the model and estimated from the data via interpolation showed that the third formulation produced better fittings (lower root-mean-square errors) between model and experimental data for all ranges of aspect ratio and crack density.

Experimental characterization of synthetic porous orthorhombic fractured medium: A physical modeling approach.

The study of fractures in subsurface is very important since they are, in some cases, the main conduits for hydrocarbon flow in a reservoir. There are many ways to study the behavior of seismic waves in different fracturing conditions, including the use of physical modeling. This method allows, among other approaches, the analysis of the behavior of seismic wave properties in complex fractured media, such as media with orthorhombic symmetry. In this work we performed ultrasonic measurements on fractured physical models with orthorhombic symmetry from which we analyzed the behavior of elastic velocities and anisotropy parameters for different number of fractures. The presented results show the efficiency of the construction methodology used in the study by presenting P- and S- wave velocity values consistent with the theory for an orthorhombic medium. It was observed that for the direction perpendicular to the fracture system the values of P and S-wave velocities were the smallest for each model, and that the velocities decreased as the number of fractures increased in all models. Furthermore, most of the ∊ and γ values show a decreasing behavior as a function of the decreasing number of cracks, being the trend curves of ∊ linear and most of the trend curves of γ quadratic. Additionally, all the ∊ parameters presented a high correlation with the γ parameters for a small number of fractures, lower than 5.

A study of ultrasonic physical modeling of isotropic media based on dynamic similitude.

For decades, seismic and ultrasonic physical modeling has been used to help the geophysicists to understand the phenomena related to the elastic wave propagation on isotropic and anisotropic media. Most of the published works related to physical modeling use physical similitudes between model and field (geological environment) only in the geometric and, sometimes, in the kinematics sense. The dynamic similitude is approximately or, most of the time, not obeyed due to the difficulty to reproduce, in laboratory, the forces and tensions excited inside the earth when elastic waves propagate. In this work, we use expressions for dynamic similitude related to the ratio between stiffness coefficients or Lamé parameters. The resulting expression for dynamic similitude shows that this type of similitude has multiple solutions in the context of dynamic stress (non-uniqueness problem). However, the regularization of this problem can be reached by controlling porosity and clay content. Ultrasonic measurements (elastic) as well as petrophysical measurements (density, porosity and clay content) in synthetic sandstone rocks show how difficult it is to reproduce experimentally the three physical similarities studied in this work.

A new ultra-high-vacuum variable temperature and high-magnetic-field X-ray magnetic circular dichroism facility at LNLS.

X-ray magnetic circular dichroism (XMCD) is one of the most powerful tools for investigating the magnetic properties of different types of materials that display ferromagnetic behavior. Compared with other magnetic-sensitive techniques, XMCD has the advantage of being element specific and is capable of separating the spin and magnetic moment contributions associated with each element in the sample. In samples involving, for example, buried atoms, clusters on surfaces or at interfaces, ultrathin films, nanoparticles and nanostructures, three experimental conditions must be present to perform state-of-the-art XMCD measurements: high magnetic fields, low temperatures and an ultra-high-vacuum environment. This paper describes a new apparatus that can be easily installed at different X-ray and UV beamlines at the Brazilian Synchrotron Light Laboratory (LNLS). The apparatus combines the three characteristics described above and different methods to measure the absorption signal. It also permits in situ sample preparation and transfer to another chamber for measurement by conventional surface science techniques such as low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). Examples are given of XMCD measurements performed with this set-up on different materials.