Nikolay Dyaur

Fluid substitution effects on seismic anisotropy

We derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil) on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P-wave moduli increase as much as 56% and P-wave velocity increases up to 19% for gas-to-brine substitution. For the direction parallel to fractures, P-wave velocity remains almost constant when porosity is low (5%) but can increase up to 4% if porosity is high (25%). Since P-waves in two different directions have different sensitivities to fluids and fractures, the Thomsens parameters (defined for HTI and orthorhombic symmetries), ϵ and δ, are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear-wave splitting (γ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures, and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation and 6% decrease for high porosity gas sands. We find that the percentage changes in gas-to-oil substitution are about half that of the gas-to-brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.

Elastic Properties of Four Shales Reconstructed From Laboratory Measurements At Unloaded Conditions

Summary The elastic properties of four shales (Barnett, Woodford, Muscogee, and Caney) are studied on 11 samples at the room conditions with the help of specially developed and constructed apparatus, allowing us to measure the elastic wave velocities (VP, VSH, and VSV) in different directions relative to the axis of cylindrical sample. The difference in the angular behavior of the velocities is attributed to the different mineral composition and microfabric of the shales. The stiffness tensors of shales are reconstructed from the measurements taking into account the experimental errors in velocities and rotation angle and possible disorientation of the sample axis relative to the elastic symmetry axis. The closest VTI stiffness tensors are inverted for all shale samples, and the accuracy of such an approximation is given in terms of relative average errors between experimental and theoretical velocities. These errors are shown to vary from 0.5% to 8%. These errors can be decreased if the tensor is approximated by the monoclinic symmetry in the laboratory coordinate system.

Influence of Source Frequency on Shear Wave Splitting - An Experimental Approach

Elastic anisotropy due to aligned cracks has been the subject of many seismic physical modeling experiments. Different experimental approaches related to sizes, shapes and density of cracks has been taken into account in earlier investigations. In this paper we present a physical study of shear-wave splitting in anisotropy induced aligned cracked media. In this experiment, rubber discs were used as inclusions in a solid epoxy resin matrix. Pulse transmission measurements were carried out on a reference model (without inclusions) and two other models with different aperture of cracks and dissimilar crack densities. The seismic records were measured using three different S-wave source transducers with dominant frequency of 0.1 MHz (low frequency), 0.5 MHz (intermediate frequency) and 1 MHz (high frequency). Crack apertures to seismic wavelength ratio were varied from 1.3 to 13.3 in one model and 2.3 to 23.5 in the second cracked model. Our results show that effects associated with acoustic scattering, attenuation and velocity dispersion interfere directly in shear wave splitting, which in turn is a function of crack aperture and source frequency.

3D Velocity Reconstruction In Shale Derived From Limited Number of Measurements

A technique allowing the determination of 3D distribution of both compressional and shear wave velocities in shales from limited number of experimental data has been developed. The technique involves an EMT (effective medium theory)-based inversion of shale’s microstructure (clay platelet orientation, crack shape and their connectivity). The knowledge of the microstructure and shale’s mineral composition makes it possible to find the shale’s stiffness tensor with the help of EMT methods. The stiffness tensor and density are necessary data to obtain the 3D distribution of velocities in shale. We demonstrate that this technique is applicable if only Vp is measured in some directions in the normal-to-bedding plane of shale, or Vp and Vs measurements are available only in the directions parallel and normal to bedding.

Physical modeling of anisotropic domains: Ultrasonic imaging of laser‐etched fractures in glass

Physical modeling, using ultrasonic sources and receivers over scaled exploration structures, plays a useful role in wave propagation and elastic property investigations. This paper explores the anisotropic response of novel fractured glass blocks created with a laser-etching technique. We compare transmitted and reflected signals for Pand Swaves from fractured and unfractured zones in a suite of ultrasonic experiments. The unaltered glass velocities are 5801 m/s and 3448 m/s for P and S waves, respectively, with fractured zones showing a small decrease (about 1%). Signals propagating through the fractured zone have decreased amplitudes and increased coda signatures. Reflection surveys (zero-offset and variable polarization and offset gathers) record significant scatter from the fractured zones. The glass specimens with laser-etched fractures display a rich anisotropic response.

Fracture Characterization from Elastic Waves - An Ultrasonic Experimental Approach

The main goal of this work was to estimate the preferential fracture orientation of a cracked medium based on cross-correlated S-wave seismograms analysis and Thomsen parameters. For this purpose, we analysed ultrasonic measurements of elastic (P and S) waves in a physical-modelling experiment with an artificially anisotropic cracked model. The solid matrix consisted of epoxy-resin, and small rubber-strip pieces simulate weakly-filled cracks. The anisotropic cracked model has three regions with three different fracture orientations. We used rotation of S1 and S2 polarizations for a cross-correlation analysis of the orientations, and P and S-wave measurements to evaluate the anisotropic parameters gamma and epsilon. The shear-wave source has a dominant frequency of 90 kHz, which corresponds to low wavelengths as compared to to the crack aperture, ensuring effective-media behaviour. Integrating the results from cross-correlation with anisotropic parameters analysis, we were able to estimate fracture orientation in anisotropic cracked model. The anisotropy parameter gamma showed good agreement with the cross-correlation analysis and, beyond that, provided additional information about the crack orientation that cross-correlation alone did not resolve.

Effect of overburden pressure on anisotropic parameters in a layered orthorhombic medium

Summary A physical modeling study of the effect of overburden pressure on anisotropic parameters was conducted using intrinsically orthorhombic phenolic boards as the model. These boards were coupled together with the help of a pressure device and uniaxial stress was gradually increased while time arrival and velocity measurements were repeated. Results show maximum increase in compressional and shear wave velocities ranging from 4% to 10% in different directions as a function of increasing uniaxial stress. Anisotropic parameter gamma generally diminished with increasing pressure and ranged from 0% to 33%. We observed anisotropic behavior attributable to both orthorhombic and VTI symmetries. Polar anisotropy behavior is due primarily to layering or bedding and tends to increase with pressure. Certain anisotropic parameters however unveil inherent orthotropic symmetry of phenolic.