Stress-dependent Crack Density Tensor Characterization and Fluid Content Identification in Cracked Medium

2017 ◽  
Author(s):  
L.H. Huang ◽  
G.Z. Zhang ◽  
X.Y. Yin ◽  
J.J. Zhang
Keyword(s):  
Crack Density ◽  
Density Tensor ◽  
Fluid Content ◽  
Content Identification ◽  
Stress Dependent

scholarly journals Exploring trends in microcrack properties of sedimentary rocks: An audit of dry-core velocity-stress measurements

Geophysics ◽  
2009 ◽  
Vol 74 (5) ◽  
pp. E193-E203 ◽  
Author(s):  
Doug A. Angus ◽  
James P. Verdon ◽  
Quentin J. Fisher ◽  
J.-M. Kendall
Keyword(s):  
Aspect Ratio ◽  
Sedimentary Rock ◽  
Crack Density ◽  
Sedimentary Rocks ◽  
Rock Physics ◽  
Initial Crack ◽  
Density Tensor ◽  
Physics Model ◽  
Rock Physics Model ◽  
Stress Dependent

Rock-physics models are used increasingly to link fluid and mechanical deformation parameters for dynamic elastic modeling. We explore the input parameters of an analytical stress-dependent rock-physics model. To do this, we invert for the stress-dependent microcrack parameters of more than 150 sedimentary rock velocity-stress core measurements taken from a literature survey. The inversion scheme is based on a microstructural effective-medium formulation defined by a second-rank crack-density tensor (scalar crack model) or by a second- and fourth-rank crack-density tensor (joint inversion model). Then the inversion results are used to explore and predict the stress-dependent elastic behavior of various sedimentary rock lithologies using an analytical microstructural rock-physics model via the initial modelinput parameters: initial crack aspect ratio and initial crack density. Estimates of initial crack aspect ratio are consistent among most lithologies with a mean of 0.0004, but for shales they differ up to several times in magnitude with a mean of 0.001. Estimates of initial aspect ratio are relatively insensitive to the inversion method, although the scalar crack inversion becomes less reliable at low values of normal-to-tangential crack compliance ratio [Formula: see text]. Initial crack density is sensitive to the degree of damage as well as the inversion procedure. An important implication is that the fourth-rank crack-density term is not necessarily negligible for most sedimentary rocks and evaluation of this term or [Formula: see text] is necessary for accurate prediction of initial crack density. This is especially important because recent studies suggest that [Formula: see text] can indicate fluid content in cracks.

EFFECTS OF POROSITY ON SEISMIC ATTENUATION

1994 ◽  
Vol 02 (01) ◽  
pp. 53-69 ◽  
Author(s):  
YUE-FENG SUN ◽  
JOHN T. KUO ◽  
YU-CHIUNG TENG
Keyword(s):  
Quality Factor ◽  
Crack Density ◽  
Seismic Attenuation ◽  
Center Frequency ◽  
Fluid Content ◽  
Intrinsic Attenuation ◽  
Resolution Mapping ◽  
Waveform Change ◽  
Optimization Schemes ◽  
Quality Factor Q

Effects of porosity on the attenuation of wave propagation are studied. The effects of pore fluids and porous structures are significant on changing the shapes of propagating wavelets. The waveform change of a propagating wavelet is much more sensitive to porosity than intrinsic attenuation. The attenuation occurred in natural rocks may largely due to these porous effects in addition to the internal friction of the solid represented by the intrinsic quality factor Q. The waveform of a propagating wavelet is quantitatively associated with attenuation, porosity, and fluid content, and is characterized by three parameters: the porosity ϕ, the quality factor Q, and the center frequency f0. Estimations of attenuation, porosity, and fluid content can be made by optimal wavelet analysis. High-resolution mapping of subsurface structures can be achieved by solving the integral equation with the nonlinear optimization of the time-variant wavelets. The inversion and the optimization schemes have been applied to study the porous sea floor and the crustal axial magma chamber (AMC) on the East Pacific Rise. These results provide porosity, attenuation information, and the highly resolved wave events, for further evaluation of compressional and shear wave velocities and other physical properties such as crack density and aspect ratio.

An innovative method to simulate stress-induced velocity changes in anisotropic rock

2021 ◽  
Vol 11 (4) ◽  
pp. 1-18
Author(s):  
Q. Bai ◽  
H. Konietzky
Keyword(s):  
Seismic Velocity ◽  
Numerical Models ◽  
Crack Density ◽  
Bedding Plane ◽  
Anisotropic Rock ◽  
Proposed Model ◽  
Isotropic Structure ◽  
Velocity Changes ◽  
Induced Velocity ◽  
Stress Dependent

This contribution proposes a numerical microstructural modeling approach to investigate stress-induced seismic velocity changes on anisotropic rocks. By introducing pre-existing cracks with preferential orientations in bonded-particle assemblies, the transverse isotropic structure of the Whitby Mudstone is simulated. Using power-law distributed aperture and calibrated micro-properties, we successfully reproduce stress-dependent velocity changes on Whitby Mudstones with different anisotropic angles in relation to the applied loads. The proposed model also duplicates the directional dependence of wave speed with respect to the bedding plane as expected theoretically. The numerical models show that velocity increase results from the closure of pre-existing cracks due to load increase. Direct relations are established between velocity changes and opened crack density (or crack closure), which displays a similar tendency compared with theoretical predictions. This relation can be used to quantify the micromechanisms behind the velocity changes. The proposed model provides the ability to directly examine the micro-processes underlying velocity changes.

Effects of fractures on NMO velocities and P‐wave azimuthal AVO response

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 711-726 ◽  
Author(s):  
Feng Shen ◽  
Xiang Zhu ◽  
M. Nafi Toksöz
Keyword(s):  
Aspect Ratio ◽  
Fractured Reservoirs ◽  
Crack Density ◽  
P Wave ◽  
Fractured Reservoir ◽  
Fracture Density ◽  
Fracture Detection ◽  
Fluid Content ◽  
Fracture Parameters ◽  
Fractured Medium

This paper attempts to explain the relationships between fractured medium properties and seismic signatures and distortions induced by geology‐related influences on azimuthal AVO responses. In the presence of vertically aligned fractures, the relationships between fracture parameters (fracture density, fracture aspect ratio, and saturated fluid content) and their seismic signatures are linked with rock physics models of fractured media. The P‐wave seismic signatures studied in this paper include anisotropic parameters (δ(v), (v), and γ(v)), NMO velocities, and azimuthal AVO responses, where δ(v) is responsible for near‐vertical P‐wave velocity variations, (v) defines P‐wave anisotropy, and γ(v) governs the degree of shearwave splitting. The results show that in gas‐saturated fractures, anisotropic parameters δ(v) and (v) vary with fracture density alone. However, in water‐saturated fractures δ(v) and (v) depend on fracture density and crack aspect ratio and are also related to Vp/VS and Vp of background rocks, respectively. Differing from δ(v) and (v), γ(v) is the parameter most related to crack density. It is insensitive to the saturated fluid content and crack aspect ratio. The P‐wave NMO velocities in horizontally layered media are a function of δ(v), and their properties are comparable with those of δ(v). Results from 3‐D finite‐difference modeling show that P‐wave azimuthal AVO variations do not necessarily correlate with the magnitude of fracture density. Our studies reveal that, in addition to Poisson's ratio, other elastic properties of background rocks have an effect on P‐wave azimuthal AVO variations. Varying the saturated fluid content of fractures can lead to azimuthal AVO variations and may greatly change azimuthal AVO responses. For a thin fractured reservoir, a tuning effect related to seismic wavelength and reservoir thickness can result in variations in AVO gradients and in azimuthal AVO variations. Results from instantaneous frequency and instantaneous bandwidth indicate that tuning can also lead to azimuthal variations in the rates of changes of the phase and amplitude of seismic waves. For very thin fractured reservoirs, the effect of tuning could become dominant. Our numerical results show that AVO gradients may be significantly distorted in the presence of overburden anisotropy, which suggests that the inversion of fracture parameters based on an individual AVO response would be biased unless this influence were corrected. Though P‐wave azimuthal AVO variations could be useful for fracture detection, the combination of other types of data is more beneficial than using P‐wave amplitude signatures alone, especially for the quantitative characterization of a fractured reservoir.

Use of AVOA data to estimate fluid indicator in a vertically fractured medium

Geophysics ◽  
2006 ◽  
Vol 71 (3) ◽  
pp. C15-C24 ◽  
Author(s):  
Ranjit K. Shaw ◽  
Mrinal K. Sen
Keyword(s):  
Reflection Coefficient ◽  
Random Noise ◽  
Crack Density ◽  
Synthetic Data ◽  
Quantitative Measure ◽  
P Wave ◽  
Fluid Content ◽  
Data Inversion ◽  
Microstructural Property ◽  
Fractured Medium

Microstructural attributes of cracks and fractures, such as crack density, aspect ratio, and fluid infill, determine the elastic properties of a medium containing a set of parallel, vertical fractures. Although the tangential weakness [Formula: see text] of the fractures does not vary with the fluid content, the normal weakness [Formula: see text] exhibits significant dependence on fluid infill. Based on linear-slip theory, we used the ratio [Formula: see text] — termed the fluid indicator — as a quantitative measure of the fluid content in the fractures, with g representing the square of the ratio of S- and P-wave velocity in the unfractured medium. We used a Born formalism to derive the sensitivity to fracture weakness of PP- and PS-reflection coefficients for an interface separating an unfractured medium from a vertically fractured medium. Our formulae reveal that the PP-reflection coefficient does not depend on the 2D microcorrugation/surface roughness with ridges and valleys parallel to the fracture strike, whereas the PS-reflection coefficient is sensitive to this microstructural property of the fractures. Based on this formulation, we developed a method to compute the fluid indicator from wide-azimuth PP-AVOA data. Inversion of synthetic data corrupted with 10% random noise reliably estimates the normal and tangential fracture weaknesses and hence the fluid indicator can be determined accurately when the fractures are liquid-filled or partially saturated. As the gas saturation in the fractures increases, the quality of inversion becomes poorer. Errors of 15%–20% in g do not affect the estimation of fluid indicator significantly in case of liquid infill or partial saturation. However, for gas-saturated fractures, incorrect values of g may have a significant effect on fluid-indicator estimates.

A Continuum Model for Dynamic Tensile Microfracture and Fragmentation

1985 ◽  
Vol 52 (3) ◽  
pp. 593-600 ◽  
Author(s):  
L. Seaman ◽  
D. R. Curran ◽  
W. J. Murri
Keyword(s):  
Continuum Model ◽  
Crack Density ◽  
Volume Growth ◽  
Material Point ◽  
Detailed Simulation ◽  
Elastoplastic Materials ◽  
Material Separation ◽  
Dependent Growth ◽  
Stress Dependent ◽  
Fracture Processes

A continuum model for dynamic tensile cleavage fracture and fragmentation has been developed for detailed simulation of brittle fracture processes in elastoplastic materials. The model includes processes for nucleation of microcracks, stress-dependent growth, coalescence and fragmentation, and stress relaxation caused by the developing damage. Fracturing is characterized by a crack density with a distribution of sizes at each material point. The model extends previous work by treating more completely full material separation and stress-free volume growth, as well as multiple loadings, unloadings, and recompaction, and by describing the damage in greater microscopic detail.

Modeling sensitivity of 3D, 9-C wide azimuth data to changes in fluid content and crack density in cracked reservoirs

Geophysics ◽  
2010 ◽  
Vol 75 (5) ◽  
pp. T155-T165 ◽  
Author(s):  
Herurisa Rusmanugroho ◽  
George A. McMechan
Keyword(s):  
Practical Importance ◽  
Crack Density ◽  
Maximum Amplitude ◽  
P Wave ◽  
Staggered Grid ◽  
Wave Reflections ◽  
S Wave ◽  
Reservoir Properties ◽  
Fluid Content ◽  
Isotropic Media

The volume density of cracks and the fluids contained in them are salient aspects of characterization of cracked reservoirs. Thus, it is of practical importance to investigate whether variations in these reservoir properties are detectable in seismic observations. Eighth-order staggered-grid, 3D finite-difference simulations generate nine-component amplitude variations with offset and azimuth (AVOAZ) for reflections from the top of a vertically cracked zone embedded in an isotropic host. The T-matrix method is used to calculate elastic stiffness tensors. Responses for various crack densities and fluid contents show sensitivity to the spatial orientation of, and variation in, anisotropy. In isotropic media, when source and receiver components have the same orientation (such as XX and YY), reflection amplitude contours align approximately perpendicular to the particle motion. Mixed components (such as XY and YX) have amplitude patterns thatare symmetrical pairs of the same, or opposite, polarity on either side of the diagonal of the 9-C response matrix. In anisotropic media, AVOAZ data show the same basic patterns and symmetries as for isotropic media but with a superimposed tendency for alignment parallel to the strike of the vertical cracks. The data contain combined effects related to the source, receiver, and crack orientations. The sensitivity of data to changes in fluid content is quantified by comparing the differences between responses to various fluid conditions, to the maximum amplitude of oil-filled crack responses. For a crack density of 0.1, amplitude differences are [Formula: see text] for oil-dry and [Formula: see text] for oil-brine. The corresponding values for S-wave reflections are [Formula: see text] for oil-dry and [Formula: see text] for oil-brine. Amplitude changes caused by changing the oil-filled crack density from 0.1 to 0.2 are [Formula: see text] for P-wave reflections and [Formula: see text] for S-wave reflections. These differences are visible in AVOAZ data and are potentially diagnostic for reservoir characterization.

Determination of the stress-dependent stiffness of plasma-sprayed thermal barrier coatings using depth-sensitive indentation

2003 ◽  
Vol 18 (8) ◽  
pp. 1975-1984 ◽  
Author(s):  
J. Malzbender ◽  
R. W. Steinbrech
Keyword(s):  
Crack Density ◽  
Strain Curve ◽  
Spherical Indenter ◽  
Atmospheric Plasma ◽  
Elastic Response ◽  
Elastic Behavior ◽  
Thermal Barrier ◽  
Additional Information ◽  
Stress Dependent ◽  
Plasma Sprayed

The elastic response of atmospheric plasma-sprayed coatings was investigated using Vickers and spherical indenter geometries. In both cases a strong dependency of the stiffness on the applied load (indentation depth) was observed. The stiffness of the coatings decreased with increasing load for a Vickers indenter, whereas it increased for a spherical indenter. This contrary behavior was related to the relative crack density in the deformed volume and to the stress dependence of the stiffness due to crack closure. The effect of annealing on the stiffness was quantified for both tip geometries. The heat treatment yielded additional information on the relationship between the indentation data and the microstructural defects. From the results it was concluded that the stiffness measured using a sharp indenter and small load reflected the elastic behavior of single spraying splats. With the relatively large spherical indenter, the average global stiffness of the thermal barrier coating was measured even at small loads. From the data obtained using the spherical indenter, a compressive stress–strain curve was suggested. Furthermore, values of the apparent crack density and yield strength were determined from the indentation tests.

Cyclic Damage Evolution in a PMC Laminate

2007 ◽  
Vol 348-349 ◽  
pp. 33-36
Author(s):  
Alan Plumtree ◽  
M. Melo
Keyword(s):  
Fatigue Damage ◽  
Damage Evolution ◽  
Damage Mechanics ◽  
Life Stage ◽  
Crack Density ◽  
Stage I ◽  
Matrix Cracks ◽  
Minimum Stress ◽  
Stress Dependent ◽  
Cyclic Damage

Using damage mechanics, cyclic damage evolution has been described and evaluated in a non-crimped glass epoxy fabric composite. A fundamental fatigue study has been carried out by progressively monitoring the fatigue damage modulus and crack density throughout the life of an [0,+45,90,-45]2 (antisymmetric) laminate cycled at a stress ratio R (minimum stress/maximum stress) of 0.1. Development of damage can be separated into two main stages. Initially, damage increases very quickly during the first 10% of life (Stage I). Afterwhich, it increases more slowly at a relatively constant rate to failure (Stage II). The changes in the fatigue modulus and crack density both show the same behaviour. A large amount of damage in the form of transverse matrix cracks develops during the first cycle. These then remain constant throughout life. By contrast, the number of shear matrix cracks increase continually. The crack density is cycle, not stress dependent. This behaviour is reflected by changes in the fatigue modulus. Using damage mechanics, a representative equation has been applied to express the progressive evolution of damage. The significance of which is that the amount of fatigue damage at the end of Stage I for any stress level can be used to predict fatigue life and the stress-life diagram for the laminate.

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