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 Contents & Abstracts Journal of Seismic Exploration



CONTENTS Volume 16, Number 2-4, December 2007

Special issue: SEISMIC ANISOTROPY - State of the Art
Part I: Fracture Characterization
E. Liu
Introduction
N. Barton
Fracture-induced seismic anisotropy when shearing is involved in production from fractured reservoirs
S. Maultzsch, M. Chapman, E. Liu and X.-Y. Li
Anisotropic attenuation in VSP data
E. Kozlov, E. Liu and I. Garagash
An integrated geophysical and geomechanical study of natural fracture characterization in a carbonate reservoir in east Siberia, Russia
M. Jakobsen, J.A. Skjervheim and S.I. Aanonsen
Characterization of fractured reservoirs by effective medium modelling and joint inversion of seismic and production data
M. Jakobsen
Effective hydraulic properties of fractured reservoirs and composite porous media
H. Mansouri
Anisotropic time-lapse estimation of geomechanical compaction
T.I. Chichinina, I.R. Obolentseva, G. Ronquillo-Jarillo, V.I. Sabinin, L.D. Gik and B.A. Bobrov
Attenuation anisotropy of P- and S-waves: theory and laboratory experiment
J. Wei, B. Di and X.-Y. Li
Effect of fracture scale length and aperture on seismic wave propagation: an experimental study
P.N.J. Rasolofosaon and B.E. Zinszner
The unreasonable success of Gassmann's theory ... Revisited
X. Zeng, K. Han and E. Liu
Numerical investigations of the limitation of Hudson's theory of cracked media using boundary element modelling
S.B. Gorshkalev, W.V. Karsten, K.A. Lebedev and I.V. Korsunov
Evidence for rapid variations of azimuthal anisotropy in the near surface: an example from eastern Siberia, Russia
D. Yang, S. Chen and J. Li
A Runge-Kutta method using high-order interpolation approximation for solving 2D acoustic and elastic wave equations
T. Chen, E. Liu and R. Cui
Application of multi-seismic attribute fusion to the detection of structural variations in a coal field in east China
J. Cheng, D. Pan and D. Li
Detection of mining-induced fracturing in the overburden of a coal field in east China
S. Crampin and Y. Gao
The new geophysics and the future of international workshops on seismic anisotropy

ABSTRACTS Vol. 16, No. 2-4, December 2007

Special issue: SEISMIC ANISOTROPY - State of the Art
Part I: Fracture Characterization
Liu, E., 2007. Introduction.

In October 2006, 120 people from over 20 countries (60 from within China) gathered at the foot of the Great Wall of China in Beijing to attend the 12th International Workshop on Seismic Anisotropy (12IWSA). About 90 papers were presented in the workshop. This special issue is Part I of the workshop proceedings and Part II will be published in early 2008. Part I contains 16 papers on seismic fracture characterization, and Part II contains 8 papers primarily on anisotropic seismic imaging. In this introduction, I will review the recent development in the theory, methodology, and applications of seismic anisotropy with special references to seismic fracture characterizations.
Barton, N., 2007. Fracture-induced seismic anisotropy when shearing is involved in production from fractured reservoirs.

Conducting, `open' joints, fractures or microcracks parallel to the classic direction sHmax are commonly referred to in the geophysics literature. They are the focus for most of the shear wave polarization studies, and are often assumed to be stress-aligned microcracks. Nevertheless, measurements in deeper wells reported during the last 10-15 years by Stanford University researchers, do not show conducting joints parallel to the `classic' direction sHmax. The non-conducting fractures in these deep wells are in the directions relative to sHmax that are normally assumed to be conducting directions in geophysics literature. The conducting joints in deep wells are found to be consistently in conjugate directions, bisected by the `classic' sHmax direction, so shear stress may therefore be acting to assist in their permeability. Numerous fractured reservoir cases in fact show 20? to 40? rotations of the polarization axes of qS1 and qS2, relative to interpreted sHmax directions, possibly because more than one set of fractures is present, as expected in most rock masses. Shearing induced by reservoir production and compaction, on one or more sets of fractures, is also known to be an important contributor to the maintenance of permeability in the face of increased effective stress. Shearing of conjugate sets of fractures is also considered by the author as a potential source of the temporal rotation of seismic anisotropy and attenuation, as recently recorded in 4D seismic at the Ekofisk and Valhall reservoirs in the North Sea.
Maultzsch, S., Chapman, M., Liu, E. and Li, X.-Y., 2007. Anisotropic attenuation in VSP data.

Observations of elastic anisotropy have proven capable of providing important information about subsurface fracture distributions, principally measures of fracture strike and intensity. Theoretical models predict that dynamic anisotropy, by which we mean attenuation and frequency-dependent anisotropy, can give important additional information regarding scale length and fluid saturation, but it is not clear whether such effects can be observed in field data. In this paper we review the analyses of a range of VSP datasets, and demonstrate systematic frequency dependence in both P- and shear-wave anisotropic attributes. These observations appear to be consistent with synthetic modelling, and we conclude that such observations have the potential to be used to infer important additional properties of the fracture system, principally scale lengths and fluid saturations.
Kozlov, E., Liu, E. and Garagash, I., 2007. An integrated geophysical and geomechanical study of natural fracture characterization in a carbonate reservoir in East Siberia, Russia.

IAs a result of extensive theoretical, experimental and field studies over the last 20 years, several widely recognized seismic characteristics of rock fracturing have been established: increased anisotropy of seismic velocity and reflectivity, decreased velocity, increased intrinsic attenuation and scattering, and anomalous frequency dependence of reflection records. More often than not, these fracture-related characteristics are weak and can be ambiguous to identify, thus causing essential difficulties in the detection of fractures and estimation of fracture parameters. We propose an integrated geophysical and geomechanical approach which aims to minimize these difficulties in order to further improve analytical and numerical modelling techniques and to better identify fracture-related anomalies in seismic data. The study presented in this paper is the first attempt to exemplify some commonly used techniques implemented at an oil/gas field in East Siberia where the hydrocarbon productivity of a carbonate reservoir is controlled by extensive fracturing.
Jakobsen, M., Skjervheim, J.A. and Aanonsen, S.I., 2007. Characterization of fractured reservoirs by effective medium modelling and joint inversion of seismic and production data.

This paper proposes a method for characterization of natural fractures in hydrocarbon reservoirs by quantitative integration of production data and (anisotropic) seismic attributes. The method is based on a unified model for the effective elastic and hydraulic properties of fractured porous media (which takes into account the effects of fracture geometry and fracture-fracture interaction in a consistent manner) and an Ensemble Kalman Filter (data assimilation) method (which provides uncertainties as well as mean values). In principle, our method can deal with fairly complex models of fractured reservoirs (e.g., involving multiple sets of vertical fractures that are embedded in a heterogeneous matrix). However, the initial inversion results presented here are based on a simplified model (involving a single set of horizontal fractures that are embedded in a homogeneous matrix). At the same time, the simplified model is both heterogeneous and anisotropic, since the fracture density was allowed to vary from grid block to grid block, in accordance with a (Gaussian) geostatistical model. An application to synthetic data suggests that one may obtain a significantly better estimate of the fracture density and permeability distributions within a fractured reservoir, by using time-lapse measurements of seismic attributes (the vertical P-wave acoustic impedance of each grid block) in addition to reservoir production data (bottomhole pressure, water cut, gas-oil ratio), in the dynamic reservoir characterization (history matching) process.
Jakobsen, M., 2007. Effective hydraulic properties of fractured reservoirs and composite porous media.

An integral equation method (Green's function technique) is used here to derive approximations for the effective (absolute) permeability tensors of fractured reservoirs and composite porous media. First, I derive an exact formal solution for the effective permeability tensor of a general (statistically homogeneous) random medium, in terms of the so-called T-matrix for the medium, which satisfies an integral equation of the Lippmann-Schwinger type (originally associated with quantum scattering theory). Then, I solve the Lippmann-Schwinger equation approximately under the assumption that all heterogeneities can be represented by ellipsoidal inclusions. It is assumed that the inclusions are distributed randomly in space in accordance with two-point correlation functions of ellipsoidal symmetry, and the interactions between more than two inclusions are ignored. On the basis of this T-matrix approach, I have derived novel expressions for the effective permeability tensors of a wide class of random media (including fractured reservoirs and sand-shale formations) of interest to the petroleum industry. The present method is extremely cheap computationally, and it complements the numerical methods of permeability upscaling (in the limit of complete scale separation) that are commonly used by industry. The effective medium approximations derived here may help us to understand the link between (frequency-dependent) seismic anisotropy and permeability, and is particularly relevant for the integration of seismic and production data from mesoscopically, heterogeneous reservoirs.
Mansouri, H., 2007. Anisotropic time-lapse estimation of geomechanical compaction.

Stress-induced velocity anisotropy is illustrated to improve the estimation of geomechanical compaction of oil and gas reservoirs. I modify a zero-offset two-way travel-time method for estimating reservoir compactions, extend it to non-zero-offset and include velocity anisotropy and demonstrate the improvement to compaction estimation of two reservoir models.
Chichinina, T.I., Obolentseva, I.R., Ronquillo-Jarillo, G., Sabinin, V.I., Gik, L.D. and Bobrov, B.A., 2007. Attenuation anisotropy of P- and S-waves: Theory and laboratory experiment.

We have developed a theory of attenuation anisotropy in transversely isotropic (TI) medium due to a single set of parallel fractures, and carried out an ultrasonic laboratory experiment on wave propagation in a thin-layered synthetic medium (dry and oil-saturated), that imitates the fractured medium. The theory predicts that Q-anisotropy is linked to velocity anisotropy. We study the directional behavior of P- and S-wave attenuation as a function of wave-propagation angle a from symmetry axis, and found out that it is similar to the behavior of the corresponding velocities. The theory also predicts a certain ratio between P-wave attenuations and velocities in two principal symmetry directions of TI medium (normal and parallel to fracture planes), as well as a certain relationship between SH- and SV-wave attenuations, and P- and SV-wave attenuations. We have compared the experimental data with the theory-predicted data and have obtained good confirmation of the theory.

We have developed a methodology for estimating anisotropy parameters for the transversely isotropic attenuative medium, using joint inversion of all data on attenuations and velocities of the three wave types (P, SH and SV). We take into account the interrelationship of P- and S-waves attenuations which depend on the same quantities - complex moduli of the stiffness matrix which include complex weaknesses characterizing fractures. In the result, for the experimental data, the complex weaknesses have been reconstructed, as well as Thomsen-style parameters for attenuation eQ, dQ, and ?Q.

We found out that eQ is independent of fracture properties, it is a simple function of the VP/VS-ratio. However, the parameter dQ may be meaningful for fracture characterization, because it is defined by the QP/QS-ratio, where Q1 and Q1 are the S-wave and P-wave attenuations in the direction orthogonal to the fracture plane. The sign of dQ may serve as an indicator of the crack-fill fluid; if dQ < 0 then QP/QS < 1, which corresponds to the case of gas-filled cracks, and vice versa if dQ > 0, then it's the case of liquid-filled cracks for which QP/QS > 1.
Wei, J., Di, B. and Li, X.-Y., 2007. Effect of fracture scale length and aperture on seismic wave propagation: an experimental study.

In this paper, we investigate the effects of fracture scale length and aperture on seismic wave propagation through seismic physical modelling. The physical models are constructed from a solid background of epoxy resin with inclusions of silicon rubber chips which come with different radius and thickness to simulate fractures with different scale length and aperture. The chips embedded in each model are of the same radius and thickness, and the fracture density is kept constant for all models in order to understand the effects of the scale length and aperture. P- and S-waves that propagate parallel and perpendicular to the fractures are then recorded using a pulse transmission method. The experimental results show that given the same fracture density the changing of radius has an only minor effect on the P-wave velocity and amplitude, and there are also little effects on the shear-wave amplitudes. The main observable effect is an increase of the slow shear-wave velocity with radius, leading to a decrease in shear-wave splitting with radius. The changing of fracture thickness has also little effects on the shear-wave amplitude except an obvious decrease in the slow shear-wave velocity, leading to an increase of shear-wave splitting with thickness. However, the increasing in fracture thickness induced a strong attenuation in the P-wave, in particularly for P-wave propagating perpendicular to the fracture. These findings may be useful for differentiating the effects of thin microcracks and large open fractures.
Rasolofosaon, P.N.J. and Zinszner, B.E., 2007. The unreasonable success of Gassmann's theory ... Revisited.

The poroelastic theory of Gassmann (1951) is widely used in fluid substitution problems of seismic monitoring in media considered as isotropic. Disagreements between experimental results and the predictions of this theory sometimes reported in the literature, at least in the laboratory, are often due to unsuitable experimental techniques. Here we focus on the importance of correct velocity measurements and demonstrate that only the careful phase velocity technique give consistent results with Gassmann's theory, in contrast with first break method or correlation technique. Using the phase velocity method we unambiguously show that Gassmann's theory, a quasi-static theory in principle, surprisingly explain experimental results outside its strict domain of applicability, for instance in the ultrasonic frequency band (0.1-1.0 MHz), which seems "unreasonable" (Rasolofosaon and Zinszner, 2002). This seems to be due to the negligible velocity dispersion due to purely poroelastic effects.

From another point of view it is not commonly appreciated that Gassmann in his original paper also dealt with anisotropic porous media, but of a special type. In his simplified theory the grain constituent is assumed isotropic, and only the rock skeleton is anisotropic. Besides the much greater simplicity of the formalism of the simplified theory, we note a substantial reduction of the number of characteristic elastic parameters to be estimated for the grain constituent, namely from 6 in the general theory to 1 in the simplified theory, which is quite convenient for practical applications. We a posteriori demonstrate the relevancy of this theory with numerical simulations and experimental ultrasonic measurements in the laboratory, which clearly provides additional credit to Gassmann's theory, emphasizing once again its "unreasonable success".
Zeng, X., Han, K. and Liu, E.R., 2007. Numerical investigations of the limitation of Hudson's Theory of cracked media using boundary element modelling.

In this paper, we model P-waves propagation in a cracked medium using the boundary element method (BEM). Two different models are considered: Hudson's effective medium model and a model with discrete crack distributions. Effects of different crack parameters, such as crack scale length, crack density, scattering, and P-wave speed are analyzed using numerical modelling with BEM. Then the limitations of these crack parameters in the Hudson's theory are quantitatively investigated. We conclude that the relative crack scale length is an important factor. If we use 10% as the acceptable relative energy scattering level, the minimum wavelength should be 7.3 times longer than the crack scale length. In view of the error in P-wave velocity, and if we use 10% relative error as the acceptable error range, the up limit for crack density is about 0.09 for Hudson's first-order theory and 0.19 for the second-order theory (for dry cracks).
Gorshkalev, S.B., Karsten, W.V., Lebedev, K.A. and Korsunov, I.V., 2007. Evidence for rapid variations of azimuthal anisotropy in the near surface: an example from Eastern Siberia, Russia.

Azimuthal anisotropy of near surface as studied with P- and S-waves data is widely spread in the Urupchen-Tokhomo Zone (UTZ) in Eastern Siberia, and the degree of this anisotropy is very high. It is characterized by rapid lateral changes not only in quantity but also in symmetry direction, being correlated to the surface topography. This causes considerable problems in the analysis of deeper reservoir intervals, especially with surface techniques, and requires special investigation of the near surface as well as creating new processing techniques.
Yang, D., Chen, S. and Li, J., 2007. A Runge-Kutta method using high-order interpolation approximation for solving 2D acoustic and elastic wave equations.

We transform the seismic wave equations in 2D inhomogeneous anisotropic media into a system of first-order partial differential equations with respect to time t. Based on the transformed equations, a new Runge-Kutta (RK) method using a high-order interpolation approximation is developed in this article. Our method enables wave propagation to be simulated in two dimensions through generally anisotropic and heterogeneous models. The high-order space derivatives are determined by using the wave displacement and its gradients simultaneously, while the time derivatives are approximated by the fourth-order RK method. On the basis of such a structure, the so-called RK-type method can suppress effectively numerical dispersions and source-noise caused by discretizing the wave equations when too-coarse grids are used, and is fourth-order accuracy in both space and time. Numerical calculations of the relative errors show that the numerical error of the RK-type method is less than those of the conventional finite-difference method (FDM) and fourth-order Lax-Wendroff correction (LWC) scheme. The three-component seismic wave-fields in an isotropic model are simulated and compared with the second-order FDM and fourth-order LWC. Meanwhile, we also present the wave-field snapshots computed by the RK-type method in a two-layered model with transversely isotropic symmetry. Promising numerical results further illustrate that the RK-type method has less numerical dispersions and can suppress effectively the source-noise.
Chen, T., Liu, E. and Cui, R., 2007. Application of multi-seismic attribute fusion to the detection of structural variations in a coal field in east China.

It is now over twenty years since the first 3D seismic experiment was carried out in coal mines in China. However, how to enhance the resolution for detecting small-scale faults and fractures in coal mines remains a challenge. In the last decade, there have been many new techniques that have been developed in oil/gas industry, which have a dramatic impact in improving structural interpretation for fault detections in coal fields. Techniques include wavelet transform, coherence cube analysis, multi-attribute analysis, and high-resolution image processes. However, interpreters face another dilemma of choosing "optimal" seismic attributes as there has been an explosive increase of the quantity of seismic attributes in the last decades. In this paper, we would like to share some of our experience in detecting faults and fracturing in a coal field in the east part of China through the use of attribute fusion techniques. Our experience shows that the use of multi-attribute fusion can enhance the detectability, reliability and efficiency of small-scale structural interpretations in coal fields.
Cheng, J., Pan, D. and Li, D., 2007. Detection of mining-induced fracturing in the overburden of a coal field in east China.

Overburden deformation and failure are commonly caused by underground mining activities. The mining-induced fracturing in the overburden occurs along with underground mining walk-ways, which is a dynamic process, and the development of mining-induced fracturing is extremely complex. We analyze 3D seismic data in order to understand the characteristic of seismic wavefield from overburden mining-induced fracturing. The information about the range of mined-out area, the development height of mining-induced fracturing, the fracture width and the mining-affected area can be obtained. Our experience shows that it is technically feasible to use 3D seismic data to detect the mining-induced fracturing in the overburden.
Crampin, S. and Gao, Y., 2007. The new geophysics and the future of international workshops on seismic anisotropy.

We suggest that, after a quarter of a century, International Workshops on Seismic Anisotropy (IWSAs) now report a mature science that needs stimulating. The stimulation may be the New Geophysics. New Geophysics is a new understanding of fluid-rock deformation, where fluid-saturated microcracks in the crust are so closely spaced they verge on fracture-criticality and failure in fracturing and earthquakes that they are critical systems. The crust as a critical system has new properties and may be considered as a New Geophysics with subtle implications for much of the behaviour of solid-earth geoscience. The key observable and diagnostic phenomenon is seismic shear-wave splitting, hence the importance for IWSAs. The New Geophysics, where low-level deformation can be monitored with shear-wave splitting, future behaviour calculated/predicted with anisotropic poro-elasticity and, in some circumstances, future behaviour potentially controlled by feedback, is a fundamental revision of classical sub-critical geophysics. We anticipate that International Workshops on Seismic Anisotropy will be invigorated as shear-wave splitting becomes the major observable for the New Geophysics and its many applications.





 

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