Tectonically deformed coal (TDC) is defined as coal formed by various tectonic stresses. As one of the key concerns in the formation and recovery of coal bed methane (CBM) from the tectonic ...deformation coal seams, the texture and fractures of TDC present various features in different deformation environments and as a result the physical properties change. Therefore, the identification of TDC is significant and can be obtained from the physical properties. In this study, fourteen representative TDC and two undeformed coal samples from the Suzhou mining area in the east of north China plate were collected to investigate the geophysical recognition parameters of TDC by elastic properties, such as the velocities (P and S-waves), elastic parameters (Vp/Vs and Poisson's ratio) and velocity anisotropies. The results show that the elastic properties of undeformed coal and TDC samples from different deformation environments exhibit significant difference. Normally, the velocities and elastic parameters of undeformed coal have the highest values, while the velocity anisotropy has the lowest value. In brittle deformation environment, the velocity and elastic parameters decrease while the velocity anisotropy ascends with increased deformation extent. For the TDC samples from shear deformation environment, the velocity anisotropies present the significant high values among the selected TDC samples. In plastic deformation environment, the elastic parameters gradually decrease with the deformation extent ascending, while the velocity anisotropies are relatively high. Among the elastic properties, the Vp/Vs, Poisson's ratio and velocity anisotropy are sensitive to deformation type and extent of coal samples. Furthermore, the development characteristics of coal structural fractures have a significant impact on the elastic properties of measured coal samples from different deformation environments. In summary, the results may provide reference parameters for future studies in CBM rock physics from the tectonic deformation coal seams.
•The elastic parameters and velocity anisotropy of tectonically deformed coal (TDC) samples exhibit significant differences.•The Vp/Vs, Poisson’s ratio and velocity anisotropy are sensitive to deformation type and extent of TDCs.•The development characteristics of fractures have a significant impact on the elastic properties of TDC samples.
SUMMARY
The squirt flow model, proposed by Mavko & Jizba, has been widely used in explaining the frequency-related modulus and velocity dispersion between ultrasonic and seismic measurements. In this ...model, the saturated bulk modulus at high frequency is obtained by taking the so-called unrelaxed frame bulk modulus into Biot's or Gassmann's formula. When using Gassmann's formula, the mineral bulk modulus is taken as matrix bulk modulus. However, the soft pores (cracks) in rocks have a weakening effect on the matrix bulk modulus. The saturated bulk modulus at high frequency calculated with mineral bulk modulus as matrix bulk modulus is higher than the real values. To overcome this shortcoming we propose a modified matrix bulk modulus based on the Betti–Rayleigh reciprocity theorem and non-interaction approximation. This modification takes the weakening effect of soft pores (cracks) into consideration and allows calculating the correct saturated bulk modulus at high frequency under different soft-pore fractions (the ratio of soft porosity to total porosity) or crack densities. We also propose an alternative expression of the modified matrix bulk modulus, which can be directly obtained from laboratory measurements. The numerical results show that the saturated bulk modulus at high frequency using the original matrix bulk modulus (i.e. mineral bulk modulus) is approximated to that using the modified one only for rocks containing a small amount of soft-pore fraction. However, as the soft-pore fraction becomes substantial, using the original bulk matrix modulus is not applicable, but the modified one is still applicable. Furthermore, the results of the modified squirt flow model show good consistency with published numerical and experimental data. The proposed modification extends the applicable range of soft-pore fraction (crack density) of the previous model, and has potential applications in media having a relatively substantial fraction of soft pores or almost only soft pores, such as granite, basalt and thermally cracked glasses.
SUMMARY
Squirt flow is an essential cause of wave dispersion and attenuation in saturated rocks. The squirt flow model, proposed by Gurevich et al., has been widely applied to explain the wave ...dispersion and associated attenuation for saturated rocks at sonic and seismic frequency bands. In this model, the saturated bulk modulus is obtained by taking the partially relaxed frame bulk modulus as the dry frame modulus into Gassmann's formula with the mineral bulk modulus as the matrix bulk modulus. However, because of the weakening effect of soft pores on rock matrix bulk modulus, the model cannot accurately predict the saturated bulk modulus when the soft-pore fraction (the ratio of the soft porosity to total porosity) becomes large. We modified this model following Gurevich et al. by setting a different boundary condition. The modified squirt flow model can obtain correct saturated bulk modulus for large soft-pore fractions in the full range of frequencies, showing excellent consistency with the predictions of Gassmann, Mavko & Jizba (modified) at both low- and high-frequency limits, respectively. Modelling results show that the saturated bulk moduli and their dispersions calculated by the original and modified models exhibit little difference when the soft-pore fraction is small. Under this condition, the original model is as effective and accurate as the modified one. When the soft-pore fraction becomes larger, the differences in the bulk moduli and their dispersions become substantial, suggesting the original model is not applicable any longer. Furthermore, the differences calculated for the intermediate frequency range is even more obvious than other ranges, suggesting that the modified model should be used to calculate the bulk modulus and the dispersion in this frequency range. In summary, the modified squirt flow model can extend the original model's applicable range in terms of soft-pore fraction and has a potential application in rocks having a relatively large amount of soft-pore fraction such as basalts.
The acoustic behavior in fluid attenuating media can be effectively simulated using a fractional Zener model (FZM). Because of the fractional time derivatives of both stress and strain in the ...constitutive relationship, this mechanism is very realistic and flexible in describing seismic attenuation. However, using conventional FZM wave equations to propagate seismic waves requires storing large amounts of previous wavefield information to calculate the fractional time derivatives, which is unacceptable in practice. In this paper, we derive a new time-domain viscoacoustic wave equation in the framework of the FZM. This new equation does not contain any fractional time derivatives; thus, it is more economical in computational costs. Furthermore, the amplitude attenuation and phase dispersion effects are separated in the newly proposed equation, which is very favorable to compensate for energy loss and correct phase dispersion in reverse-time migration. To improve the accuracy, we incorporate a wave number (
k
)-space operator into the decoupled FZM wave equation to compensate for temporal dispersion errors caused by the second-order finite-difference discretization. Therefore, a high-temporal-accuracy viscoacoustic wave equation is derived to simulate nearly constant-
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wavefields in attenuating media. In the implementation, a low-rank decomposition method is introduced to solve the mixed-domain operators. Numerical analysis and modeling results demonstrate the effectiveness and applicability of the proposed method for simulating the decoupled viscoacoustic wavefield with high accuracy.
Pore pressure prediction is critical for shale gas reservoir characterization and simulation. The Wufeng–Longmaxi shale, in the southeastern margin of the Sichuan Basin, is identified as a complex ...reservoir affected by overpressure generation mechanisms and variability in lithification. Thus, standard methods need to be adapted to consistently evaluate pore pressure in this basin. Based on wireline logs, formation pressure tests, and geological data, this study applied the Eaton–Yale approach, which extends the theoretical basis of Eaton and Bowers methods to reservoir geological conditions and basin history. The method was developed by integrating petrophysical properties, rock physics interpretations, and geology information. The essential steps include (1) a multi-mineral analysis to determine mineral and fluid volumes; (2) a determination of the normal pressure trend line and extending it to overpressured sections; (3) predicting pore pressure using the basic Eaton approach and identifying overpressured zones; (4) correcting compressional velocity using lithology logs and a rock physics model; (5) determining the Biot Alpha coefficient and vertical-effective stress and estimating the new pore pressure values using the Eaton–Yale method. Overpressure zones were corrected, and reservoir pore pressure varied between 30.354 and 34.959 MPa in the wells. These research results can provide a basis for building reservoir simulation models, identifying reservoir boundaries, and predicting relative permeability.
Coal-hosted gallium-rich ores are mainly explored with geochemical analyses, and their elasticities lack research. This paper incorporated core testing, rock-physics modeling, and Monte Carlo ...simulations to characterize the elastic parameters of gallium-rich cores and discuss whether coal-hosted gallium-rich ores are elastically detectable. The measured cores from No. 6 coal in the Heidaigou mine showed that the gallium contents strongly correlate to the boehmite contents with a 0.96 correlation coefficient. The rock-physics modeling results showed that mineral compositions and contents are critical factors influencing elastic parameters, and elastic parameters in No. 6 coal showed profound heterogeneities as mineral compositions and contents. The preferred parameters for classifying and grouping different mineral-rich cores are the bulk modulus and moduli ratio. Cross-plotting bulk modulus vs. moduli ratio can qualitatively group measured cores and Monte-Carlo simulated realizations into different mineral-rich and saturation states properly. Concerning the factors of boehmite content, porosity, and saturation state, an interpretation template for boehmite-rich coal was proposed and used. As the template interpreted readings close to the measured contents, the built templates can quantitatively interpret boehmite and gallium contents in coal-hosted ores with high precision. In summary, the coal-hosted gallium-rich ores are elastically detectable.
The absorption (anelastic attenuation) and anisotropy properties of subsurface media jointly affect the seismic wave propagation and the quality of migration imaging. Anisotropic viscoelastic model ...can effectively describe seismic velocity and attenuation anisotropy effects. To reduce the computational cost and complexity of elastic wave modes decoupling for seismic imaging in anisotropic attenuating media, we have developed a pure-viscoacoustic transversely isotropic (TI) wave equation starting from the complex-valued velocity dispersion relation of quasi-compressional (qP) wave. The wave equation involving fractional Laplacians has advantages of being able to describe the constant-
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(frequency-independent quality factor) attenuation, arbitrary TI velocity and attenuation, decoupled amplitude loss and velocity dispersion effects. Numerical analyses showed that the simplified equation can accurately hold the velocity and attenuation anisotropy of qP-wave in viscoelastic anisotropic media in the range of moderate anisotropy. Compared to previous pseudo-viscoacoustic equations, the pure-viscoacoustic equation can be completely free from undesirable S-wave artifacts and behaves good numerical stability in tilted transversely isotropic (TTI) attenuating media. There are obvious wavefield differences between isotropic attenuation and anisotropic attenuation cases especially in the direction perpendicular to the axis of symmetry. Furthermore, to mitigate the influences of velocity and attenuation anisotropy on migrated seismic images, we have developed an anisotropic attenuation (
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) compensated reverse time migration (AQ-RTM) approach based on the new propagator. The compensation can be implemented by reversing the sign of the dissipation terms and keeping the dispersion terms unchanged during wavefields extrapolation. Synthetic example from a Graben model illustrated that the anisotropic
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-compensated RTM scheme can produce images with more balanced amplitude and accurate position of reflecters compared with conventional RTM methods under assumptions of acoustic anisotropic (uncompensated) and isotropic attenuating media. Results from a Marmousi-II model demonstrated that the new methodology is applicable for complicated geological model to significantly improve imaging resolution of the target area and deep layers.
Previous studies demonstrated that seismic attenuation and anisotropy can significantly affect the kinematic and dynamic characteristics of wavefields. If these effects are not incorporated into ...seismic migration, the resolution of the imaging results will be reduced. Considering the anisotropy of velocity and attenuation, we derive a new pure-viscoacoustic wave equation to simulate P wave propagation in transversely isotropic (TI) attenuating media by combining the complex dispersion relation and modified complex modulus. Compared to the conventional complex modulus, the modified modulus is derived from the optimized relationship between angular frequency and wavenumber, which can improve the modeling accuracy in strongly attenuating media. Wavefield comparisons illustrate that our pure-viscoacoustic wave equation can simulate stable P wavefields in complex geological structures without S-wave artifacts and generate similar P wave information to the pseudo-viscoacoustic wave equation. During the implementation, we introduce two low-rank decompositions to approximate the real and imaginary parts and then use the pseudo-spectral method to solve this new equation. Since the proposed equation can simulate decoupled amplitude attenuation and phase dispersion effects, it is used to perform
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-compensated reverse-time migration (Q-RTM). Numerical examples demonstrate the accuracy and robustness of the proposed method for pure-viscoacoustic wavefield simulations and migration imaging in transversely isotropic attenuating media.
Because high stress anomalies can induce dynamic failures in the underground panels of coal mines, the identification of high stress areas is critically important for mining safety. We have ...identified areas of high vertical stress of an in-situ longwall panel with seismic refraction tomography, and interpreted our results using rock physics models constrained by core measurements. Since P- and S-velocities in rocks are stress sensitive, mapping of them can provide useful information to identify areas of potential high stress. We have inverted P- and S-velocities of main roof of an in situ panel with refraction tomography, and converted them into vertical stress estimates using a calibration function from core measurements. Those estimates are classified into three groups (high, medium, and low), respectively. Identified zones of high vertical stress show qualitative agreement with monitored in-panel microseismic data. Zones derived from P-waves give a more plausible assessment than S-waves. We also discuss the link between high vertical stress areas and in-panel faults. Because of the impact of the regional stress field and previously mined out voids, the existing fault zone affects the redistribution of vertical stress in the underground panel.
•We observe refracted P- and S-waves simultaneously in an underground mine panel.•We identify high vertical stress areas of the panel using tomographic velocities.•Classified high vertical stress areas match high-energy in-panel microseisms.•Fault zone and mined out voids affect vertical stress redistribution of the panel.•Interpreted results from P-wave are more reliable than those from S-wave.
Tectonically deformed coal (TDC) is closely related to gas outbursts. Since TDC exploration is an essential objective for coalfield exploration, it is of great significance to study the petrophysical ...properties of TDCs and explore their differences. This study collected 17 TDCs and undeformed coal samples from the Huaibei coalfield and ultrasonically tested their petrophysical parameters, including densities, P- and S-wave velocities, and their derived petrophysical parameters (VP/VS ratio, P- and S-wave impedances). Undeformed coal and TDCs with different deformation types (brittle, shear, and plastic deformations) show significant differences in their petrophysical parameters, and cross-plot analysis can directly differentiate them. As with traditional geological methods, acoustically measured petrophysical parameters are good indicators to determine the type of coal deformation. However, the TDCs with the same deformation type have similar petrophysical parameters; it is not easy to distinguish them directly. Instead, the proposed method incorporating principal component analysis and clustering can accurately distinguish up to five classes of TDCs. Different types of tectonic deformation environments and their intensities are highly correlated with the clustering results. This paper also provides essential petrophysical parameters for undeformed coal and TDCs in the Huaibei coalfield, and these parameters can help interpret undeformed coal and TDCs using wireline logs and seismic data.
