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To improve our understanding of wave dispersion and adsorption signature of organic shales, we select a set of vertical shale samples cored from different formations and conduct the forced-oscillation measurements in seismic frequencies of 2–800 Hz at the oven-dried conditions. The results demonstrated that both Young's modulus and Poisson's ratio of immature Barnett and Eagle Ford shales and mature Mancos shale remain roughly constant and the extensional attenuations are also insignificant at the entire frequency band. However, the mature Jungar shale exhibits strong dispersions with the magnitude of 23.4 per cent for Young's modulus at the confining pressure of 3.4 MPa. The dispersion and magnitude of Jungar shale overall exhibit a decreasing trend with increasing pressure, but still maintain strong attenuation at the confining pressure of 20.8 MPa. It is found that the single parameter of TOC, thermal maturity and clay content are not the controlling factors of wave dispersion and adsorption signatures. Instead, the geochemical index |${\rm{TOC}} \times {S_1}/({S_1}{\rm{ + }}{S_2})$| is positively correlated with dispersion and attenuation magnitude, suggesting that organic richness and maturity level jointly control the wave adsorption characteristics. Geochemical analysis and phenomenological modelling imply that such kinds of energy dissipation in organic shales are possibly caused by the viscous friction at interfaces between inorganic grains and the surface of liquefied or bitumen-filled organic matter. Besides, a modified standard linear solid model containing multiple sets of a characteristic frequency is proposed and well captures the continuous dispersion signatures considering that the Jungar shale contains various maturity-induced viscous organic matter. [ABSTRACT FROM AUTHOR]

Understanding the relationship between compressional and shear wave velocities and their anisotropic characteristics in self-resourcing reservoir shales is of considerable interest for petrophysical, geophysical, and geomechanical applications of unconventional resources exploration and production. We compile laboratory measurements on the ultrasonic velocity of known shale reservoirs worldwide. Their compressional and shear wave velocities propagating in the vertical and horizontal directions can be characterized by a linear model with the coefficients of determination (R2) close to 0.9. The P-to-S-wave velocity ratio in the vertical direction is overall higher than that in the horizontal direction. The P-to-S-wave velocity ratio decreases with decreasing P-wave velocity due to the increasing organic matter content and hydrocarbon-filled porosity. Mineralogy influences the relationship between compressional and shear wave velocities to a minor degree. Carbonate-rich shales have a higher P-to-S-wave velocity ratio than silica-rich shale when the P-wave velocity is lower than 4 km/s and a lower ratio when the P-wave velocity is higher than 5 km/s. By employing anisotropic fluid substitution, we justify that the model derived from the laboratory measurements can apply to shale reservoirs containing gas or volatile oil. We apply the linear models to predict the shear wave velocity in two vertical wells of shale reservoirs. The predictions of shear wave velocity in the targeted reservoirs have less than 3% errors, demonstrating that the proposed linear models, to the first order, can predict shear wave velocity in unconventional shale reservoirs. • P- and S-wave velocity relationships in anisotropic organic shales are characterized with a linear model. • P-to-S wave velocity ratio decreases with decreasing P-wave velocity due to the increasing TOC and porosity. • The linear relationship is applicable for prediction of S-wave velocity in shale reservoir containing volatile oil and gas. [ABSTRACT FROM AUTHOR]

Understanding and modeling the elastic and anisotropic responses of organic shale is crucial for interpreting sonic and seismic data, and hence play a vital role in many fields of Earth and Energy science such as hydrocarbon source rock evaluation, reservoir characterization of unconventional shale plays, cap rock assessment for geological storage of CO 2 and nuclear waste disposal. The thermal maturity level significantly controls the geochemistry signature, geological characteristics and geophysical responses of organic shale. We review the rock physics modeling that links the geological feature with elastic responses of organic shale in general. However, current rock physics modeling of organic shales either ignore or qualitative consider the effects of thermal maturity. To construct a quantitatively maturity-constrained rock physics model for shale, we review the evolutionary characteristics of two types of organic matter (kerogen and bitumen) and organic porosity during thermal maturation. We summarize and quantitatively characterize the maturity-dependent elastic properties of organic matter and fluids. We propose a joint geochemistry-rock physics modeling that use various effective medium theories to quantify the effects of thermal maturity level (Vitrinite reflectance, % R o) on the elastic signatures of inorganic frame, fluids filled porous organic matter as well as their elastic interactions. In particular, the role of kerogen and bitumen on the elastic properties of the organic shale are separately incorporated into the rock physics modeling. The developed rock physics model demonstrates that reservoir parameters (e.g. TOC, porosity and clay content) have distinct impacts on the elastic and anisotropic behaviors of shale reservoirs at different maturity levels. We also use several ultrasonic and sonic logging data sets at various maturity levels, including Jimsar shale formation in NW China, Qingshankou shale formation in NE China and Longmaxi shale formation in SW China, to benchmark the effectiveness of the proposed model. The modeling results show satisfactory agreement with the P-wave velocity, S-wave velocity, density, P-wave anisotropy magnitude, and S-wave anisotropy magnitude, only when the appropriate maturity level is given, illustrating the importance of combining organic geochemistry with rock physics modeling. We also discussed the influence of organic matter morphology, the degree of clay alignment as well as overpressure-induced micro-fractures on the elastic and anisotropic responses of organic shale, which can be also considered within the developed rock physics modeling framework. [ABSTRACT FROM AUTHOR]

Heterogeneity is an inherent feature of subsurface rocks. Unconventional reservoir shales, a group of typically fined-grained sedimentary rocks, usually exhibit strong heterogeneities due to the diversity of their constituents, including stiff silicate and carbonate minerals, ductile clay minerals, and organic matter. Understanding the influence of heterogeneities on the mechanical response as well as the failure behavior of reservoir shales plays a crucial role in numerous fields of Earth and Energy sciences, such as unconventional reservoir exploration, sealing of CO 2 sequestration, nuclear waste disposal, and hydrogen storage. In this study, we propose two heterogeneity indexes, intending to quantitatively characterize the heterogeneity of mineralogical compositions in unconventional reservoir shales. The first index takes into account the heterogeneity of mineral content, and the other one comprehensively considers the mineralogical heterogeneity and its corresponding mechanical properties. We apply these heterogeneities indexes to two experimental datasets of marine and lacustrine shales. The results show that the heterogeneity index significantly influences the mechanical properties and deformation behaviors of reservoir shales. As the increment of heterogeneous indexes, the ultimate strength and static Young's modulus show a general decreasing trend. However, the effect of the heterogeneity index on the static Poisson's ratio is weak. The numerical simulation results also confirm our experimental analysis, where the ultimate strength and Young's modulus of rock have a negative correlation with the heterogeneity indexes. The proposed heterogeneity indexes provide insights into understanding the mechanical behaviors and fracability assessment of reservoir shales. [ABSTRACT FROM AUTHOR]