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Seismic and electromagnetic explorations are two of the most successful geophysical applications for understanding the subsurface earth, and the joint interpretation of seismic and electromagnetic survey data can help to better characterize the rocks because they contain independent and complementary information about the rocks. However, the success of the joint interpretation depends on the understanding of the correlations between the elastic and electrical rock properties and their influencing factors. Confining pressure is an important geological parameter that has been found to give rise to linear elastic–electrical correlations in sandstones. However, it is still poorly known what controls the slopes of the pressure-dependent linear correlations, even though slope is one of the most important parameters determining the linear correlation. We make artificial sandstones with controlled porosity and permeability, respectively, and measure their pressure-dependent elastic (electrical resistivity) and electrical (P -wave velocity) properties simultaneously, as well as porosity. We show that the slopes of the measured electrical resistivity versus P -wave velocity as an implicit function of confining pressure correlate positively with the compliant porosity in all the samples. The results not only reveal the petrophysical parameter that controls the slopes of the pressure-dependent linear elastic–electrical correlations in sandstones, but also provide a basis for the discrimination of the slope-controlling parameter from the simultaneously measured elastic and electrical properties. [ABSTRACT FROM AUTHOR]

Fractured hydrate-bearing reservoirs are extensively discovered worldwide and show notable anisotropic geophysical properties. Hydrate distribution in fractures significantly affects the anisotropic properties, and hence plays an important role in the accurate assessment of hydrate resources. However, the knowledge about how the hydrate distribution impacts the anisotropic geophysical properties of fractured reservoirs, which is the premise for the identification and quantification of hydrate in fractured reservoirs, is still poorly understood. To obtain such knowledge, we forward study the effects of various hydrate distribution (including floating, bridging and evolving hydrate distribution) in aligned fractures on the anisotropic elastic, electrical and joint elastic–electrical properties of a digital core using validated numerical methods. We show that for all the hydrate distribution, the anisotropic velocities increase, while the conductivities decrease with increasing hydrate saturation, with the effects of the floating and bridging distribution being the least and greatest, respectively. We also show that the anisotropic velocities and conductivities for the floating and bridging distribution vary approximately linearly with hydrate saturation, leading to linear correlations between the elastic and electrical properties. Further investigation illustrates that the difference in the slopes of the linear joint correlations between the two distribution is significantly greater than that of the individual properties as a function of hydrate saturation. The results have revealed the distinct effects of hydrate distribution on the anisotropic elastic and electrical properties of fractured reservoirs, and have confirmed the superiority of the joint elastic–electrical properties for the distinguishment of hydrate distribution in fractures over individual physical properties. [ABSTRACT FROM AUTHOR]

Joint seismic and controlled‐source electromagnetic surveys can reduce uncertainty and improve hydrocarbon quantification. However, the successful interpretation of joint seismic and controlled‐source electromagnetic survey data requires a valid rock physics model to link the elastic and electrical rock properties. A cross‐property differential effective medium is recently proposed with pores embedded in the mineral matrix. We have presented a technique to test the applicability of the cross‐property differential effective medium model to the joint elastic‐electrical properties of reservoir sandstones. The method is based on the common employment of measured sample porosity and inverted equivalent pore aspect ratio from cementation exponent for the modelling. Application of the modelling approach to a laboratory joint elastic‐electrical dataset shows that there is a systematic discrepancy in the modelled and measured joint elastic–electrical properties. It is also found that an equivalent pore aspect ratio is existing to reasonably predict the trend of the joint elastic–electrical data; however, the porosity used in the modelling is far from their measured values. Conversely, if the measured sample porosity is employed, the mentioned equivalent pore aspect ratio fails to fit either of the measured elastic and electrical rock properties. The results indicate the incapability of the cross‐property differential effective medium model for the joint elastic–electrical modelling of reservoir rocks and suggest the requirement of a new model to link the elastic and electrical properties. [ABSTRACT FROM AUTHOR]

Temperature- and pressure-dependent elastic properties (static and dynamic shear and bulk moduli) of porous rocks are investigated in terms of the characteristics of pore microstructures (i.e., crack density, crack porosity, and crack aspect-ratio distribution). Based on the Mori–Tanaka (MT) and David–Zimmerman (DZ) models, we correlated elastic properties with these microstructural characteristics and indicated that these microstructural properties can be estimated from dynamic and static moduli that are measured by considering the joint effect of thermal stress and confining pressure. Experiments with a tight sandstone under the saturated condition show that the cumulative crack density and porosity obtained from static moduli are higher than those from dynamic moduli. We demonstrated that the MT and DZ models can be used to describe the temperature- and pressure-dependent microstructures using static and dynamic properties. The dynamic bulk compressibility and shear compliance decrease with increasing confining pressures and decreasing temperatures, whereas the corresponding static values experience complex variations due to the joint effect of thermal stress and confining pressure. For the domination of thermal stress exerted by an increase in temperature, the widening of earlier formed micro-cracks during heating reduces the static moduli. However, during the domination by confining pressure, pre-existing micro-cracks will be closed with increasing temperatures, which increases the static moduli. These static elastic properties (Young's modulus, bulk modulus, and Poisson's ratio) share a similar exponential trend with the ratio of thermal stresses to confining pressures, which can be used to correlate dynamic and static moduli. Highlights: We illustrate temperature- and pressure-dependent pore microstructures jointly using static and dynamic moduli based on the Mori-Tanaka and David-Zimmerman models. Experiments with a saturated tight sandstone show that the cumulative crack density and porosity obtained from static moduli are higher than those from dynamic moduli. Static moduli share a similar exponential trend with the ratio of thermal stresses to confining pressures based on which we propose an approach to estimate static moduli from dynamic moduli. [ABSTRACT FROM AUTHOR]

Elastic and electrical properties can be jointly interpreted for better characterizations of rocks with cracks that are common in geological rocks. However, the cross-property relationship between the elastic and electrical properties of cracked rocks, which forms the key to the successful joint elastic–electrical interpretation, remains poorly understood. We investigate the pressure effects on the joint elastic–electrical properties in brine-saturated artificial sandstones with aligned non-interacting penny-shaped cracks that are far from percolation. We measured and compared the anisotropic electrical conductivity and ultrasonic velocity in the artificial sandstones with and without aligned cracks under applied effective stress from 5 to 50 MPa. We showed that the existence of aligned cracks significantly enhanced the elastic and electrical anisotropies of the rocks, and the difference in the elastic and electrical anisotropies between the cracked and intact samples reduced as the effective stress increased. We also showed that the pressure-dependent electrical conductivity and ultrasonic velocity exhibited strong linear correlations in both the intact and cracked samples cored in different directions, and a difference existed in the slopes between the samples with and without aligned cracks. The distinct contributions of the pressure-induced deformation of cracks to the anisotropic elastic and electrical properties, as well as their different sensitivity to the cracks, were found to plausibly explain the observed experimental results. Theoretical modelling for quantitative interpretation of the experimental data is presented in a companion paper. [ABSTRACT FROM AUTHOR]

Cracks are widely existing in natural rocks, and the effects of cracks on the elastic and electrical properties of rocks make it possible to establish appropriate joint interpretation to detect the crack characteristics more accurately. However, the premise is the knowledge of the joint elastic–electrical properties of cracked rocks. We aim to study theoretically the pressure-dependent joint elastic–electrical properties of porous rocks with aligned penny-shaped cracks. To do so, we developed a model describing the deformation of cracks in a transversely isotropic (TI) background with stress, and combined with existing models for the anisotropic elastic and electrical properties of TI rocks with aligned cracks. The impacts of crack deformation on the anisotropic elastic and electrical properties of cracked rocks, and the effects of crack porosity and crack aspect ratio on the pressure-dependent joint elastic–electrical properties were then studied. We showed that the modelling results compared satisfactorily with experimental data. The modelling results demonstrated that the crack compaction with stress reduced the elastic and electrical anisotropy. We also found that the anisotropic elastic wave velocities and electrical conductivities of cracked rocks exhibited approximately negative linear correlations when linked by the effective stress, and the slopes and intercepts of the linear relationships were shown to be systematically dependent on the porosity and aspect ratio of the aligned cracks. The above knowledge of the pressure-dependent joint elastic–electrical properties of TI rocks with aligned cracks can be employed for quantitative interpretation of the joint elastic and electrical exploration data. [ABSTRACT FROM AUTHOR]

Natural fractures are usually multiscale in size, orientations and distribution, resulting in complex anisotropic characteristics. Analytical methods for quantifying the associated effective elastic properties are based on some assumptions, such as dilute fracture concentration and regular-shaped fractures, which do not occur in actual reservoirs. Here, we conduct anisotropic finite-element modeling of effective elastic properties of complex fractured rocks using the least-square fitting method. The algorithm is developed for a 3D case and validated by classical effective medium theories for models with different fracture densities. The results of the 3D numerical method agree well with the theoretical predictions at low fracture density. The model also considers the interactions among fractures to calculate equivalent stiffness tensors at high fracture density. Three 2D fracture models are simulated to demonstrate the basic behavior of stress interactions and their effect on the overall elasticity under different fracture densities. We applied the developed model to 3D natural fractures built from a real outcrop, and we found that the fracturing pattern significantly affects the effective anisotropy properties. The resultant P-wave phase velocities as functions of the incidence angle and frequency are anisotropic. This study provides a great potential to calculate equivalent stiffness tensors and anisotropic properties of 3D multiscale natural fractures. [ABSTRACT FROM AUTHOR]

Seismic characterization of fractures is of great importance in many disciplines, for which rock physics models provide the basis to link fracture properties to seismic attributes. However, to date, most rock physics models neglect the background anisotropy that may exist in many fractured formations. Hence, in this work, we developed a theoretical model for rock effective elastic properties with penny‐shaped cracks embedded in the transversely isotropic (TI) background medium. We first derived analytical solutions for the case with dry or fluid‐filled penny‐shaped cracks parallel to the isotropic plane of TI background medium. Further, the results were extended to the case with cracks inclined to the isotropic plane with any angles. We then studied, based on the developed theoretical model, two tight sand samples with TI and isotropic background, respectively, to illustrate effects of background anisotropy on effective elastic properties of fractured rocks. The results show that the background anisotropy has significant influence on P and S wave velocity anisotropy, as well as on shear wave splitting. The background anisotropy can either increase or decrease P and SH wave velocity anisotropy depending on crack inclination angles, whereas it has relatively small influence on SV wave velocity anisotropy. To further illustrate the important effects of background anisotropy, we compared theoretical predictions to ultrasonic measurements on a synthetic fractured sandstone sample with TI background medium. The results show notable improvement of the agreement between theoretical predictions and experimental results after considering background anisotropy. Key Points: A theoretical model is developed to study effects of background anisotropy on elastic properties of fractured rocksBackground anisotropy has significant influence on the P and S wave velocity anisotropy, as well as on the shear wave splittingEffects of background anisotropy are also illustrated by comparing theoretical predictions to experimental data [ABSTRACT FROM AUTHOR]

The dependence of elastic moduli of shales on the mineralogy and microstructure of shales is important for the prediction of sweet spots and shale gas production. Based on 3D digital images of the microstructure of Longmaxi black shale samples using X-ray CT, we built detailed 3D digital images of cores with porosity properties and mineral contents. Next, we used finite-element (FE) methods to derive the elastic properties of the samples. The FE method can accurately model the shale mineralogy. Particular attention is paid to the derived elastic properties and their dependence on porosity and kerogen. The elastic moduli generally decrease with increasing porosity and kerogen, and there is a critical porosity (0.75) and kerogen content (ca. ≤3%) over which the elastic moduli decrease rapidly and slowly, respectively. The derived elastic moduli of gas- and oil-saturated digital cores differ little probably because of the low porosity (4.5%) of the Longmaxi black shale. Clearly, the numerical experiments demonstrated the feasibility of combining microstructure images of shale samples with elastic moduli calculations to predict shale properties. [ABSTRACT FROM AUTHOR]

Montmorillonites are commonly existing in shales and understanding the anisotropic elastic properties of montmorillonite is very crucial because they provide important geomechanical information for the discovery and development of shale gas. Layer charges with varying densities and locations are naturally associated with montmorillonites and it has been demonstrated that these layer charge characteristics can dramatically influence the anisotropic elastic properties of dry montmorillonites. However, montmorillonites are generally hydrated by water under reservoir conditions and whether the existence of water will impact the effect of the layer charge characteristics on the anisotropic elastic properties is still poorly understood. To gain such knowledge, this paper investigated the anisotropic elastic parameters of hydrated montmorillonites with various layer charge characteristics via molecular dynamic (MD) simulations. The results demonstrated that the existence of water reduces all the elastic factors of montmorillonites, and weakens the variation of the elastic factors with different layer charge locations (LCLs), in comparison with dry montmorillonites. It was also found that when hydrated, the influence of layer charge density (LCD) on the in-plane compressional factors tends to be diminished, and the out-of-plane compressional and shear factors tend to increase consistently with increasing LCD, whereas their dependence on the LCD can be disordered in different dry montmorillonite models. The variations in the basal spacing and the interactions between the atoms caused by different layer charge characteristics when water is present were found to plausibly explain and interpret the simulation results. The results not only illustrate the link between the anisotropic elastic properties and layer charge characteristics in hydrated montmorillonites but also provide crucial geomechanical information to help the successful discovery and development of shale gas reservoirs containing hydrated montmorillonite. [ABSTRACT FROM AUTHOR]

Cracks that are widely existing in geological formations play an important role in affecting the elastic and electrical rock properties, which are remotely measured for detecting and characterizing cracked rocks. Joint interpretation of the complementary elastic and electrical data is expected to give a better assessment of the cracks. However, the correlations between the elastic and electrical rock properties, which are the key to the successful joint interpretation, remain poorly understood in layered porous rocks with cracks of varying orientations. We reviewed and summarized the most recent advances in the theoretical models for the elastic and electrical properties of layered porous rocks with inclined and rotated cracks as well as with randomly orienting cracks. The limitations of the models were identified by comparing the modeling results with available experimental data, and we applied the models within their validity range to obtaining theoretically the correlations between the elastic and electrical properties in layered porous rocks with cracks of the above-mentioned orientations, in the horizontal and vertical directions, respectively. We showed that the elastic and electrical properties are systematically correlated with cracks of varying crack densities and aspect ratios, different cracks orientations affect the correlations in varying ways. The results not only illustrate the link between the elastic and electrical properties in cracked rocks but also have important practical applications when the seismic and electromagnetic exploration data are jointly interpreted to gain better knowledge about the subsurface Earth, although future work is required to extend the models to more realistic cracked formations and to experimentally test the validity of the models and the theoretically obtained joint correlations. [ABSTRACT FROM AUTHOR]