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Geothermal energy is increasingly important for the global environment and for the sustainable development of our society. Electrical surveys are widely employed for the exploration of geothermal energy, because the electrical geophysical properties provide useful information about the fluids at depth. However, although quantitative interpretation of electrical survey data relies on the knowledge about the effects of temperature on the electrical properties of fluid-bearing rocks, it remains poorly understood about how temperature affects the electrical conductivity of clay-free porous rocks. We bridge this knowledge gap by measuring the electrical conductivity and porosity of five brine saturated clean Berea sandstones with temperature ranging between 25 and 140 °C, and analysing all the factors that impact the rock conductivity. We showed that the effects of surface conductivity on the temperature-dependent electrical conductivity can be negligible, whereas the temperature induced variation in the porosity and pore structure quantitatively characterized in terms of cementation exponent can be more significant. We also found that temperature affects the electrical conductivity of brine saturated Berea sandstones by impacting the brine conductivity, and the pore structure and porosity of the samples, with their importance in a descending order. The results have provided new insights into how temperature affects the electrical conductivity of clay-free porous rocks, and will help to improve the quantitative interpretation of electrical survey data for the exploration of geothermal energy. [ABSTRACT FROM AUTHOR]

This article explores the advancements and challenges in seabed prospecting technology, emphasizing the importance of seabed geophysical data in understanding the planet and driving marine energy resource exploration and deep earth science research. It calls for further technical refinement in ocean bottom geophysical prospecting techniques and the integration of seabed exploration technology with big data and artificial intelligence. The article also introduces a special issue on seabed prospecting technology, which includes research papers covering a range of prospecting methods and geophysical applications. The document provides summaries of various research studies related to seismic data analysis and exploration, offering key findings and methodologies used in each study. [Extracted from the article]

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]

SUMMARY: Geophysical exploration technologies are developed not only to detect and quantify the geofluid in a reservoir but also with the expectation to identify the reservoir's permeability that determines its exploitability. However, the knowledge about whether there is a geophysically invertible pore parameter that determines the permeability of a rock, the key to the geophysical identification of permeability, remains poorly understood. We address this knowledge gap by establishing, through dedicated laboratory measurements and theoretical modelling, the correlations between permeability and the porosity and aspect ratios of the dual-pore structure in artificial sandstones with approximately the same porosity but with different permeability. Permeability is found to increase with increasing compliant porosity but show no apparent relations with the dual-pore aspect ratios, indicating compliant porosity is the key parameter determining the permeability of rocks with the same porosity. The results reveal the key factor that influences permeability in sandstones with the same porosity, and provide a potential way to simply identify the permeability to indicate the exploitability of a geofluid reservoir. [ABSTRACT FROM AUTHOR]

Differential effective medium (DEM) model has been widely employed for the interpretation of electrical survey data. However, the contact of grains that is inevitably happening in reservoir rocks is not taken into account by the DEM model, making the model prediction underestimate the measured formation factor of clean granular rocks. We have developed a modified DEM model by introducing a geometric factor to account for the contact of grains that complicates the pore network and thus deviates the transportation of electrical current. The geometric factor is derived by fitting the measured formation factor from a large published data set of 111 clean sandstone samples, and is found to decrease exponentially with rock porosity. Comparison with laboratory data sets of various artificial and real clean sandstones shows that the modified DEM model improves the modelling results and fits satisfactorily the measured formation factor with varying porosity. The results illustrate the practical applicability of the developed closed-form model for the improved simulation of electrical properties of clean granular rocks, and suggest grain contact as a potential link for the joint elastic-electrical characterization of granular rocks through integrated seismic and electromagnetic surveys. [ABSTRACT FROM AUTHOR]

Cracks widely exist in rocks and are important in various geophysical applications. However, although electrical methods are conventionally employed for the detection and characterization of cracks, the fundamental question whether cracks improve the conductive ability of porous rocks (a quantification of their electrical conductivity if their porosity and saturant are the same) remains largely unaddressed. We address this knowledge gap through theoretical models with confirmed validity. We show that the conductive ability of a rock containing non‐interacting penny‐shaped cracks with random orientation will be improved only in the case when the aspect ratio of the cracks is below a certain value, which is referred to as the critical crack aspect ratio. We also show that the critical crack aspect ratio is uniquely determined by the porosity and electrical conductivity (or cementation exponent) of the porous rock where the cracks reside. We further demonstrate that the critical crack aspect ratio is some representation of the pore structure, and using two times the critical crack aspect ratio as the pore geometry can give rise to a reasonable agreement between modelled and measured P‐ and S‐wave velocities. The critical crack aspect ratio offers a consistent microstructure for the joint elastic–electrical modelling for the improved characterization of cracks through integrated seismic and electrical surveys. [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]

Understanding the electrical properties of reservoir rocks has important applications in oil and gas exploration. Small‐scale fractures that are widely existing in reservoir rocks are usually aligned along a certain preferential direction due to reasons such as tectonic movement, making the electrical conductivity of the rocks anisotropic. Since all rocks are experiencing geological pressure, it is theoretically and practically important to study the influence of pressure on the fractures and its control on the anisotropic conductivity of reservoir rocks. We first prepared artificial porous sandstone samples with and without penny‐shaped fractures and experimentally measured the anisotropic conductivity of the samples under differential pressure (the difference between confining pressure and pore fluid pressure). Then, we inverted from the experimental data for the fracture porosity and fracture aspect ratio at each differential pressure. The variation of the fracture parameters caused by the differential pressure and their influence on the anisotropic electrical conductivity of the rocks were further analysed theoretically. The results show that as the differential pressure increases, the measured anisotropic conductivity of the artificial samples with and without fractures all decreases exponentially. We also show that both the inverted fracture porosity and fracture aspect ratio decrease exponentially with the increase of differential pressure, and the fracture porosity and fracture aspect ratio are linearly correlated as an implicit function of differential pressure. The modelling results show that the differential pressure‐induced decrease in the fracture porosity reduces the electrical conductivity in all directions, and the conductivity reduction in the rock parallel to the direction of the fractures is most significant. Comparing with the influence of the fracture porosity resulting from the applied differential pressure, the pressure‐caused variation in the fracture aspect ratio was shown to play a secondary role on affecting the pressure‐dependent anisotropic electrical properties. [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]

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]

Seismic waves propagating in a fractured porous rock with fluids can be greatly attenuated due to the wave‐induced fluid flow, including the macroscopic Biot flow, the mesoscopic interlayer fluid flow and the microscopic squirt flow. However, the comprehensive effects of the above wave‐induced fluid flows on the frequency‐dependent seismic properties in layered and fractured rocks with partial saturation are still poorly understood. To obtain such knowledge, we present a superposition approach accounting for the dispersion and attenuation induced by the multi‐scale wave‐induced fluid flows in the above‐mentioned target rocks, and study the influences of the water saturation, layer thickness ratio and fracture properties (i.e. fracture density and fracture aspect ratio) on the frequency‐dependent vertical P‐wave velocity (VP0). The results demonstrate that the increasing water saturation enhances the dispersion and attenuation of VP0 in the full frequency band, and the varying layer thickness ratio not only affects the dispersion and attenuation induced by the interlayer fluid flow, but also influences those caused by the Biot flow and squirt flow. The fracture properties are also found to significantly impact the seismic properties of the target rock. In addition to its effects on the squirt flow, the increasing fracture density is shown to weaken the dispersion and attenuation induced by the Biot flow, and enhance those caused by the interlayer fluid flow. The obtained results provide new insights into the frequency‐dependent seismic properties in the partially saturated layered rocks with fractures, and the knowledge gained from these results has important practical applications for the successful interpretation of fluid and fracture properties applying the seismic exploration data acquired in fractured reservoirs. [ABSTRACT FROM AUTHOR]

Seismic properties of hydrate-bearing reservoirs that are affected significantly by the hydrate distribution are key for quantitative assessment of the reservoir. The knowledge of hydrate distribution in fractured reservoirs remains poorly understood. To obtain such knowledge, we measured and analyzed five anisotropic velocities needed to fully characterize the seismic anisotropy in an artificial sandstone with aligned fractures during hydrate formation associated with varying distribution. We showed that while the formation of hydrate improved the velocities, the improvement was more significant for hydrate saturation above 10%. We also showed that the increasing trends varied among the anisotropic velocities when hydrate saturation was above 10%. Specifically, the compressional wave velocity travelling vertical to the bedding plane and the shear wave velocity with polarization perpendicular to the bedding plane increased more rapidly than the other compressional and shear wave velocities, respectively. Interpretation of the anisotropic seismic results suggested that the hydrate tends to bind to the grains in the fractures at low hydrate saturation, and becomes to bridge the fracture surfaces when the hydrate saturation exceeds 10%. The results have provided new insights into the hydrate distribution and its resulting anisotropic seismic properties in fractured reservoirs. This will pave the way for the successful assessment of hydrate in fractured reservoirs. [ABSTRACT FROM AUTHOR]

Fractures can widely exist in hydrate‐bearing reservoirs and can give rise to anisotropic acoustic properties that play an important role in the accurate assessment of fractured reservoirs. However, the knowledge of the responses of acoustic anisotropy in fractured hydrate‐bearing reservoirs is still poorly understood. To obtain such understanding, we designed and implemented dedicated laboratory experiments to measure the anisotropic acoustic velocities in an artificial sandstone with aligned penny‐shaped fractures parallel to the layers during hydrate formation and dissociation at the confining pressures of 20 and 50 MPa, respectively. We showed that although the velocities at higher confining pressure were systematically greater, all the velocities at both confining pressures increased and decreased during hydrate formation and dissociation, respectively. We also showed that the velocities during hydrate formation were significantly higher than those during dissociation at hydrate saturation greater than 10%. It was found that all the velocities smoothly reduced with decreasing saturation during dissociation, while the varying velocities exhibited distinct increasing gradients during formation, where the compressional wave anisotropy reduced dramatically at saturation between 10% and 25%. Analyses of the experimental results suggested that the floating hydrate evolved to bridge the surfaces of fractures when hydrate saturation exceeded 10% during hydrate formation, and the bridging hydrate gradually transformed to floating in the pore system during dissociation. The results provide new insights into hydrate distribution and its effects on acoustic anisotropy in fractured reservoirs during hydrate formation and dissociation and will contribute to improving the accuracy of hydrate assessment in such reservoirs. Key Points: We study the evolution of hydrate formation and dissociation on the acoustic anisotropy in an artificial sandstone with aligned fracturesThe dependences of anisotropic velocities on hydrate saturation are smooth for hydrate dissociation but are distinct for hydrate formationThe anisotropic results suggest that hydrate floats in the fractures for saturation up to 10% and then bridges fractures during formation [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]

Wave‐induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks. While numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks, most of them neglect the wave‐induced fluid flow resulting from the background anisotropy (e.g. the interlayer fluid flow between different layers) that can be normal in real reservoirs. Here, according to the theories of poroelasticity, we present an approach to study the frequency‐dependent seismic properties of more realistic and complicated rocks, i.e. horizontally and periodically layered porous rock with horizontal and randomly orienting fractures, respectively, distributed in one of the two periodical layers. The approach accounts for the dual effects of the wave‐induced fluid flow between the fractures and the background pores and between different layers (the interlayer fluid flow). Because C33 (i.e., the modulus of the normally incident P‐wave) is directly related to the P‐wave velocity widely measured in the seismic exploration, and its comprehensive dispersion and attenuation are found to be most significant, we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of C33. The results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave‐induced fluid flow between the fractures and the background pores, especially for the layered porous rock with horizontal fractures. The modelling results also demonstrate that for the considered rock construction, the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture‐relevant frequency band. Increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture‐relevant frequency band only, especially for the layered porous rock with horizontal fractures. [ABSTRACT FROM AUTHOR]

The existence of cracks can significantly influence the electrical properties of rocks, and electrical exploration has been applied for the detection and characterization of cracks. However, the premise is the knowledge of the quantitative effects of cracks on the electrical properties of fractured rocks, and theoretical models have provided means to obtain such knowledge. In this work, we derive explicit theoretical models for the anisotropic electrical conductivity of transversely isotropic (TI) rocks with inclined and rotated cracks, as well as cracks with random orientation distribution. Based on the developed models, we study systematically and quantitatively the effects of the crack properties and distributions on the anisotropic electrical conductivity of cracked rocks. We show that for TI rocks with aligned cracks, the inclination angle of the cracks affects both the vertical and the horizontal electrical conductivities, whereas the rotation angle only influences the horizontal electrical conductivity, although both the two angles can lead to the global rotations of the principal axes of the electrical conductivity tensor. On the other hand, the conductivity of TI rocks containing randomly distributed cracks remains transversely isotropic. For both aligned and randomly distributed cracks, the increasing crack volume fraction can linearly increase the electrical conductivity of the effective rocks, and cracks with lower aspect ratio show more significant effects on the electrical conductivity. The knowledge of the effects of the cracks on the TI rocks obtained in this study can be applied for more accurate interpretation of the electrical exploration data to better characterize cracked formations. Key Points: We develop theoretical models for the electrical conductivity of transversely isotropic rocks with inclined cracks of varying orientationsThe effects of crack parameters and crack orientation distribution on the anisotropic electrical conductivity of cracked rocks are studiedThe models can be applied for more accurate interpretation of electrical exploration data to better characterize cracked formations [ABSTRACT FROM AUTHOR]

Electrical surveys are among the most important geophysical applications to understand the subsurface Earth, where anisotropic rock matrix is frequently encountered. Knowledge of the electrical properties of such rocks and their relationships with information of geological features included in the anisotropic rocks is crucial for the interpretation of electrical survey data. For this purpose, we first derive rigorous theoretical models for the electrical anisotropy of transversely isotropic (TI) rocks with spheroidal inclusions. The models assume the anisotropic background behaves isotropically in the direction whey the electrical field is applied parallel to the TI axes, and account for both aligned and randomly distributed inclusions and reduce to classical electrical models in the case of isotropic background. We then study systematically, through the obtained new models, the combined effects of geometry, concentration and alignment of both conductive and insulating spheroidal inclusions on the electrical anisotropy of TI rocks. We show that for aligned inclusions, the impact of inclusion volume fraction and aspect ratio is most significant in the direction when the electrical conductivity is measured along and perpendicular to the inclusions for conductive and insulating inclusions, respectively. For randomly distributed inclusions, both horizontal and vertical conductivity increases with decreasing aspect ratio of conductive inclusions and decreases with reducing aspect ratio of insulating inclusions. Most importantly, it is shown that the effects of randomly distributed conductive and insulating inclusions on the electrical anisotropy are similar although the rocks with insulating inclusions exhibit greater electrical anisotropy. The results provide new insights into the roles of spheroidal inclusions on the electrical anisotropy of TI rocks and pave a way for the successful interpretation of electrical survey data acquired in anisotropic rocks. [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]

Abstract Understanding the dielectric properties of reservoir rocks has important applications in various aspects in the petroleum industry. Fractures are common features in sedimentary porous rocks that not only provide space for the hydrocarbon to accumulate and reside but also are important pathways for the hydrocarbon to flow and therefore have a controlling impact on the formation and distribution of the hydrocarbon reservoirs. However, the dielectric properties of fractured reservoir rocks are still poorly understood. We investigated, through dedicated laboratory experiments, the dielectric behaviors of synthetic porous sandstones with aligned fractures and their relationships with the salinity of the electrolyte saturating the samples, the measurement frequency of the applied electrical field and the directions relative to the fractures. It showed that the fractures have significant effects on the dielectric properties of porous sandstones and their dependence on the brine salinity and measurement frequency reveals the various polarization mechanisms occurring at different frequency ranges. The results offer a preliminary basis for the characterization of fractured rocks using the dielectric properties, which has great potential applicability in oil and gas exploration and exploitation. Highlights • The dielectric properties of synthetic porous sandstones with aligned fractures are studied. • The fractures have great effects on the dielectric behaviors of porous sandstones. • The results reveal various polarization mechanisms occurring at different frequency ranges. [ABSTRACT FROM AUTHOR]

Abstract Accurate determination of the dielectric properties of porous rocks is important for the dielectric exploration methods in a range of applications from water resources to petroleum industry. Carefully controlled laboratory measurements offer the best way of obtaining the dielectric behaviors but they will require relatively large quantity of sample materials prepared in specific shapes, which is not always available. Numerical and theoretical simulations compensate for this weakness and can compute the dielectric dispersion at the pore-scale level on small fragments of rocks. However, whether consistent dielectric results can be obtained from the numerical computation and from the properly developed theoretical models still needs investigation. We introduced in this paper the numerical model based on the three-dimensional finite difference method (3D-FDM) and a range of theoretical models on basis of interfacial polarization for the calculation of the frequency dependent dielectric properties in porous rocks of complex geometry. The numerical and theoretical models were applied to a hypothetical porous rock with ideal shaped grains and to a real synthetic sandstone sample with complex pore and grain structure. Comparison of the simulation results from the two methods showed excellent agreement with each other with squared correlation coefficients better than R 2 = 0.98 for both relative permittivity and conductivity of the two example rocks. The consistent numerical and theoretical results provide a complementary way for the numerical and theoretical models to work together for a better simulation of the dielectric properties of porous rocks. Highlights • A finite difference method is introduced for modeling the dielectric properties of reservoir rocks. • Theoretical models for the frequency dependent dielectric properties of porous rocks are analyzed. • The numerical and theoretical modeling results are compared for two examples. [ABSTRACT FROM AUTHOR]

Pressure dependency of seismic properties of porous rocks is important for a range of geophysical applications. Recently, a theoretical model has been developed to interpret the pressure and frequency dependent squirt-flow attenuation in fluid saturated porous rocks based on the dual porosity concept. We implemented laboratory measurements of dry rock porosity and ultrasonic (compressional and shear wave) velocity and saturated rock velocity and attenuation on two sandstone samples with varying confining pressures from 5 to 60 MPa to test the pressure dependent attenuation of the squirt-flow model. Based on the experimental data, the pressure dependent compliant and stiff porosity were obtained by fitting the dual porosity model to the measured variation of total porosity with pressure, and the compliant pore aspect ratio was estimated as a free parameter to make the model predicted velocity of the saturated rocks best fit their laboratory data. Comparison of the modelled attenuation with the measured variation of attenuation with pressure showed that the squirt-flow attenuation is consistent with the measurement at low-pressure regime but underestimates the experimental data when the confining pressure becomes higher. The results confirm the validation of the squirt-flow model to explain the attenuation at low pressures and suggest the requirement of the combination with other theoretical models to fully interpret the attenuation at high confining pressures. [ABSTRACT FROM AUTHOR]

Abstract: Seismic and electromagnetic surveys are among the most important geophysical applications to understand the subsurface Earth. Joint interpretation of the independent but complementary exploration data can provide geophysical parameters needed to better assess the rocks. However, cross‐property relationship between the elastic and electrical properties of fractured porous rocks, the key to the successful joint interpretation of seismic and electromagnetic data in anisotropic formations, is poorly understood. Here we use dedicated laboratory experiments and theoretical modeling to show how elastic velocity and electrical conductivity are linked in porous sandstone with aligned fractures and how the joint properties are affected by the fractures. The P and SH wave velocities are found to be approximately linearly correlated with electrical conductivity when measured in varying directions, and the linear trends are determined by the geometry and amount of the fractures in the rocks. The results provide a pathway to successful joint interpretation in fractured rocks. [ABSTRACT FROM AUTHOR]

We used a common microstructural model to investigate the cross-property relations between elastic wave velocities and dielectric permittivity in carbonate rocks. A unified model based on validated self-consistent effective medium theory was used to quantify the effects of porosity and water saturation on both elastic properties (compressional and shear wave velocities) and electromagnetic properties (dielectric permittivity). The results of the forward models are presented as a series of cross-plots covering a wide range of porosities and water saturations and for microstructures that correspond to different predominant aspect ratios. It was found that dielectric permittivity correlated approximately linearly with elastic wave velocity at each saturation stage, with slopes varying gradually from positive at low saturation conditions to negative at higher saturations. The differing sensitivities of the elastic and dielectric rock properties to changes in porosity, pore morphology and water saturation can be used to reduce uncertainty in subsurface fluid saturation estimation when co-located sonic and dielectric surveys are available. The joint approach is useful for cross-validation of rock physics models for analysing pore structure and saturation effects on elastic and dielectric responses. [ABSTRACT FROM AUTHOR]

Modeling the pressure dependency of rock velocity is important for interpreting and comparing the seismic and earthquake data from different depths. This study develops a multicomponent differential effective medium model for the elastic properties of porous rocks with two types of pores in the grain background without mixing order. The developed model is applied to modeling the pressure dependent elastic velocity of porous rocks by incorporating the variation of stiff and compliant porosity as a function of pressure. The pressure dependent stiff and compliant porosity were inverted from the measured total porosity under pressure using a dual porosity model, and the unknown constant stiff and compliant pore aspect ratios were inverted by best fitting the modeled velocity to the measured data. Application of the approach to a low porosity granite and a medium porosity sandstone sample showed that the pressure dependency of rock velocity can be satisfactorily modeled by the developed model using the pressure dependent stiff and compliant porosity and carefully estimated stiff and compliant pore aspect ratio values. [ABSTRACT FROM AUTHOR]

Knowledge about the pressure dependency of elastic and electrical properties is important for a variety of geophysical applications. We present a technique to invert for the stiff and compliant porosity from velocity measurements made as a function of differential pressure on saturated sandstones. A dual porosity concept is used for dry rock compressibility and a squirt model is employed for the pressure and frequency dependent elastic properties of the rocks when saturated. The total porosity obtained from inversion shows satisfactory agreement with experimental results. The electrical cementation factor was determined using the inverted porosity in combination with measured electrical conductivity. It was found that cementation factor increased exponentially with increasing differential pressure during isostatic loading. Elastic compressibility, electrical cementation factor and electrical conductivity of the saturated rocks correlate linearly with compliant porosity, and electrical cementation factor and electrical conductivity exhibit linear correlations with elastic compressibility of the saturated rocks under loading. The results show that the dual porosity concept is sufficient to explain the pressure dependency of elastic, electrical and joint elastic-electrical properties of saturated porous sandstones. [ABSTRACT FROM AUTHOR]

Marine controlled source electromagnetic sounding has developed rapidly in recent years to complement conventional seismic method for better discrimination of pore fluid. To obtain quantitative and reliable interpretation of the CSEM data, a robust rock physics model is vital to link the bulk rock electrical conductivity to the electrical properties and microstructure of the rock constituents, especially when some of the constituents exhibit extreme electrical behaviours (e.g., pyrite, a common mineral associated with reservoir rocks). Based on the multi-phase incremental model validated on published experimentally measured electrical conductivity of pyrite-bearing sandstones, the effects of key parameters, i.e., pyrite content, porosity, pyrite conductivity, grain aspect ratio and water saturation on the electrical conductivity of pyrite-bearing sandstones and their corresponding CSEM responses were comprehensively studied. The results are expected to assist in the CSEM data interpretation when a pyrite-bearing sandstone reservoir is encountered in the future. [ABSTRACT FROM AUTHOR]

The improved interpretation of marine controlled source electromagnetic (CSEM) data requires knowledge of the inter-relationships between reservoir parameters and low frequency electrical resistivity. Hence, the electrical resistivities of 67 brine (35 g/l) saturated sandstone samples with a range of petrophysical properties (porosity from 2% to 29%, permeability from 0.0001 mD to 997.49 mD and volumetric clay content from 0 to 28%) were measured in the laboratory at a frequency of 2 Hz using a four-electrode circumferential resistivity method with an accuracy of ± 2%. The results show that sandstones with porosity higher than 9% and volumetric clay content up to 22% behave like clean sandstones and follow Archie's law for a brine concentration of 35 g/l. By contrast, at this brine salinity, sandstones with porosity less than 9% and volumetric clay content above 10% behave like shaly sandstones with non-negligible grain surface conductivity. A negative, linear correlation was found between electrical resistivity and hydraulic permeability on a logarithmic scale. We also found good agreement between our experimental results and a clay pore blocking model based on pore-filling and load-bearing clay in a sand/clay mixture, variable (non-clay) cement fraction and a shaly sandstone resistivity model. The model results indicate a general transition in shaly sandstones from clay-controlled resistivity to sand-controlled resistivity at about 9% porosity. At such high brine concentrations, no discernible clay conduction effect was observed above 9% porosity. [ABSTRACT FROM AUTHOR]

Highlights: [•] The in situ acoustic properties are acquired using a completely new apparatus. [•] Relations between acoustic properties and the geotechnical parameters are obtained. [•] Reasons contribute to the relations are analyzed. [•] Velocity shows good correlation with geotechnical parameters. [•] Attenuation shows weak correlation with geotechnical parameters. [Copyright &y& Elsevier]

SUMMARY: The pressure sensitivity of stiffness in fractured rocks is closely related to fracture-surface geometries. The resulting stress-dependence of stiffness can be represented by the third-order elastic constants (3oECs). Fracture surfaces are generally rough at various scales, and can significantly affect the 3oECs of pre-stressed fractures as well as the wave-induced fluid flow (WIFF) induced by the Biot slow P -wave between fractures and the background medium. The WIFF usually depends on the fracture width relative to the slow P -wavelength and the fracture-surface roughness. We generate various fracture-surface geometries at different scales of random roughnesses parametrized by the surface standard deviation (SSD) of fracture-surface heights. With theoretical analyses and numerical simulations, we investigate the effect of fracture-surface geometries on the stress- and frequency-dependent stiffness through the 3oECs for pre-stressed rocks with aligned fractures. For the elastic wave in the low-frequency regime of Biot theory with the fracture scale much less than the wavelength, the induced WIFF significantly enhances the effect of fracture-surface geometries on the 3oECs and P - and S -wave moduli. The stiffness of fractured rocks increases with increasing SSDs, yielding a high sensitivity to pre-stresses. Toward the high-frequency limit, however, the fluid diffusion between fractures and the porous background decreases, which reduces the influence of fracture-surface roughnesses with the 3oECs much less than that in the low-frequency regime. The resulting P -wave modulus of aligned fluid-saturated fractures approximates to the background value. [ABSTRACT FROM AUTHOR]

We measured in the laboratory compressional wave velocity and electrical resistivity on 434 sediment samples collected from the Yellow Sea to study the joint elastic-electrical properties of marine sediments. Porosity was found to reduce both elastic velocity and electrical resistivity of the marine sediments in a non-linear fashion; velocity showed an approximate linear increase with increasing logarithm of resistivity. Various effective medium models either implemented or developed were compared with the new dataset. The model results showed that the combined self-consistent approximation and differential effective medium model using critical porosity of 0.6 and 0.5 for velocity and resistivity respectively gave a reasonable description of the joint elastic-electrical behaviors of the marine sediments. The joint elastic-electrical properties of the marine sediments established would be used to estimate resistivity from measured velocity and vice versa, and could also be suitable for detection of gas hydrate or other suitable targets from joint seismic-resistivity surveys. [ABSTRACT FROM AUTHOR]

Improvements in the joint inversion of seismic and marine controlled source electromagnetic data sets will require better constrained models of the joint elastic-electrical properties of reservoir rocks. Various effective medium models were compared to a novel laboratory data set of elastic velocity and electrical resistivity (obtained on 67 reservoir sandstone samples saturated with 35 g/l brine at a differential pressure of 8 MPa) with mixed results. Hence, we developed a new three-phase effective medium model for sandstones with pore-filling clay minerals based on the combined self-consistent approximation and differential effective medium model. We found that using a critical porosity of 0.5 and an aspect ratio of 1 for all three components, the proposed model gave accurate model predictions of the observed magnitudes of P-wave velocity and electrical resistivity and of the divergent trends of clean and clay-rich sandstones at higher porosities. Using only a few well-constrained input parameters, the new model offers a practical way to predict in situ porosity and clay content in brine saturated sandstones from co-located P-wave velocity and electrical resistivity data sets. [ABSTRACT FROM AUTHOR]

We measured in the laboratory ultrasonic compressional and shear-wave velocity and attenuation (0.7-1.0 MHz) and low-frequency (2 Hz) electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity, permeability and clay content on the joint elastic-electrical properties of reservoir sandstones. P- and S-wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi-logarithmic scale for both clean and clay-rich sandstones; P- and S-wave attenuations showed a bell-shaped correlation (partial for S-waves) with apparent electrical formation factor. The joint elastic-electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents. The laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co-located seismic and controlled source electromagnetic surveys. [ABSTRACT FROM AUTHOR]

We conducted a laboratory study of the joint elastic-electrical properties of sixty-three brine-saturated sandstone samples to assess the likely impact of differential pressure (confining minus pore fluid pressures) in the range 8-60 MPa on the joint interpretation of marine seismic and controlled-source electromagnetic survey data. The samples showed a wide range of petrophysical properties representative of most sandstone reservoirs. We found that a regression equation comprising both a constant and an exponential part gave a good fit to the pressure dependence of all five measured geophysical parameters (ultrasonic P- and S-wave velocity, attenuation and electrical resistivity). Electrical resistivity was more pressure-sensitive in clay-rich sandstones with higher concentrations of low aspect ratio pores and micropores than in clean sandstones. Attenuation was more pressure-sensitive in clean sandstones with large open pores (macropores) than in clay-rich sandstones. Pore shape did not show any influence on the pressure sensitivity of elastic velocity. As differential pressure increases, the effect of the low aspect ratio pores and micropores on electrical resistivity becomes stronger than the effect of the macropores on attenuation. Further analysis of correlations among the five parameters as a function of pressure revealed potentially diagnostic relationships for geopressure prediction in reservoir sandstones. [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]

Fractures are commonly existing in the subsurface rocks, and understanding the seismic anisotropy in fractured rocks is of great importance in a range of geophysical applications. However, although all the fractured rocks are suffering from geological stresses and most of them are experiencing partial water saturation, the combined effects of confining pressure and water saturation on the seismic anisotropy in fractured rocks remain poorly understood. To obtain such knowledge, we measured and compared the anisotropic velocities and corresponding seismic anisotropy in two artificial porous sandstone samples with and without aligned fractures, respectively, as functions of varying confining pressure and degree of water saturation. We showed that the existence of aligned fractures significantly reduced the five measured velocities but dramatically enhanced the seismic anisotropy in the fractured sample. We also showed that confining pressure and water saturation had an interdependent complex effect on the seismic anisotropy of the fractured sample. The seismic anisotropy of the dry sample showed the highest‐pressure sensitivity, and the saturation effect was most significant at low confining pressure. Integrated interpretation of the experimental data indicated that wave‐induced fluid flow due to pore‐scale and mesoscale fluid distribution in the rocks is responsible for most of the observed pressure and saturation‐dependent anisotropic behaviors in the rocks. The results have helped to gain new knowledge on the seismic anisotropy in partially saturated fractured rocks subject to varying pressures and suggest the requirement for advanced theoretical models to fully understand the acquired data. Key Points: Combined pressure and water saturation effects on the seismic anisotropy in artificial porous sandstone with aligned fractures are studiedExistence of aligned fractures reduces the measured velocities but enhances the seismic anisotropy in the fractured sampleSeismic anisotropy of dry sample shows the highest pressure sensitivity and saturation effect is most significant at low confining pressure [ABSTRACT FROM AUTHOR]

In principle, wave propagation in porous media can simultaneously trigger macroscopic fluid flow and thermal flow, which can be described by Biot's poroelasticity and Lord–Shulman thermoelasticity, respectively. The physical processes of those effects are significantly different, but phenomenologically, they can lead to identical wave attenuation and dispersion and are hard to be distinguished. By using Biot's virtual concept entropy flow, the Biot-consistent General Linear Solid (the GLS) framework and matrix notation, a rigorous and convenient tool is provided to reveal the similarities and disparities between poroelasticity and thermoelasticity. By using the same framework, a Biot-consistent thermo-poroelastic model is proposed to consider macroscopic effects of fluid and thermal flows simultaneously in an elegant way. These similarities allow us to directly translate many of the available results in poroelasticity to thermoelasticity and vice versa by a simple change of notation. The disparities indicate a fundamental difference in physical mechanisms. Plane-wave analysis shows that the primary P -wave modes of thermoelasticity and poroelasticity are all GSLS-equivalent (Generalized Standard Linear Solid) and can be identical if the model parameters are selected properly. However, the corresponding slow-wave modes have significantly different phase velocity dispersion although the attenuation spectra of which are identical. Such a surprising result can be explained by the GSLS non-equivalence of the slow-wave modes and the fundamentally different mechanisms. As expected, the thermo-poroelastic model predicts four wave modes, which are the fast- and slow- P , temperature (T wave) and S waves. Two attenuation peaks due to, respectively, the thermal- and fluid-flow effects are predicted for the fast- P wave. The slow- P wave mode due to fluid flow is influenced by the thermal effects, but the T wave seems unaffected by the fluid flow. The thermo-poroelastic model is then applied to laboratory observations at 200–106 Hz for the brine-saturated tight sandstone under 35 MPa effective pressure. The unified model provides a convenient framework for studying geothermal exploration, thermal-enhanced oil recovery and other applications involving temperature variations within the porous rock. [ABSTRACT FROM AUTHOR]

P and SV wave scattering by thermally constrained saturated cracks couples elastic deformation with temperature. We estimate the scattering attenuation and velocity dispersion by aligned fluid‐saturated cracks with finite thickness, which are sparsely and randomly embedded in an isothermal and isotropic elastic medium. By incorporating thermoelastic effects into the representation theorem with the non‐interaction Foldy approximation, we formulate the dispersion and attenuation of P and SV waves induced by displacement discontinuities across aligned cracks constrained by a thermoelastic boundary condition. The frequency‐dependent response as a function of temperature is calculated for various P and SV wave incidence angles. The resulting scattering attenuation and dispersion are compared with those of the conventional aligned‐crack model. The examples show that the dissipation increases with increasing temperature, but the scattering is inhibited at a given temperature. The P wave shows a strong sensitivity to temperature along the direction normal to the cracks, whereas the SV wave has an opposite behavior. This theory has the potential to assess the distribution of temperature from seismic attributes. Key Points: We study the thermoelastic dispersion and attenuation of P and SV wave scattering by aligned‐crack in an isothermal elastic mediumThe dissipation increases with increasing temperature, but the scattering is inhibited at a given temperatureThe P wave shows a strong sensitivity to temperature along the crack normal, whereas the SV wave behaves in opposite manner [ABSTRACT FROM AUTHOR]

Classical third‐order thermoelastic constants are generally derived from the theory of small‐amplitude acoustic waves in isotropic materials during heat treatments. Investigating higher‐order thermoelastic constants for higher temperatures is challenging owing to the involvement of the number of unknown parameters. These Taylor‐type thermoelastic constants from the classical thermoelasticity theory are formulated based on the Taylor series of the Helmholtz free energy density for preheated crystals. However, these Taylor‐type thermoelastic models are limited even at low temperatures in characterizing the temperature‐dependent velocities of elastic waves in solid rocks as a polycrystal compound of different mineral lithologies. Thus, we propose using the Padé rational function to the total thermal strain energy function. The resulting Padé thermoelastic model gives a reasonable theoretical prediction for acoustic velocities of solid rocks at a higher temperature. We formulate the relationship between the third‐order Padé thermoelastic constants and the corresponding higher‐order Taylor thermoelastic constants with the same accuracy. Two additional Padé coefficients α1 ${\mathit{\alpha }}_{1}$ and α2 ${\mathit{\alpha }}_{2}$ can be calculated using the second‐, third‐, and fourth‐order Taylor thermoelastic constants associated with the Brugger's constants, which are consistent with those obtained by fitting the experimental data of polycrystalline material. The third‐order Padé thermoelastic model (with four constants) is validated by the fourth‐order Taylor thermoelastic prediction (with six constants) with ultrasonic measurements for polycrystals (olivine samples) and solid rocks (sandstone, granite, and shale). The results demonstrate that the third‐order Padé thermoelastic model can characterize thermally induced velocity changes more accurately than the conventional third‐order Taylor thermoelastic prediction (with four constants), especially for solid rocks at high temperatures. The Padé approximation could be considered a more accurate and universal model in describing thermally induced velocity changes for polycrystals and solid rocks. Plain Language Summary: Using the Taylor series for thermoelastic constants at higher temperatures is challenging owing to the involvement of the number of unknown parameters. Thus, we propose the third‐order Padé approximation to replace the Taylor series. Applications to laboratory measurements show that, with much less complexity, the Padé approximation presents a tendency of higher accuracy than the third‐order Taylor series and has similar precision with the higher‐order Taylor series at higher temperatures (up to 1500 K for polycrystals and 1000°C for solid rocks). Then we use such a model to predict the temperature‐dependent velocities of elastic waves. Finally, we analyze the physics of two additional Padé coefficients by associating them with the thermal expansion mismatch due to rock heterogeneities in lithology and with the thermally induced deformation of microcracks. Key Points: We apply the Padé rational function to the total thermal strain energy functionWe correlate the Padé and higher‐order Taylor thermoelastic constants through the equivalency of approximation accuraciesThe physics of the Padé coefficients is relevant to the thermal expansion mismatch and thermally induced deformation of microcracks [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]

Generally, local stress induced by individual crack hardly disturbs their neighbours for small crack densities, which, however, could not be neglected as the crack density increases. The disturbance becomes rather complex in saturated porous rocks due to the wave‐induced diffusion of fluid pressures. The problem is addressed in this study by the comparison of two solutions: the analytical solution without stress interactions and the numerical method with stress interactions. The resultant difference of effective properties can be used to estimate the effect of stress interactions quantitatively. Numerical experiments demonstrate that the spatial distribution pattern of cracks strongly affects stress interactions. For regularly distributed cracks, the resulting stress interaction (shielding or amplification) shows strong anisotropy, depending on the arrangement and density of cracks. It has an important role in the estimation of effective anisotropic parameters as well as the incident‐angle‐dependency of P‐ and SV‐wave velocities. Contrarily, randomly distributed cracks with a relative small crack density generally lead to a strong cancellation of stress interactions across cracks, where both the numerical and analytical solutions show a good agreement for the estimation of effective parameters. However, for a higher crack density, the incomplete cancellation of stress interactions is expected, exhibiting an incidence‐angle dependency, slightly affecting effective parameters, and differentiating the numerical and analytical solutions. [ABSTRACT FROM AUTHOR]

Poro-acoustoelastic theory has made a great progress in both theoretical and experimental aspects, but with no publications on the joint research from theoretical analyses, experimental measurements, and numerical validations. Several key issues challenge the joint research with comparisons of experimental and numerical results, such as digital imaging of heterogeneous poroelastic properties, estimation of acoustoelastic constants, numerical dispersion at high frequencies and strong heterogeneities, elastic nonlinearity due to compliant pores, and contamination by boundary reflections. Conventional poro-acoustoelastic theory, valid for the linear elastic deformation of rock grains and stiff pores, is modified by incorporating a dual-porosity model to account for elastic nonlinearity due to compliant pores subject to high-magnitude loading stresses. A modified finite-element method is employed to simulate the subtle effect of microstructures on wave propagation in prestressed digital cores. We measure the heterogeneity of samples by extracting the autocorrelation length of digital cores for a rough estimation of scattering intensity. We conductexperimental measurements with a fluid-saturated sandstone sample under a constant confining pressure of 65 MPa and increasing pore pressures from 5 to 60 MPa. Numerical simulations for ultrasound propagation in the prestressed fluid-saturated digital core of the sample are followed based on the proposed poro-acoustoelastic model with compliant pores. The results demonstrate a general agreement between experimental and numerical waveforms for different stresses, validating the performance of the presented modeling scheme. The excellent agreement between experimental and numerical coda quality factors demonstrates the applicability for the numerical investigation of the stress-associated scattering attenuation in prestressed porous rocks. [ABSTRACT FROM AUTHOR]

Abstract Understanding the adsorption mechanisms of different gas molecules on various minerals is crucial for accurately modelling shale gas adsorption behaviors and for objectively evaluating adsorbed gas contents under geological conditions. We simulated the adsorption behaviors of CH 4 , CO 2 and N 2 on both organic matter and inorganic minerals at 60 °C and 90 °C over a range of pressures up to 50 MPa by using the Grand Canonical Monte Carlo (GCMC) method. It has been found that both the comprehensive effect of the adsorption sites with differential adsorption capacity and the distribution density of the adsorption sites on the organic matter and inorganic mineral surfaces control the adsorption capacity in terms of per unit surface area of minerals. For individual minerals with a certain adsorption capacity in terms of per unit surface area, the specific surface area of individual minerals is the critical factor that determines the adsorption capacity in terms of per unit mass of the minerals. The interaction among gas molecules also affects the adsorption behavior slightly. We further compared the adsorption capacity among various gas molecules on both organic matter and inorganic minerals by inspecting the strength and distribution density of the adsorption sites on mineral surfaces, the specific surface area of the minerals and the interaction strength among gas molecules. These investigations allowed us to identify the key factors controlling shale gas adsorption mechanisms on different minerals, which provide some helpful insights for both of the exploration and the development of shale gas. Highlights • Both strength and density of adsorption sites control adsorption capacity of mineral surface. • Strengths of adsorption sites on clay surface are different, but similar on kerogen surface. • Specific surface area is one of the main factors controls adsorption behaviors. • Density of adsorption sites and specific surface area of kerogen are larger than that of clay. • Interactions among gas molecules also affect the adsorption behaviors. [ABSTRACT FROM AUTHOR]

Understanding the adsorption mechanism of shale gas is an essential pre-requisite for establishing models to evaluate the adsorbed gas amount under geological conditions quantitatively and to guide shale gas exploration and development. By using the Grand Canonical Monte Carlo (GCMC) method, we simulated the adsorption behavior of CH 4 and CO 2 in K-illite slit pores, and revealed the key gas adsorption mechanisms and discussed their implications on shale gas exploration and development by analyzing the distribution of gas mole concentration, gas-surface interaction energy, density field and etc. It is found that even with the presence of weak adsorption layers in meso and macro pores, the adsorption behavior of both CH 4 and CO 2 is dominated by the strong adsorption layers and thus can only be approximated but not strictly described by the classic Langmuir model. However, the micro-pore filling effect leads to the overlap of adsorption layers in micro pores, causing more deviation when using the classic Langmuir model to evaluate the adsorption behavior. Even though the adsorption behavior is not affected by pore size dimensions in meso and macro pores, the proportion of the adsorbed gas increases with the decreasing pore size. Both CH 4 and CO 2 are adsorbed in the center of the six-membered oxygen ring on the silicon oxygen tetrahedron surface. The CH 4 molecules (with no polarity) are at the center of the ring, but the CO 2 molecules (with electric quadrupole moment) are closer to the oxygen atom with polarity in the ring. The electric quadrupole moment makes the adsorption capacity of CO 2 much stronger than that of CH 4 in K-illite pores, providing a theoretical basis for enhancing CH 4 recovery efficiency by injecting CO 2 in the development of shale gas. [ABSTRACT FROM AUTHOR]

A good understanding of oil-bearing pore size distribution (PSD) in shales is crucial for determining reservoir sweet-spots. In this paper, we determined the oil-bearing PSD in lacustrine shales at peak oil generation by comparing PSDs between the extracted and un-extracted shale samples, and by relating the difference in PSDs between the extracted and un-extracted shale samples to the oil content in shales. It is found that while the specific pore volume (SPV) of meso pores are nearly 8 times as large as that of micro pores, macro pores contribute to the majority of the pore volume in lacustrine shales at peak oil generation. SPVs of the extracted samples are generally larger than that of the un-extracted ones. The oil-bearing PSD in pores with sizes up to 80 nm were obtained by subtracting SPVs of the un-extracted samples from that of the extracted ones within specific pore size intervals. The oil-bearing mechanism was further investigated by linking the oil-bearing PSD to the total oil content obtained from pyrolysis experiments. It is found that the content of oil residing in the micro and meso pores is significantly less than the total oil content, indicating that the oil is mainly contained in the macro pores. Therefore, the abundance and distribution of macro pores in lacustrine shales at peak oil generation are critical factors of a shale oil reservoir. [ABSTRACT FROM AUTHOR]

Geomechanical properties are of great importance in coal mining exploration, (enhanced) coal bed methane production and carbon geosequestration in deep unmineable coal seams. However, coal highly heterogeneous rocks, and conventional experimental methods (e.g. acoustic, seismic or unconfined compressive strength tests) only measure the cm-m scale bulk properties. Thus, we measured the geomechanical properties at nanoscale of Western Australian Collie coal. We mapped the nanoscale mechanical heterogeneity and correlated it with the sample's morphology (measured by SEM-EDS). It was found that the quartz and siderite in the coal had higher Young's moduli (up to 28.6 GPa), while the organic coal matrix had lower Young's moduli (from 3 GPa to 5 GPa), while the soft material kaolinite had the lowest Young's moduli (<3 GPa). We conclude that it is necessary to measure the geomechanical coal properties at nano/micro scale reliably assess and eventually facilitate field scale CBM/ECBM or CO 2 storage. [ABSTRACT FROM AUTHOR]

A good consistence between the grand canonical Monte Carlo (GCMC) simulation results and the adsorption experimental measurements is an important precondition to reveal the shale gas adsorption mechanisms by the GCMC method. To better link the simulations and the experiments, we investigated the expression of the excess adsorption amount and the reasonability of selecting the critical parameters by performing the GCMC simulations of CH 4 in the Na-Montmorillonite simulation cell with the pore size of 4 nm at the temperature of 90 °C under varying pressures. It is found that the excess adsorption amount in the nanopore in the simulations and between the simulations and the experiments are comparable by expressing it in per unit surface area of the adsorbent. The accessible volume probed by the corresponding gas molecule is the theoretical value of the free volume, and the determination of the bulk gas density from the GCMC method, which keeps the same method with the calculation of the absolute loading number of gas molecules, will eliminate the system error. We expect the findings are useful in the further investigation on the shale gas adsorption mechanisms by combing the GCMC simulations and the adsorption experiments. [ABSTRACT FROM AUTHOR]

Total organic carbon (TOC) estimation is significantly crucial for shale reservoir characterization. Traditional TOC estimation methods (such as Passey and Schmoker method) do not provide accurate TOC predictions in shale gas reservoirs especially for the self-generated and self-stored reservoirs. This study proposes, for the first time, a new TOC prediction method based on Gaussian Process Regression (GPR) bridging geostatistics and machine learning technique. The method utilizes a non-parametric regression approach in shale TOC predictions, and not only provides the expert solutions in high-dimension processing, small samples and non-linear problems, but also has a better adaptation and generalization ability compared with other machine learning methods. The approach accounts for all the well logging attributes and chooses the relevant logs to build TOC estimation model, and 7 different kernel functions and 5 attributes groups are analyzed to get the optimized hyperparameters in practice. Application of the developed model to two shale gas reservoirs showed that the model predicted TOC matched well with that from the laboratory measurements. The proposed model based on GPR method provides an accurate way for the TOC prediction in the tight shale gas reservoirs. [ABSTRACT FROM AUTHOR]

Rock mechanical properties are of crucial importance to shale reservoirs as they directly affect the production efficiency and hydraulic fracturing design. However, limited data exists for rock mechanical properties in lacustrine shales. We thus conducted a series of petrophysical and rock mechanical tests on lacustrine shales to study the difference between argillaceous shales and silty laminae shales. We observed distinctly different rock mechanical properties for these two types shales. Moreover, these two lithologies also showed significantly different Young's moduli and Ultimate strength. Mixed illite/smectite and organic matter content strongly affected the rock mechanical properties of the lacustrine shales and 3D x-ray imaging indicated that the argillaceous shales contained more open bedding fractures, transverse sub-fractures, and a complicated fracture network. In contrast, the silty laminae shales only contained a few open bedding fractures and sub-fractures propagating throughout the samples. It is concluded that the texture and micro-structure in the study shale reservoir determines their fracture complexity. We also conclude that lacustrine shales with high clay content can be extensively fractured and potentially achieve economical production, especially in case of argillaceous shales. The results of this study can therefore help optimize hydraulic fracturing and improve enhanced gas recovery in lacustrine shale reservoirs. • Geomechanical properties are controlled by chemical composition. • 3D morphology of the failed samples varies with lithology. • Geological processes play a critical role in determining geomechanical properties. [ABSTRACT FROM AUTHOR]