Alexander Rozhko - Academia.edu (original) (raw)

Papers by Alexander Rozhko

Rock Mechanics and Rock Engineering, 2024

Negative compressibility (NC) refers to the counterintuitive behaviour where a material expands w... more Negative compressibility (NC) refers to the counterintuitive behaviour where a material expands when subjected to compressive stress. In rock mechanics, NC can be linked to failure. During the onset of failure, a slight increase in amplitude of mean compressive stress from axial loading, with constant radial stresses, can lead to dilation due to microcrack growth and grain sliding. Post-peak softening then results in reduced deviator stress as axial compressive strain amplitude increases. This study unveils a novel form of NC in poroelastic materials containing microcracks with liquid bridges, a phenomenon distinct from known failure mechanisms. This new manifestation of NC is associated with changes in the capillary force of the liquid bridge, leading to the NC of a microfracture without failure occurring. To validate the new concept of NC in rocks during elastic deformation, we formulated a theoretical model and conducted laboratory experiments on partially saturated Mancos shale. The experiments involved the simultaneous increase in amplitude of confining (axial and radial) stresses and pore pressure in the gas phase by equal magnitudes. We observed extensional volumetric strains during compression at constant net stress, indicating negative shale stiffness. Using a micromechanical model, we simulated liquid bridge hysteresis in microfractures by coupling the wettability, interfacial tension, and elastic properties of the rock, liquid, and gas phases. Theoretical modelling reveals that capillary forces may dominate over elastic forces under specific conditions defined by two dimensionless parameters. This study enhances our understanding of the static and dynamic response of the overburden at partial saturation with free gas that may contribute to carbon storage and monitoring efforts. Moreover, the concept of negative compressibility and liquid bridge phenomena has potential applications in underground hydrogen and ammonia storage. Understanding microfracture behaviour under varying pressures is crucial for developing safe and efficient storage solutions for these emerging energy carriers. Highlights • Novel finding: Porous materials with microcracks and liquid bridges can exhibit negative compressibility due to capillary force changes. • Two dimensionless parameters define conditions when capillary forces dominate over elastic forces, producing negative compressibility. • Experiments on partially saturated Mancos shale show negative compressibility during elastic deformation, not linked to failure mechanisms. • Significance: Insights gained have implications for carbon storage, subsurface hydrogen and ammonia storage.

Proceedings, Jun 11, 2018

Low-velocity drop-weight impact experiments on individual and multiple Cyclotetramethylene tetran... more Low-velocity drop-weight impact experiments on individual and multiple Cyclotetramethylene tetranitramine (HMX) energetic particles were performed using a modified drop-weight machine equipped with high-speed photography components. Multiple particles experienced more severe burning reactions than an individual particle. Comparisons between impacted salt and HMX particle show that jetting in HMX is mainly due to the motion of fragmented particles driven by gaseous reaction products. Velocity of jetting, flame propagation, and area expansion were measured via image processing, making it possible to quantify the chemical reaction or mechanical deformation violence at different stages.

Proceedings, Jun 11, 2018

Proceedings, Oct 4, 2017

Summary Effect of small scale parameters on micro sweep efficiency in fractured carbonate reservo... more Summary Effect of small scale parameters on micro sweep efficiency in fractured carbonate reservoirs is studied. Parameters with most significant impact are listed and effect of those are taken into account using different upscaling techniques.

Proceedings, Sep 20, 2015

Faults (shear cracks) are formed by the coalescence of tensile microcracks in the brittle caprock... more Faults (shear cracks) are formed by the coalescence of tensile microcracks in the brittle caprock. The damage zone of the fault consisting of microcracks can serve as a permeable pathway within sealing formations. Like macrofractures, microfractures occur in populations that exhibit well-defined statistical properties such as their size distribution. The are many publications explaining how the aperture size distribution can be related to the sealing and transport properties, described by capillary pressure and effective phase permeabilities, however there are no models which explain how the aperture distribution depends the effective confining stress and on the saturation degree. Available models, developed for porous rock are not applicable to fractured rock, because fracture is much more compressible than the pore throat. In this paper the author proposed a new theoretical model, applied in two steps to the damage zone of the fault. In the first step the author developed new analytical solutions for the effective phase permeabilities and capillary pressure of the rock possessing a single deformable crack. In the second step the author applied the analytical solutions to multi-scale microcrack system of the damage zone of the fault to predict the stress and pressure dependence of the sealing and transport properties of the fault.

Proceedings, Oct 4, 2017

In this work, we investigate the effect of alteration of fracture total permeability, relative pe... more In this work, we investigate the effect of alteration of fracture total permeability, relative permeability and capillary pressure with the change of normal net stress (due to fluid injection or depletion), on estimated oil recovery factor from fractured carbonate reservoir. The numerical experiments are performed in two steps: first, we predict two-phase fluid-flow properties of fracture for different levels of normal effective stress using our in-house geomechanical model; and second, we study the effect of normal net stress in the reservoir on oil recovery factor, using conceptual simulation models and a commercial simulator. Our main conclusions of this study: • Oil recovery and water production in fractured carbonate reservoirs is controlled by, not only the fracture permeability (and other matrix related parameters), but also fracture rock curves. • Complex behavior of fluid flow in fractures cannot be captured only by varying fracture permeability. Shape of relative permeability and its dependence on the effective stress have a significant impact on flow of water vs. oil.

Proceedings, Apr 24, 2017

Fluid-flow in fractured reservoirs is highly sensitive to the change of effective stress during f... more Fluid-flow in fractured reservoirs is highly sensitive to the change of effective stress during fluid injection or production. Permeability, capillary pressure and relative permeability of rock fractures to oil and water directly impact the amount of hydrocarbons that can be ultimately recovered; however, these parameters are difficult to measure in the lab as a function of effective stress. This stimulates development of computational algorithms to predict the impact of stress changes on two-phase fluid-flow properties of fractures at depth. In this work, we developed a numerical approach for determining relationships between normal effective stress, elastic rock properties, fracture aperture distribution, aspect ratio scaling, oil/water interfacial tension, contact angle and two-phase fluid-flow characteristics of rough-walled fractures. We extended a well-established approach developed for modeling of single-phase fluid-flow in rough-walled fractures. According to this approach, the aperture distribution is replaced by a network of elliptical cavities forming connected pathway from the inlet to the outlet. The extension towards two-phase flow is based on our previous analytical model, in which a two-phase fluid-flow is calculated in a deformable elliptical cavity. The numerical algorithm developed in this work allows quick computation of the impact of the stress-change on two-phase fluid-flow properties of fractured rock. Relative permeabilities of fractures are shown to be non-linear functions of water saturation dependent on the effective normal stress. The capillary pressure-saturation curve for rough-walled fracture is shown to be a function of the effective normal stress. The dependency of fracture permeability, fracture porosity and surface area of open/closed fracture on the effective normal stress is also predicted by the model, which can be used as input parameters for reservoir simulators.

The borehole fluid injection is commonly used for CO2 storage and during stimulation of hydrocarb... more The borehole fluid injection is commonly used for CO2 storage and during stimulation of hydrocarbon and geothermal reservoirs. Usually this process induces micro-earthquakes with magnitude up to 4. The spatio-temporal distribution of the induced seismicity can be used for monitoring of fluid movement in the reservoir (passive seismicity) and for characterization of physical properties of the reservoir. Here we propose the method to explain and predict the shape and the spatio-temporal distribution of the induced seismicity cloud. This method is based on calculation of propagation of critical Coulomb Failure stress perturbation, which, as demonstrated, triggers the micro-seismicity. It is shown that non-linear field observations are well explained by coupling of linear fluid diffusion into the deformation of linear poroelastic rocks. It is demonstrated that depending on borehole fluid injection conditions, the seismicity triggering fronts can be scaled with different scaling laws; from the cubic root of time to almost a time-independent function. The proposed analytical solution can also be used potentially to predict the shape of back-triggering (suppression) front due to ceasing of fluid injection and to predict the conditions for induced seismicity during drawdown of pressure at borehole.

45th U.S. Rock Mechanics / Geomechanics Symposium, Jun 26, 2011

54th U.S. Rock Mechanics/Geomechanics Symposium, Jun 28, 2020

The influence of drilling fluids on the mechanical strength of shale is already a well known phen... more The influence of drilling fluids on the mechanical strength of shale is already a well known phenomenon in the oil industry. It is well realized that the strength of shales depends on relative saturation of wetting and non-wetting fluids (e.g. water & gas or water & oil). The strength of shales drastically decreases with increase in saturation of a wetting fluid phase [1] due to changes in capillary pressure. Also the experimental data suggest that the strength of rocks is lower for higher values of interfacial tension between wetting and non-wetting fluids [2]. The influence of fluid salinity on the strength of shales is also well-documented [3]. Depending on salt concentration, the amount of water content (and pressure) in shales can either decrease or increase due to osmotic diffusion and cause the increase or decrease in the strength of shale, respectively. Using the fracture mechanics approach [4 & 5] and a published analytical solution [6], I have calculated the change in stress intensity factor, for the elliptical crack shown on Figure 1.

Journal of Petroleum Science and Engineering

Proceedings, 2017

A large proportion of carbonate reservoirs contain natural open fractures, which have a significa... more A large proportion of carbonate reservoirs contain natural open fractures, which have a significant impact reservoir fluid flow and ultimate recovery. In this study, high resolution X-ray computed tomography (μCT) has been used for imaging, interpretation and analysis of open fracture in carbonate in true 3D. The samples are form the Shetland Group in the Gullfaks Field, which has produced oil since late 2012. The μCT data shows that the tensile fractures occur as calcite veins or are partly filled with calcite cement. Fractures tend to be most frequent in low porosity parts of the formation, and bioturbations also have a significant effect on the shape, orientation and occurrence of fractures. The μCT gives the detailed geometry and aperture distribution of the fractures in 3D.

The influence of drilling fluids on the mechanical strength of shale is already a well known phen... more The influence of drilling fluids on the mechanical strength of shale is already a well known phenomenon in the oil industry. It is well realized that the strength of shales depends on relative saturation of wetting and non-wetting fluids (e.g. water & gas or water & oil). The strength of shales drastically decreases with increase in saturation of a wetting fluid phase [1] due to changes in capillary pressure. Also the experimental data suggest that the strength of rocks is lower for higher values of interfacial tension between wetting and non-wetting fluids [2]. The influence of fluid salinity on the strength of shales is also well-documented [3]. Depending on salt concentration, the amount of water content (and pressure) in shales can either decrease or increase due to osmotic diffusion and cause the increase or decrease in the strength of shale, respectively. Using the fracture mechanics approach [4 & 5] and a published analytical solution [6], I have calculated the change in stre...

GEOPHYSICS, 2021

Low-frequency shadows are frequently interpreted as attenuation phenomena due to partial saturati... more Low-frequency shadows are frequently interpreted as attenuation phenomena due to partial saturation with free gas. However, several researchers have argued that shadows are not necessarily a simple attenuation phenomenon because low-frequency energy must have been added or amplified by some physical or numerical process. Attenuation alone should attenuate higher frequencies, not boost lower frequencies. The physical or numerical effects explaining this phenomenon are still debatable in the literature. To better understand the elastic wave energy’s spectral changes in partially saturated rock, we have considered the hysteresis of liquid bridges phenomena inside the crack. We determine that liquid bridges’ hysteresis leads to the nonlinear energy exchange between frequencies, explaining the wave energy boost at lower frequencies. We find that the energy exchange between different frequencies depends on the wave amplitude and the seismic wave spectrum. The low-frequency energy boost is...

GEOPHYSICS, 2020

<jats:p> I this paper I raise three comments on the recently-published model of: Zeng, C., ... more <jats:p> I this paper I raise three comments on the recently-published model of: Zeng, C., W. Deng, W., J. Fan, and K. H., Liu, 2020, Investigation of the resonance of nonwetting droplets in constricted capillary tubes: Geophysics, 85, no 2, ID1-ID17. </jats:p>

Journal of Geophysical Research: Solid Earth, 2020

Frequency dispersion is a well-known effect in geophysics, which means that waves of different wa... more Frequency dispersion is a well-known effect in geophysics, which means that waves of different wavelengths propagate at different velocities. Amplitude dispersion is a less-known effect, which means that waves of different amplitudes propagate at different velocities. Herewith, we consider the alteration of the interfacial energy during wave-induced two-phase fluid flow in a partially saturated rock and demonstrate that this leads to a nonlinear amplitude dispersion effect. When the wave amplitude is small, seismic waves cause bending of the interface menisci between immiscible fluids at the pore scale. However, when the wave amplitude is sufficiently large, the interface menisci will slip at the pore scale, causing attenuation of the elastic energy by the contact line friction mechanism. At the zero frequency limit, all viscous dissipation models predict zero attenuation of the elastic wave energy, while this approach predicts a nonzero attenuation due to a static contact angle hysteresis effect. Herein, we extend the Gassmann's theory with three extra terms, which can be obtained from standard laboratory tests: pore-size distribution and interfacial tension between immiscible fluids and rock wettability (advancing and receding contact angles). We derive closed-form analytical expressions predicting the effective fluid modulus in partially saturated rock, which falls between Voigt and Reuss averages. Next, we demonstrate that the nonlinear amplitude dispersion effect leads to energy transfer between different frequencies. This may explain the low-frequency microtremor anomalies, frequently observed above hydrocarbon reservoirs, when the low-frequency energy of ocean waves (0.1-1 Hz) is converted to higher frequencies (2-6 Hz) by partially saturated reservoirs.

Geophysical Prospecting, 2018

The effect of surface phenomena occurring at the interfaces between immiscible fluids and a solid... more The effect of surface phenomena occurring at the interfaces between immiscible fluids and a solid on the seismic attributes of partially saturated rocks has not yet been fully studied. Meanwhile, over the past two decades considerable progress has been made in the physics of wetting to understand effects such as contact line friction, contact line pinning, contact angle hysteresis, and equilibrium contact angle. In this paper, we developed a new rock physics model considering the aforementioned effects on seismic properties of the rock with a partially saturated plane-strain crack. We demonstrated that for small wave-induced stress perturbations, the contact line of the interface meniscus will remain pinned, while the meniscus will bulge and change its shape through the change of the contact angles. When the stress perturbation is larger than a critical value, the contact line will move with advancing or receding contact angle depending on the direction of contact line motion. A critical stress perturbation predicted by our model can be in the range of 102−104 Pa, that is typical for linear seismic waves. Our model predicts strong seismic attenuation in the case when the contact line is moving. When the contact line is pinned, the attenuation is negligibly small. Seismic attenuation is associated with the hysteresis of loading and unloading bulk moduli, predicted by our model. The hysteresis is large when the contact line is moving and negligibly small when the contact line is pinned. Furthermore, we demonstrate that the bulk modulus of the rock with a partially saturated crack depends also on the surface tension and on the contact angle hysteresis. These parameters are typically neglected during calculation of the effecting fluid moduli by applying different averaging techniques. We demonstrate that contact line friction may be a dominant seismic attenuation mechanism in the low frequency limit (<10 Hz) when capillary forces dominate over viscous forces during wave-induced two-phase fluid flow.

Journal of Petroleum Science and Engineering, 2016

Two-phase fluid flow in tight rocks containing cracks is not fully understood. Published experime... more Two-phase fluid flow in tight rocks containing cracks is not fully understood. Published experimental studies performed on water-saturated tight rocks often associate gas flow with instabilities. A common observation is that gas begins to flow once it has reached a given breakthrough pressure. Pronounced sample dilation associated with the gas breakthrough is frequently observed experimentally, which is difficult to explain using standard two-phase flow models developed for porous and fractured rocks. Previously it was suggested that such two-phase fluid-flow mechanism is possible via network of connected dilatant crack-like pathways, which opened and closed depending on the magnitude of gas pressure, stress, and water pressure. In order to better understand the hydro-mechanical coupling between two-phase flow and deformation of dilatant crack-like pathway, a new theoretical model is proposed in this paper. We model two-phase fluid flow in a representative volume element (RVE) containing a deformable crack. The initial geometry of the crack is approximated by an elliptical cavity of high aspect ratio (major to minor axes of ellipse). The final spindle-like cross-section of the crack is calculated analytically by two-way coupling of the capillary pressure with the deformation of fracture aperture. It depends on pressure in the wetting fluid phase occupying crack tips; pressure in the non-wetting fluid phase occupying central part of the crack, and far-field rock stresses. This approximation can accurately predict the fluid flux through tortuous and rough-walled network of microfractures if the appropriate hydraulic aperture of elliptical crack is considered. Using the model, we analyzed conditions controlling formation of dilatant crack-like pathways, which were found to depend on a dimensionless parameter. The deformation of fracture aperture and tensile fracturing are shown to be controlled by generalized effective stress similar to Bishop's effective stress, experimentally introduced for partially saturated soils and rocks. The conceptual model provides an insight into the stress and pressure effects on two-phase permeabilities and the capillary pressure of tight rocks containing cracks.

Rock Mechanics and Rock Engineering, 2024

Negative compressibility (NC) refers to the counterintuitive behaviour where a material expands w... more Negative compressibility (NC) refers to the counterintuitive behaviour where a material expands when subjected to compressive stress. In rock mechanics, NC can be linked to failure. During the onset of failure, a slight increase in amplitude of mean compressive stress from axial loading, with constant radial stresses, can lead to dilation due to microcrack growth and grain sliding. Post-peak softening then results in reduced deviator stress as axial compressive strain amplitude increases. This study unveils a novel form of NC in poroelastic materials containing microcracks with liquid bridges, a phenomenon distinct from known failure mechanisms. This new manifestation of NC is associated with changes in the capillary force of the liquid bridge, leading to the NC of a microfracture without failure occurring. To validate the new concept of NC in rocks during elastic deformation, we formulated a theoretical model and conducted laboratory experiments on partially saturated Mancos shale. The experiments involved the simultaneous increase in amplitude of confining (axial and radial) stresses and pore pressure in the gas phase by equal magnitudes. We observed extensional volumetric strains during compression at constant net stress, indicating negative shale stiffness. Using a micromechanical model, we simulated liquid bridge hysteresis in microfractures by coupling the wettability, interfacial tension, and elastic properties of the rock, liquid, and gas phases. Theoretical modelling reveals that capillary forces may dominate over elastic forces under specific conditions defined by two dimensionless parameters. This study enhances our understanding of the static and dynamic response of the overburden at partial saturation with free gas that may contribute to carbon storage and monitoring efforts. Moreover, the concept of negative compressibility and liquid bridge phenomena has potential applications in underground hydrogen and ammonia storage. Understanding microfracture behaviour under varying pressures is crucial for developing safe and efficient storage solutions for these emerging energy carriers. Highlights • Novel finding: Porous materials with microcracks and liquid bridges can exhibit negative compressibility due to capillary force changes. • Two dimensionless parameters define conditions when capillary forces dominate over elastic forces, producing negative compressibility. • Experiments on partially saturated Mancos shale show negative compressibility during elastic deformation, not linked to failure mechanisms. • Significance: Insights gained have implications for carbon storage, subsurface hydrogen and ammonia storage.

Proceedings, Jun 11, 2018

Low-velocity drop-weight impact experiments on individual and multiple Cyclotetramethylene tetran... more Low-velocity drop-weight impact experiments on individual and multiple Cyclotetramethylene tetranitramine (HMX) energetic particles were performed using a modified drop-weight machine equipped with high-speed photography components. Multiple particles experienced more severe burning reactions than an individual particle. Comparisons between impacted salt and HMX particle show that jetting in HMX is mainly due to the motion of fragmented particles driven by gaseous reaction products. Velocity of jetting, flame propagation, and area expansion were measured via image processing, making it possible to quantify the chemical reaction or mechanical deformation violence at different stages.

Proceedings, Jun 11, 2018

Proceedings, Oct 4, 2017

Summary Effect of small scale parameters on micro sweep efficiency in fractured carbonate reservo... more Summary Effect of small scale parameters on micro sweep efficiency in fractured carbonate reservoirs is studied. Parameters with most significant impact are listed and effect of those are taken into account using different upscaling techniques.

Proceedings, Sep 20, 2015

Faults (shear cracks) are formed by the coalescence of tensile microcracks in the brittle caprock... more Faults (shear cracks) are formed by the coalescence of tensile microcracks in the brittle caprock. The damage zone of the fault consisting of microcracks can serve as a permeable pathway within sealing formations. Like macrofractures, microfractures occur in populations that exhibit well-defined statistical properties such as their size distribution. The are many publications explaining how the aperture size distribution can be related to the sealing and transport properties, described by capillary pressure and effective phase permeabilities, however there are no models which explain how the aperture distribution depends the effective confining stress and on the saturation degree. Available models, developed for porous rock are not applicable to fractured rock, because fracture is much more compressible than the pore throat. In this paper the author proposed a new theoretical model, applied in two steps to the damage zone of the fault. In the first step the author developed new analytical solutions for the effective phase permeabilities and capillary pressure of the rock possessing a single deformable crack. In the second step the author applied the analytical solutions to multi-scale microcrack system of the damage zone of the fault to predict the stress and pressure dependence of the sealing and transport properties of the fault.

Proceedings, Oct 4, 2017

In this work, we investigate the effect of alteration of fracture total permeability, relative pe... more In this work, we investigate the effect of alteration of fracture total permeability, relative permeability and capillary pressure with the change of normal net stress (due to fluid injection or depletion), on estimated oil recovery factor from fractured carbonate reservoir. The numerical experiments are performed in two steps: first, we predict two-phase fluid-flow properties of fracture for different levels of normal effective stress using our in-house geomechanical model; and second, we study the effect of normal net stress in the reservoir on oil recovery factor, using conceptual simulation models and a commercial simulator. Our main conclusions of this study: • Oil recovery and water production in fractured carbonate reservoirs is controlled by, not only the fracture permeability (and other matrix related parameters), but also fracture rock curves. • Complex behavior of fluid flow in fractures cannot be captured only by varying fracture permeability. Shape of relative permeability and its dependence on the effective stress have a significant impact on flow of water vs. oil.

Proceedings, Apr 24, 2017

Fluid-flow in fractured reservoirs is highly sensitive to the change of effective stress during f... more Fluid-flow in fractured reservoirs is highly sensitive to the change of effective stress during fluid injection or production. Permeability, capillary pressure and relative permeability of rock fractures to oil and water directly impact the amount of hydrocarbons that can be ultimately recovered; however, these parameters are difficult to measure in the lab as a function of effective stress. This stimulates development of computational algorithms to predict the impact of stress changes on two-phase fluid-flow properties of fractures at depth. In this work, we developed a numerical approach for determining relationships between normal effective stress, elastic rock properties, fracture aperture distribution, aspect ratio scaling, oil/water interfacial tension, contact angle and two-phase fluid-flow characteristics of rough-walled fractures. We extended a well-established approach developed for modeling of single-phase fluid-flow in rough-walled fractures. According to this approach, the aperture distribution is replaced by a network of elliptical cavities forming connected pathway from the inlet to the outlet. The extension towards two-phase flow is based on our previous analytical model, in which a two-phase fluid-flow is calculated in a deformable elliptical cavity. The numerical algorithm developed in this work allows quick computation of the impact of the stress-change on two-phase fluid-flow properties of fractured rock. Relative permeabilities of fractures are shown to be non-linear functions of water saturation dependent on the effective normal stress. The capillary pressure-saturation curve for rough-walled fracture is shown to be a function of the effective normal stress. The dependency of fracture permeability, fracture porosity and surface area of open/closed fracture on the effective normal stress is also predicted by the model, which can be used as input parameters for reservoir simulators.

The borehole fluid injection is commonly used for CO2 storage and during stimulation of hydrocarb... more The borehole fluid injection is commonly used for CO2 storage and during stimulation of hydrocarbon and geothermal reservoirs. Usually this process induces micro-earthquakes with magnitude up to 4. The spatio-temporal distribution of the induced seismicity can be used for monitoring of fluid movement in the reservoir (passive seismicity) and for characterization of physical properties of the reservoir. Here we propose the method to explain and predict the shape and the spatio-temporal distribution of the induced seismicity cloud. This method is based on calculation of propagation of critical Coulomb Failure stress perturbation, which, as demonstrated, triggers the micro-seismicity. It is shown that non-linear field observations are well explained by coupling of linear fluid diffusion into the deformation of linear poroelastic rocks. It is demonstrated that depending on borehole fluid injection conditions, the seismicity triggering fronts can be scaled with different scaling laws; from the cubic root of time to almost a time-independent function. The proposed analytical solution can also be used potentially to predict the shape of back-triggering (suppression) front due to ceasing of fluid injection and to predict the conditions for induced seismicity during drawdown of pressure at borehole.

45th U.S. Rock Mechanics / Geomechanics Symposium, Jun 26, 2011

54th U.S. Rock Mechanics/Geomechanics Symposium, Jun 28, 2020

The influence of drilling fluids on the mechanical strength of shale is already a well known phen... more The influence of drilling fluids on the mechanical strength of shale is already a well known phenomenon in the oil industry. It is well realized that the strength of shales depends on relative saturation of wetting and non-wetting fluids (e.g. water & gas or water & oil). The strength of shales drastically decreases with increase in saturation of a wetting fluid phase [1] due to changes in capillary pressure. Also the experimental data suggest that the strength of rocks is lower for higher values of interfacial tension between wetting and non-wetting fluids [2]. The influence of fluid salinity on the strength of shales is also well-documented [3]. Depending on salt concentration, the amount of water content (and pressure) in shales can either decrease or increase due to osmotic diffusion and cause the increase or decrease in the strength of shale, respectively. Using the fracture mechanics approach [4 & 5] and a published analytical solution [6], I have calculated the change in stress intensity factor, for the elliptical crack shown on Figure 1.

Journal of Petroleum Science and Engineering

Proceedings, 2017

A large proportion of carbonate reservoirs contain natural open fractures, which have a significa... more A large proportion of carbonate reservoirs contain natural open fractures, which have a significant impact reservoir fluid flow and ultimate recovery. In this study, high resolution X-ray computed tomography (μCT) has been used for imaging, interpretation and analysis of open fracture in carbonate in true 3D. The samples are form the Shetland Group in the Gullfaks Field, which has produced oil since late 2012. The μCT data shows that the tensile fractures occur as calcite veins or are partly filled with calcite cement. Fractures tend to be most frequent in low porosity parts of the formation, and bioturbations also have a significant effect on the shape, orientation and occurrence of fractures. The μCT gives the detailed geometry and aperture distribution of the fractures in 3D.

The influence of drilling fluids on the mechanical strength of shale is already a well known phen... more The influence of drilling fluids on the mechanical strength of shale is already a well known phenomenon in the oil industry. It is well realized that the strength of shales depends on relative saturation of wetting and non-wetting fluids (e.g. water & gas or water & oil). The strength of shales drastically decreases with increase in saturation of a wetting fluid phase [1] due to changes in capillary pressure. Also the experimental data suggest that the strength of rocks is lower for higher values of interfacial tension between wetting and non-wetting fluids [2]. The influence of fluid salinity on the strength of shales is also well-documented [3]. Depending on salt concentration, the amount of water content (and pressure) in shales can either decrease or increase due to osmotic diffusion and cause the increase or decrease in the strength of shale, respectively. Using the fracture mechanics approach [4 & 5] and a published analytical solution [6], I have calculated the change in stre...

GEOPHYSICS, 2021

Low-frequency shadows are frequently interpreted as attenuation phenomena due to partial saturati... more Low-frequency shadows are frequently interpreted as attenuation phenomena due to partial saturation with free gas. However, several researchers have argued that shadows are not necessarily a simple attenuation phenomenon because low-frequency energy must have been added or amplified by some physical or numerical process. Attenuation alone should attenuate higher frequencies, not boost lower frequencies. The physical or numerical effects explaining this phenomenon are still debatable in the literature. To better understand the elastic wave energy’s spectral changes in partially saturated rock, we have considered the hysteresis of liquid bridges phenomena inside the crack. We determine that liquid bridges’ hysteresis leads to the nonlinear energy exchange between frequencies, explaining the wave energy boost at lower frequencies. We find that the energy exchange between different frequencies depends on the wave amplitude and the seismic wave spectrum. The low-frequency energy boost is...

GEOPHYSICS, 2020

<jats:p> I this paper I raise three comments on the recently-published model of: Zeng, C., ... more <jats:p> I this paper I raise three comments on the recently-published model of: Zeng, C., W. Deng, W., J. Fan, and K. H., Liu, 2020, Investigation of the resonance of nonwetting droplets in constricted capillary tubes: Geophysics, 85, no 2, ID1-ID17. </jats:p>

Journal of Geophysical Research: Solid Earth, 2020

Frequency dispersion is a well-known effect in geophysics, which means that waves of different wa... more Frequency dispersion is a well-known effect in geophysics, which means that waves of different wavelengths propagate at different velocities. Amplitude dispersion is a less-known effect, which means that waves of different amplitudes propagate at different velocities. Herewith, we consider the alteration of the interfacial energy during wave-induced two-phase fluid flow in a partially saturated rock and demonstrate that this leads to a nonlinear amplitude dispersion effect. When the wave amplitude is small, seismic waves cause bending of the interface menisci between immiscible fluids at the pore scale. However, when the wave amplitude is sufficiently large, the interface menisci will slip at the pore scale, causing attenuation of the elastic energy by the contact line friction mechanism. At the zero frequency limit, all viscous dissipation models predict zero attenuation of the elastic wave energy, while this approach predicts a nonzero attenuation due to a static contact angle hysteresis effect. Herein, we extend the Gassmann's theory with three extra terms, which can be obtained from standard laboratory tests: pore-size distribution and interfacial tension between immiscible fluids and rock wettability (advancing and receding contact angles). We derive closed-form analytical expressions predicting the effective fluid modulus in partially saturated rock, which falls between Voigt and Reuss averages. Next, we demonstrate that the nonlinear amplitude dispersion effect leads to energy transfer between different frequencies. This may explain the low-frequency microtremor anomalies, frequently observed above hydrocarbon reservoirs, when the low-frequency energy of ocean waves (0.1-1 Hz) is converted to higher frequencies (2-6 Hz) by partially saturated reservoirs.

Geophysical Prospecting, 2018

The effect of surface phenomena occurring at the interfaces between immiscible fluids and a solid... more The effect of surface phenomena occurring at the interfaces between immiscible fluids and a solid on the seismic attributes of partially saturated rocks has not yet been fully studied. Meanwhile, over the past two decades considerable progress has been made in the physics of wetting to understand effects such as contact line friction, contact line pinning, contact angle hysteresis, and equilibrium contact angle. In this paper, we developed a new rock physics model considering the aforementioned effects on seismic properties of the rock with a partially saturated plane-strain crack. We demonstrated that for small wave-induced stress perturbations, the contact line of the interface meniscus will remain pinned, while the meniscus will bulge and change its shape through the change of the contact angles. When the stress perturbation is larger than a critical value, the contact line will move with advancing or receding contact angle depending on the direction of contact line motion. A critical stress perturbation predicted by our model can be in the range of 102−104 Pa, that is typical for linear seismic waves. Our model predicts strong seismic attenuation in the case when the contact line is moving. When the contact line is pinned, the attenuation is negligibly small. Seismic attenuation is associated with the hysteresis of loading and unloading bulk moduli, predicted by our model. The hysteresis is large when the contact line is moving and negligibly small when the contact line is pinned. Furthermore, we demonstrate that the bulk modulus of the rock with a partially saturated crack depends also on the surface tension and on the contact angle hysteresis. These parameters are typically neglected during calculation of the effecting fluid moduli by applying different averaging techniques. We demonstrate that contact line friction may be a dominant seismic attenuation mechanism in the low frequency limit (<10 Hz) when capillary forces dominate over viscous forces during wave-induced two-phase fluid flow.

Journal of Petroleum Science and Engineering, 2016

Two-phase fluid flow in tight rocks containing cracks is not fully understood. Published experime... more Two-phase fluid flow in tight rocks containing cracks is not fully understood. Published experimental studies performed on water-saturated tight rocks often associate gas flow with instabilities. A common observation is that gas begins to flow once it has reached a given breakthrough pressure. Pronounced sample dilation associated with the gas breakthrough is frequently observed experimentally, which is difficult to explain using standard two-phase flow models developed for porous and fractured rocks. Previously it was suggested that such two-phase fluid-flow mechanism is possible via network of connected dilatant crack-like pathways, which opened and closed depending on the magnitude of gas pressure, stress, and water pressure. In order to better understand the hydro-mechanical coupling between two-phase flow and deformation of dilatant crack-like pathway, a new theoretical model is proposed in this paper. We model two-phase fluid flow in a representative volume element (RVE) containing a deformable crack. The initial geometry of the crack is approximated by an elliptical cavity of high aspect ratio (major to minor axes of ellipse). The final spindle-like cross-section of the crack is calculated analytically by two-way coupling of the capillary pressure with the deformation of fracture aperture. It depends on pressure in the wetting fluid phase occupying crack tips; pressure in the non-wetting fluid phase occupying central part of the crack, and far-field rock stresses. This approximation can accurately predict the fluid flux through tortuous and rough-walled network of microfractures if the appropriate hydraulic aperture of elliptical crack is considered. Using the model, we analyzed conditions controlling formation of dilatant crack-like pathways, which were found to depend on a dimensionless parameter. The deformation of fracture aperture and tensile fracturing are shown to be controlled by generalized effective stress similar to Bishop's effective stress, experimentally introduced for partially saturated soils and rocks. The conceptual model provides an insight into the stress and pressure effects on two-phase permeabilities and the capillary pressure of tight rocks containing cracks.