Geomechanical properties are of great importance in coal mining exploration, (enhanced) coal bed ... more 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 CO2 storage.
Low permeability of coal has been a constant obstacle to economic production from coalbed methane... more Low permeability of coal has been a constant obstacle to economic production from coalbed methane reservoirs, and liquid nitrogen (LN2) treatment has been investigated as one approach to address this issue. This study examined LN2 fracturing of a bituminous coal at pore-scale through 3D X-ray micro-computed tomography. For this purpose, a cylindrical sample was immersed into LN2 for 60 min. The micro-CT results clearly showed that the rapid freezing of the coal with LN2 generated fracture planes with large apertures originating from the pre-existing cleats in the rock. This treatment also connected original cleats with originally isolated pores and micro-cleats, thereby increasing pore network connectivity. Moreover, scanning electron microscopy highlighted the appearance of continuous wide conductive fractures with a maximum opening size of 9 µm. Furthermore, a nano-indentation technique was used to test the effect of LN2 on coal mechanical properties. The indentation moduli decrea...
Summary We studied the influence of water saturation on the elastic and anisotropy parameters of ... more Summary We studied the influence of water saturation on the elastic and anisotropy parameters of the Wellington shale in the range of the seismic frequencies from 0.1 Hz to 100 Hz at a confining pressure of 6 MPa. The measurements were conducted on three specimens retrieved at 0°, 45° and 90° angles with respect to the formation bedding. The specimens were saturated with water at four relative humidities of 12, 44, 72 and 97.5%. The measurements show a decrease in the components of the stiffness tensor with increasing saturation as well as a decrease of Thomsen’s parameters e and γ, whereas parameter δ remains virtually unchanged. We assume that the obtained results can be explained by the presence of a high percentage of smectite in the shale.
The petrophysical properties can be proper indicators to identify oil and gas reservoirs, since t... more The petrophysical properties can be proper indicators to identify oil and gas reservoirs, since the pore fluids have significant effects on the wave response. We have performed ultrasonic measurements on two sets of tight siltstones and dolomites at partial saturation. P- and S-wave velocities are obtained by the pulse transmission technique, while attenuation is calculated using the centroid-frequency shift and spectral-ratio methods. The fluid sensitivities of different properties (i.e., P- and S-wave velocities, impedances and attenuation, Poisson's ratio, density, and their combinations) are quantitatively analyzed by considering the data distribution, based on the crossplot technique. The result shows that the properties (P- to S-wave velocity and attenuation ratios, Poisson's ratio, and first to second Lamé constant ratio) with high fluid-sensitivity indicators successfully distinguish gas from oil and water, unlike oil from water. Moreover, siltstones and dolomites ca...
Summary Estimating the effects of pore filling material on the elastic moduli or velocities of po... more Summary Estimating the effects of pore filling material on the elastic moduli or velocities of porous and fractured rocks attracts widespread attention. This effect can be modeled by a recently proposed triple-porosity scheme, which quantifies this effect from parameters of the pressure dependency of the elastic properties of the dry rock. This scheme divides total porosity into three parts: compliant, intermediate, and stiff. Each type of pores is assumed to be spheroidal and characterized by a single aspect ratio. However, the implementation of this model requires the asymptotic values of the elastic moduli at much higher pressures where only non-closable pores remain open. Those pressures are beyond the capacity of most rock physics laboratories and can even crush typical sandstone samples. Experimental data at such pressures are usually unavailable. To address this issue, we introduce pore-scale numerical simulations in conjunction with effective medium theories (EMT) to compute...
SPE Asia Pacific Oil and Gas Conference and Exhibition
The gas content in shale reservoir is of great importance in reservoir evaluation. Shale reservoi... more The gas content in shale reservoir is of great importance in reservoir evaluation. Shale reservoir has various gas including free gas, adsorpted gas and soluted gas. Free gas take an important part for the total gas content. Hence, we investigated three equations for water saturation calculating and compared and improved them based on theoretical analysis in order to find a siutable one for the shale reservoir characterization. The results indicate that the Archie formula has several limitations applied to complex pore structure, which leads to high water saturation. Since the Archie formula was proposed by experimental data in pure sandstone without enough consideration about the clay of shale reservoir. The Waxman-Smits is suitable to shale gas reservoirs through theoretical analysis, but there are several uncertain parameters. The conductivity of formation water is necessary parameter in calculation of formation water saturation, but calculating the conductivity of formation wate...
Carbon dioxide (CO 2) injection into deep depleted hydrocarbon reservoirs or saline aquifers is c... more Carbon dioxide (CO 2) injection into deep depleted hydrocarbon reservoirs or saline aquifers is currently considered the best approach to large-scale CO 2 storage. Importantly, the pore structure and permeability of the storage rock are affected by fines release, migration, and reattachment in the initial stage of CO 2 injection, especially in unconsolidated sandstone reservoirs. It is thus necessary to better understand the pore structure changes and the associated permeability evolution during and after CO 2 injection. We thus imaged an unconsolidated sandstone at reservoir conditions before and after CO 2-saturated brine ("live brine") injection in situ via X-ray microcomputed tomography to explore the effects of fines migration and mineral dissolution induced by CO 2 injection. We found that in the examined sample, large pores dominated the total porosity, and porosity slightly increased after live-brine flooding. Moreover, and importantly, the pore structure changed significantly: large pores were further enlarged while small pores shrank or even disappeared. These structural changes in the tested sample were caused by mobilized fines due to the high-fluid interstitial velocity, which eventually reattached to the grains further downstream. Furthermore, the impact of the pore structural changes on permeability were analyzed in detail numerically. These permeability results are consistent with a fines migration mechanism where reattached fines block pore throats and thus decrease permeability drastically. We therefore can conclude that live brine injected into the examined unconsolidated sandstone will slightly improve storage space (porosity slightly increased); however, injectivity may be severely impaired by the permeability reduction.
There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yie... more There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yield phase or group velocities. The main problem here is that the size of the transducers used in the laboratory for ultrasonic measurements is comparable with the size of the rock sample or propagation distance. As a result, it is not clear how to interpret the measured traveltimes of ultrasonic waves. The main question is how to define an appropriate effective travel path, its length, and orientation. To answer this question, we look in detail at the full wavefront generated by a finite-size transducer using a specifically designed experimental setup, and a synthetic material (phenolic grade CE). We devise an algorithm for the interpretation of traveltimes measured in the laboratory with finite-size transducers arbitrarily oriented with respect to the symmetry planes of the material. To illustrate/validate the proposed algorithm, we designed a special testing program. The results of the ...
Abstract Porosity, fluid type and rock texture significantly affect acoustic wave propagation, si... more Abstract Porosity, fluid type and rock texture significantly affect acoustic wave propagation, since velocity dispersion and attenuation in fluid saturated rocks are mainly caused by wave-induced local fluid flow between microcracks and intergranular pores. We analyze P-wave velocity dispersion as a function of porosity to obtain information about the rock microstructure. The P-wave velocities in water-saturated rocks are predicted from measurements in gas-saturated rocks, using the Gassmann fluid substitution equation (the relaxed state). The dispersion is estimated from the difference between this predicted velocity and the measured one, where the latter corresponds to the unrelaxed state. We evaluate the wave dispersion as a function of porosity for 112 carbonates, 128 sandstones and 56 volcanic rocks, including our measurements for 86 tight rocks, showing that dispersion increases with porosity in the low porosity range, but decreases in the high porosity range. The dispersion peak occurs at a porosity of approximately 15%. Double-porosity poroelasticity modeling based on the local fluid-flow mechanism confirms this behavior. The microcrack radius has a peak in the porosity range 15–19% for all the lithologies from our collection, while the behavior of microcrack porosity is less evident. The dispersion peak may reveal the characteristics of lithological units, in particular porosity, fluid type and rock microstructure.
The effect of the amplitude of ultrasonic waves propagating through a sample is not often taken i... more The effect of the amplitude of ultrasonic waves propagating through a sample is not often taken into account in laboratory experiments. However, ultrasonic waves can produce relatively large strain inside the sample, and thus change the properties of the sample. To investigate the effect of strain amplitude on the P-wave velocity, a series of ultrasonic wave propagation experiments were carried out on three different media. All measurements were performed at 1 MHz central frequency and at the strain levels inside propagating waves of 3.0 × 10−6 to 6.0 × 10−5 without applying confining pressure to the sample. Strains in the waves were measured by a laser Doppler interferometer upon wave arrival on a free surface of the sample. The ultrasonic velocities were measured by a pair of P-wave transducers located at the same measuring point as the laser beam of the LDI. The effect of strain on P-wave velocity varied for different material. The P-wave velocity was calculated using both a first arrival and a first maximum peak at different applied voltage. The P-wave velocity remained unchanged for a pure elastic medium (aluminium); however, the velocity increased continuously with the increasing of the strain for polymethylmethacrylate and Gosford sandstone. For Gosford sandstone, velocity increases up to 0.8% with strain increase from 7.0 × 10−6 to 2.0 × 10−5. This effect of velocity increase with the strain induced by an ultrasonic wave can be explained by the in-elasticity of both the polymethylmethacrylate and Gosford sandstone samples.
Summary We present a methodology and describe a set-up that allows simultaneous acquisition of al... more Summary We present a methodology and describe a set-up that allows simultaneous acquisition of all five elastic coefficients of a transversely isotropic (TI) medium and its permeability in the direction parallel to the symmetry axis during mechanical compaction experiments. We apply the approach to synthetic shale samples and investigate the role of composition and applied stress on their elastic and transport properties. Compaction trends for the five elastic coefficients that fully characterize TI anisotropy of artificial shales are obtained for a porosity range from 40 per cent to 15 per cent. A linear increase of elastic coefficients with decreasing porosity is observed. The permeability acquired with the pressure-oscillation technique exhibits exponential decrease with decreasing porosity. Strong correlations are observed between an axial fluid permeability and seismic attributes, namely, VP/VS ratio and acoustic impedance, measured in the same direction. These correlations mig...
We use full wave forward and inversion modelling to estimate the elastic properties of rock sampl... more We use full wave forward and inversion modelling to estimate the elastic properties of rock samples from ultrasonic waveforms. The finite element algorithm (ABAQUS modelling software) is used to model a forward wave propagation within a homogeneous medium. For 19 mm diameter P-wave transducers, the result of the displacement waveform for a uniform source signal is obtained using both a linear and radial (about 2 mm) receiver arrays. Also, the use of a non-uniform source amplitude such as Gaussian distribution improves the displacement waveforms by few percent. The results accuracy is increased with increasing values of Gaussian standard deviation. However, for a nominal frequency of 1MHz, the same error increases with the decreasing frequencies. Additionally, our inversion algorithm (written in Python) searches for the best Young modulus (E) and Poison ratio (ν) of the medium iteratively. Finally, without prior knowledge of any threshold, the elastic parameters are estimated, and the results are consistent with the experimental measurements. These results provide a new modelling workflow to estimate the elastic parameters of the homogeneous and isotropic sample.
It is common to use ultrasonic techniques to measure elastic properties of the porous media. Howe... more It is common to use ultrasonic techniques to measure elastic properties of the porous media. However, conventional methods are unable to measure local strain in ultrasonic wave. This is not clear how the velocity of wave depends on its amplitude. In this work we, 1) measured the particle displacement in the ultrasonic wave using a Laser Doppler Interferometry (LDI) and 2) measured changes of Pwave velocities with wave amplitude for elastic (Aluminium), viscoelastic (Polymethylmethacrylate), and granular media (dried Gosford sandstone). We checked this phenomena using a conventional ultrasonic receiver and linked this changes to a local strain in wave. The study indicated that for a sandstone sample by increasing of local strain produced by an ultrasonic wave from 7*10-6 to 2*10-5 the P wave velocity increase by 0.7%. We also analysed the accuracy of velocity measured using LDI as a receiver and compare the results with that using conventional transducers. Moreover, the effect of proper couplant of the sample and the transducer was investigated in details.
Understanding of shales elastic properties behaviour with saturation changes is important for geo... more Understanding of shales elastic properties behaviour with saturation changes is important for geological storage of nuclear waste, CO2 sequestration as well as for development of conventional and unconventional shale oil and gas reservoirs. Existing data describing effects of saturation on elastic properties of shales are sparse and contradictory. To improve understanding of the effects of changing water content on elastic properties in shales, we conduct an experimental study on Opalinus shale samples. We measure vertical and horizontal ultrasonic P-and S-wave velocities of the same set of samples with controlled water content. The measured velocities are used to calculate components of elastic stiffness tensor in the shale at different saturations assuming its vertical transverse isotropy. Obtained results show increasing C11 and decreasing C33 with drying of the samples. Moreover, we observe 80% and 60% increase of shear moduli C44 and C66, respectively, with reduction of the water content from 5.5% weight in the preserved state to 0.3%. Conventional rock physics models are not designed to explain the observed dynamics. Here we perform a theoretical investigation of the influence on the shale moduli of following factors: (1) mechanical softening of the rock with the decrease of water saturation; (2) shrinkage of clay leading to reduction of porosity; (3) chemical hardening of clay particles; and (4) enhancing stiffness of contacts due to removing of water between clay particles.
Increasing energy demand and associated global warming are unarguably the two major challenges th... more Increasing energy demand and associated global warming are unarguably the two major challenges that the world currently faces. One of the ideas to reduce the carbon footprint while increasing the efficiency of the energy extraction is CO2 sequestration in coal seams. This can additionally enhance the coal-bed methane production. However, this process depends on many factors, among which coal wettability is of particular importance especially because of its pressure and temperature dependency. To evaluate this process, coal wettability was tested by measuring the contact angle of CO2 and water as a function of pressure, temperature, and salinity (DI water and brine (5 wt % NaCl + 1 wt % KCl), i.e., wt % is the weight percentage of salt to water. The results show that the CO2–water contact angle increases significantly, with increasing pressure, temperature, and salinity indicating more-effective CO2 wetness of coal. This, in turn, can reduce the CO2 residual trapping capacities and increase methane recover...
Permeability variation is one of the key factors influencing the injectivity of CO2 in CO2 seques... more Permeability variation is one of the key factors influencing the injectivity of CO2 in CO2 sequestration projects. Despite the research carried out on the subject, the results are highly inconsistent. In this study, the injection of brine (5 wt % NaCl + 1 wt % KCl), CO2-saturated (live) brine, and supercritical CO2 was performed on three homogeneous Berea sandstone plugs with a low clay content and two Bandera Gray sandstone plugs with a high clay content at reservoir conditions (10 MPa and 323 K). Porosity and permeability of the samples were measured using nuclear magnetic resonance (T2 relaxation time), and a dynamic (during flooding) permeability measurement technique, respectively, at different injection rates and injection durations. The mercury intrusion test was also performed on each sample to further evaluate its pore throat size distributions. From the results of this study, it was revealed that the CO2 injection rate is unlikely to affect the permeability significantly. It was also shown that ...
Abstract Pore-scale analysis of carbonate rock is of great relevance to the oil and gas industry ... more Abstract Pore-scale analysis of carbonate rock is of great relevance to the oil and gas industry owing to their vast application potentials. Although, efficient fluid flow at pore scale is often disrupted owing to the tight rock matrix and complex heterogeneity of limestone microstructures, factors such as porosity, permeability and effective stress greatly impact the rock microstructures; as such an understanding of the effect of these variables is vital for various natural and engineered processes. In this study, the Savonnieres limestone as a carbonate mineral was evaluated at micro scales using X-ray micro-computed tomography at high resolutions (3.43 μm and 1.25 μm voxel size) under different effective stress (0 MPa, 20 MPa) to ascertain limestone microstructure and gas permeability and porosity effect. The waterflooding (5 wt% NaCl) test was conducted using microCT in-situ scanning and nanoindentation test was also performed to evaluate microscale geomechanical heterogeneity of the rock. The nanoindentation test results showed that the nano/micro scale geomechanical properties are quite heterogeneous where the indentation modulus for the weak consolidated area was as low as 1 GPa. We observed that the fluid flow easily broke some less-consolidated areas (low indentation modulus) area, coupled with increase in porosity; and consistent with fines/particles migration and re-sedimentation were identified, although the effective stress showed only a minor effect on the rock microstructure.
Abstract Nanofluids, liquid suspensions of nanoparticles (Np), are an effective agent to alter th... more Abstract Nanofluids, liquid suspensions of nanoparticles (Np), are an effective agent to alter the wettability of oil-wet reservoirs to water-wet thus promoting hydrocarbon recovery. It can also have an application to more efficient carbon storage. We present a series of contact angle (θ) investigations on initially oil-wet calcite surfaces to quantify the performance of hydrophilic silica nanoparticles for wettability alteration. These tests are conducted at typical in-situ high pressure (CO 2 ), temperature and salinity conditions. A high pressure–temperature (P/T) optical cell with a regulated tilted surface was used to measure the advancing and receding contact angles at the desired conditions. The results showed that silica nanofluids can alter the wettability of oil-wet calcite to strongly water-wet at all operational conditions. Although limited desorption of silica nanoparticles occurred after exposure to high pressure (20 MPa), nanoparticle adsorption on the oil-wet calcite surface was mainly irreversible. The nanofluid concentration and immersion time played crucial roles in improving the efficiency of diluted nanofluids while salinity was less significant at high pressure and temperature. The findings provide new insights into the potential for nanofluids being applied for improved enhanced oil recovery and carbon sequestration and storage.
Geomechanical properties are of great importance in coal mining exploration, (enhanced) coal bed ... more 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 CO2 storage.
Low permeability of coal has been a constant obstacle to economic production from coalbed methane... more Low permeability of coal has been a constant obstacle to economic production from coalbed methane reservoirs, and liquid nitrogen (LN2) treatment has been investigated as one approach to address this issue. This study examined LN2 fracturing of a bituminous coal at pore-scale through 3D X-ray micro-computed tomography. For this purpose, a cylindrical sample was immersed into LN2 for 60 min. The micro-CT results clearly showed that the rapid freezing of the coal with LN2 generated fracture planes with large apertures originating from the pre-existing cleats in the rock. This treatment also connected original cleats with originally isolated pores and micro-cleats, thereby increasing pore network connectivity. Moreover, scanning electron microscopy highlighted the appearance of continuous wide conductive fractures with a maximum opening size of 9 µm. Furthermore, a nano-indentation technique was used to test the effect of LN2 on coal mechanical properties. The indentation moduli decrea...
Summary We studied the influence of water saturation on the elastic and anisotropy parameters of ... more Summary We studied the influence of water saturation on the elastic and anisotropy parameters of the Wellington shale in the range of the seismic frequencies from 0.1 Hz to 100 Hz at a confining pressure of 6 MPa. The measurements were conducted on three specimens retrieved at 0°, 45° and 90° angles with respect to the formation bedding. The specimens were saturated with water at four relative humidities of 12, 44, 72 and 97.5%. The measurements show a decrease in the components of the stiffness tensor with increasing saturation as well as a decrease of Thomsen’s parameters e and γ, whereas parameter δ remains virtually unchanged. We assume that the obtained results can be explained by the presence of a high percentage of smectite in the shale.
The petrophysical properties can be proper indicators to identify oil and gas reservoirs, since t... more The petrophysical properties can be proper indicators to identify oil and gas reservoirs, since the pore fluids have significant effects on the wave response. We have performed ultrasonic measurements on two sets of tight siltstones and dolomites at partial saturation. P- and S-wave velocities are obtained by the pulse transmission technique, while attenuation is calculated using the centroid-frequency shift and spectral-ratio methods. The fluid sensitivities of different properties (i.e., P- and S-wave velocities, impedances and attenuation, Poisson's ratio, density, and their combinations) are quantitatively analyzed by considering the data distribution, based on the crossplot technique. The result shows that the properties (P- to S-wave velocity and attenuation ratios, Poisson's ratio, and first to second Lamé constant ratio) with high fluid-sensitivity indicators successfully distinguish gas from oil and water, unlike oil from water. Moreover, siltstones and dolomites ca...
Summary Estimating the effects of pore filling material on the elastic moduli or velocities of po... more Summary Estimating the effects of pore filling material on the elastic moduli or velocities of porous and fractured rocks attracts widespread attention. This effect can be modeled by a recently proposed triple-porosity scheme, which quantifies this effect from parameters of the pressure dependency of the elastic properties of the dry rock. This scheme divides total porosity into three parts: compliant, intermediate, and stiff. Each type of pores is assumed to be spheroidal and characterized by a single aspect ratio. However, the implementation of this model requires the asymptotic values of the elastic moduli at much higher pressures where only non-closable pores remain open. Those pressures are beyond the capacity of most rock physics laboratories and can even crush typical sandstone samples. Experimental data at such pressures are usually unavailable. To address this issue, we introduce pore-scale numerical simulations in conjunction with effective medium theories (EMT) to compute...
SPE Asia Pacific Oil and Gas Conference and Exhibition
The gas content in shale reservoir is of great importance in reservoir evaluation. Shale reservoi... more The gas content in shale reservoir is of great importance in reservoir evaluation. Shale reservoir has various gas including free gas, adsorpted gas and soluted gas. Free gas take an important part for the total gas content. Hence, we investigated three equations for water saturation calculating and compared and improved them based on theoretical analysis in order to find a siutable one for the shale reservoir characterization. The results indicate that the Archie formula has several limitations applied to complex pore structure, which leads to high water saturation. Since the Archie formula was proposed by experimental data in pure sandstone without enough consideration about the clay of shale reservoir. The Waxman-Smits is suitable to shale gas reservoirs through theoretical analysis, but there are several uncertain parameters. The conductivity of formation water is necessary parameter in calculation of formation water saturation, but calculating the conductivity of formation wate...
Carbon dioxide (CO 2) injection into deep depleted hydrocarbon reservoirs or saline aquifers is c... more Carbon dioxide (CO 2) injection into deep depleted hydrocarbon reservoirs or saline aquifers is currently considered the best approach to large-scale CO 2 storage. Importantly, the pore structure and permeability of the storage rock are affected by fines release, migration, and reattachment in the initial stage of CO 2 injection, especially in unconsolidated sandstone reservoirs. It is thus necessary to better understand the pore structure changes and the associated permeability evolution during and after CO 2 injection. We thus imaged an unconsolidated sandstone at reservoir conditions before and after CO 2-saturated brine ("live brine") injection in situ via X-ray microcomputed tomography to explore the effects of fines migration and mineral dissolution induced by CO 2 injection. We found that in the examined sample, large pores dominated the total porosity, and porosity slightly increased after live-brine flooding. Moreover, and importantly, the pore structure changed significantly: large pores were further enlarged while small pores shrank or even disappeared. These structural changes in the tested sample were caused by mobilized fines due to the high-fluid interstitial velocity, which eventually reattached to the grains further downstream. Furthermore, the impact of the pore structural changes on permeability were analyzed in detail numerically. These permeability results are consistent with a fines migration mechanism where reattached fines block pore throats and thus decrease permeability drastically. We therefore can conclude that live brine injected into the examined unconsolidated sandstone will slightly improve storage space (porosity slightly increased); however, injectivity may be severely impaired by the permeability reduction.
There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yie... more There is an ongoing debate on whether laboratory ultrasonic measurements in anisotropic media yield phase or group velocities. The main problem here is that the size of the transducers used in the laboratory for ultrasonic measurements is comparable with the size of the rock sample or propagation distance. As a result, it is not clear how to interpret the measured traveltimes of ultrasonic waves. The main question is how to define an appropriate effective travel path, its length, and orientation. To answer this question, we look in detail at the full wavefront generated by a finite-size transducer using a specifically designed experimental setup, and a synthetic material (phenolic grade CE). We devise an algorithm for the interpretation of traveltimes measured in the laboratory with finite-size transducers arbitrarily oriented with respect to the symmetry planes of the material. To illustrate/validate the proposed algorithm, we designed a special testing program. The results of the ...
Abstract Porosity, fluid type and rock texture significantly affect acoustic wave propagation, si... more Abstract Porosity, fluid type and rock texture significantly affect acoustic wave propagation, since velocity dispersion and attenuation in fluid saturated rocks are mainly caused by wave-induced local fluid flow between microcracks and intergranular pores. We analyze P-wave velocity dispersion as a function of porosity to obtain information about the rock microstructure. The P-wave velocities in water-saturated rocks are predicted from measurements in gas-saturated rocks, using the Gassmann fluid substitution equation (the relaxed state). The dispersion is estimated from the difference between this predicted velocity and the measured one, where the latter corresponds to the unrelaxed state. We evaluate the wave dispersion as a function of porosity for 112 carbonates, 128 sandstones and 56 volcanic rocks, including our measurements for 86 tight rocks, showing that dispersion increases with porosity in the low porosity range, but decreases in the high porosity range. The dispersion peak occurs at a porosity of approximately 15%. Double-porosity poroelasticity modeling based on the local fluid-flow mechanism confirms this behavior. The microcrack radius has a peak in the porosity range 15–19% for all the lithologies from our collection, while the behavior of microcrack porosity is less evident. The dispersion peak may reveal the characteristics of lithological units, in particular porosity, fluid type and rock microstructure.
The effect of the amplitude of ultrasonic waves propagating through a sample is not often taken i... more The effect of the amplitude of ultrasonic waves propagating through a sample is not often taken into account in laboratory experiments. However, ultrasonic waves can produce relatively large strain inside the sample, and thus change the properties of the sample. To investigate the effect of strain amplitude on the P-wave velocity, a series of ultrasonic wave propagation experiments were carried out on three different media. All measurements were performed at 1 MHz central frequency and at the strain levels inside propagating waves of 3.0 × 10−6 to 6.0 × 10−5 without applying confining pressure to the sample. Strains in the waves were measured by a laser Doppler interferometer upon wave arrival on a free surface of the sample. The ultrasonic velocities were measured by a pair of P-wave transducers located at the same measuring point as the laser beam of the LDI. The effect of strain on P-wave velocity varied for different material. The P-wave velocity was calculated using both a first arrival and a first maximum peak at different applied voltage. The P-wave velocity remained unchanged for a pure elastic medium (aluminium); however, the velocity increased continuously with the increasing of the strain for polymethylmethacrylate and Gosford sandstone. For Gosford sandstone, velocity increases up to 0.8% with strain increase from 7.0 × 10−6 to 2.0 × 10−5. This effect of velocity increase with the strain induced by an ultrasonic wave can be explained by the in-elasticity of both the polymethylmethacrylate and Gosford sandstone samples.
Summary We present a methodology and describe a set-up that allows simultaneous acquisition of al... more Summary We present a methodology and describe a set-up that allows simultaneous acquisition of all five elastic coefficients of a transversely isotropic (TI) medium and its permeability in the direction parallel to the symmetry axis during mechanical compaction experiments. We apply the approach to synthetic shale samples and investigate the role of composition and applied stress on their elastic and transport properties. Compaction trends for the five elastic coefficients that fully characterize TI anisotropy of artificial shales are obtained for a porosity range from 40 per cent to 15 per cent. A linear increase of elastic coefficients with decreasing porosity is observed. The permeability acquired with the pressure-oscillation technique exhibits exponential decrease with decreasing porosity. Strong correlations are observed between an axial fluid permeability and seismic attributes, namely, VP/VS ratio and acoustic impedance, measured in the same direction. These correlations mig...
We use full wave forward and inversion modelling to estimate the elastic properties of rock sampl... more We use full wave forward and inversion modelling to estimate the elastic properties of rock samples from ultrasonic waveforms. The finite element algorithm (ABAQUS modelling software) is used to model a forward wave propagation within a homogeneous medium. For 19 mm diameter P-wave transducers, the result of the displacement waveform for a uniform source signal is obtained using both a linear and radial (about 2 mm) receiver arrays. Also, the use of a non-uniform source amplitude such as Gaussian distribution improves the displacement waveforms by few percent. The results accuracy is increased with increasing values of Gaussian standard deviation. However, for a nominal frequency of 1MHz, the same error increases with the decreasing frequencies. Additionally, our inversion algorithm (written in Python) searches for the best Young modulus (E) and Poison ratio (ν) of the medium iteratively. Finally, without prior knowledge of any threshold, the elastic parameters are estimated, and the results are consistent with the experimental measurements. These results provide a new modelling workflow to estimate the elastic parameters of the homogeneous and isotropic sample.
It is common to use ultrasonic techniques to measure elastic properties of the porous media. Howe... more It is common to use ultrasonic techniques to measure elastic properties of the porous media. However, conventional methods are unable to measure local strain in ultrasonic wave. This is not clear how the velocity of wave depends on its amplitude. In this work we, 1) measured the particle displacement in the ultrasonic wave using a Laser Doppler Interferometry (LDI) and 2) measured changes of Pwave velocities with wave amplitude for elastic (Aluminium), viscoelastic (Polymethylmethacrylate), and granular media (dried Gosford sandstone). We checked this phenomena using a conventional ultrasonic receiver and linked this changes to a local strain in wave. The study indicated that for a sandstone sample by increasing of local strain produced by an ultrasonic wave from 7*10-6 to 2*10-5 the P wave velocity increase by 0.7%. We also analysed the accuracy of velocity measured using LDI as a receiver and compare the results with that using conventional transducers. Moreover, the effect of proper couplant of the sample and the transducer was investigated in details.
Understanding of shales elastic properties behaviour with saturation changes is important for geo... more Understanding of shales elastic properties behaviour with saturation changes is important for geological storage of nuclear waste, CO2 sequestration as well as for development of conventional and unconventional shale oil and gas reservoirs. Existing data describing effects of saturation on elastic properties of shales are sparse and contradictory. To improve understanding of the effects of changing water content on elastic properties in shales, we conduct an experimental study on Opalinus shale samples. We measure vertical and horizontal ultrasonic P-and S-wave velocities of the same set of samples with controlled water content. The measured velocities are used to calculate components of elastic stiffness tensor in the shale at different saturations assuming its vertical transverse isotropy. Obtained results show increasing C11 and decreasing C33 with drying of the samples. Moreover, we observe 80% and 60% increase of shear moduli C44 and C66, respectively, with reduction of the water content from 5.5% weight in the preserved state to 0.3%. Conventional rock physics models are not designed to explain the observed dynamics. Here we perform a theoretical investigation of the influence on the shale moduli of following factors: (1) mechanical softening of the rock with the decrease of water saturation; (2) shrinkage of clay leading to reduction of porosity; (3) chemical hardening of clay particles; and (4) enhancing stiffness of contacts due to removing of water between clay particles.
Increasing energy demand and associated global warming are unarguably the two major challenges th... more Increasing energy demand and associated global warming are unarguably the two major challenges that the world currently faces. One of the ideas to reduce the carbon footprint while increasing the efficiency of the energy extraction is CO2 sequestration in coal seams. This can additionally enhance the coal-bed methane production. However, this process depends on many factors, among which coal wettability is of particular importance especially because of its pressure and temperature dependency. To evaluate this process, coal wettability was tested by measuring the contact angle of CO2 and water as a function of pressure, temperature, and salinity (DI water and brine (5 wt % NaCl + 1 wt % KCl), i.e., wt % is the weight percentage of salt to water. The results show that the CO2–water contact angle increases significantly, with increasing pressure, temperature, and salinity indicating more-effective CO2 wetness of coal. This, in turn, can reduce the CO2 residual trapping capacities and increase methane recover...
Permeability variation is one of the key factors influencing the injectivity of CO2 in CO2 seques... more Permeability variation is one of the key factors influencing the injectivity of CO2 in CO2 sequestration projects. Despite the research carried out on the subject, the results are highly inconsistent. In this study, the injection of brine (5 wt % NaCl + 1 wt % KCl), CO2-saturated (live) brine, and supercritical CO2 was performed on three homogeneous Berea sandstone plugs with a low clay content and two Bandera Gray sandstone plugs with a high clay content at reservoir conditions (10 MPa and 323 K). Porosity and permeability of the samples were measured using nuclear magnetic resonance (T2 relaxation time), and a dynamic (during flooding) permeability measurement technique, respectively, at different injection rates and injection durations. The mercury intrusion test was also performed on each sample to further evaluate its pore throat size distributions. From the results of this study, it was revealed that the CO2 injection rate is unlikely to affect the permeability significantly. It was also shown that ...
Abstract Pore-scale analysis of carbonate rock is of great relevance to the oil and gas industry ... more Abstract Pore-scale analysis of carbonate rock is of great relevance to the oil and gas industry owing to their vast application potentials. Although, efficient fluid flow at pore scale is often disrupted owing to the tight rock matrix and complex heterogeneity of limestone microstructures, factors such as porosity, permeability and effective stress greatly impact the rock microstructures; as such an understanding of the effect of these variables is vital for various natural and engineered processes. In this study, the Savonnieres limestone as a carbonate mineral was evaluated at micro scales using X-ray micro-computed tomography at high resolutions (3.43 μm and 1.25 μm voxel size) under different effective stress (0 MPa, 20 MPa) to ascertain limestone microstructure and gas permeability and porosity effect. The waterflooding (5 wt% NaCl) test was conducted using microCT in-situ scanning and nanoindentation test was also performed to evaluate microscale geomechanical heterogeneity of the rock. The nanoindentation test results showed that the nano/micro scale geomechanical properties are quite heterogeneous where the indentation modulus for the weak consolidated area was as low as 1 GPa. We observed that the fluid flow easily broke some less-consolidated areas (low indentation modulus) area, coupled with increase in porosity; and consistent with fines/particles migration and re-sedimentation were identified, although the effective stress showed only a minor effect on the rock microstructure.
Abstract Nanofluids, liquid suspensions of nanoparticles (Np), are an effective agent to alter th... more Abstract Nanofluids, liquid suspensions of nanoparticles (Np), are an effective agent to alter the wettability of oil-wet reservoirs to water-wet thus promoting hydrocarbon recovery. It can also have an application to more efficient carbon storage. We present a series of contact angle (θ) investigations on initially oil-wet calcite surfaces to quantify the performance of hydrophilic silica nanoparticles for wettability alteration. These tests are conducted at typical in-situ high pressure (CO 2 ), temperature and salinity conditions. A high pressure–temperature (P/T) optical cell with a regulated tilted surface was used to measure the advancing and receding contact angles at the desired conditions. The results showed that silica nanofluids can alter the wettability of oil-wet calcite to strongly water-wet at all operational conditions. Although limited desorption of silica nanoparticles occurred after exposure to high pressure (20 MPa), nanoparticle adsorption on the oil-wet calcite surface was mainly irreversible. The nanofluid concentration and immersion time played crucial roles in improving the efficiency of diluted nanofluids while salinity was less significant at high pressure and temperature. The findings provide new insights into the potential for nanofluids being applied for improved enhanced oil recovery and carbon sequestration and storage.
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Papers by Maxim Lebedev