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Quantitative analysis of collocated seismic velocity tomography and electromagnetic experiments is developed to elucidate the structure of the deep extension of the Nagamachi-Rifu fault, northeastern Japan. P and S wave seismic velocities obtained from a dense seismic network are examined and a ratio of spatial variation in P and S wave velocities d ln Vs/d ln Vp is chosen as a proxy for the influence of pore geometry. The analysis shows that the deep extension of the Nagamachi-Rifu fault reveals the d ln Vs/d ln Vp values exceeded 1.1. Such large values of d ln Vs/d ln Vp cannot correspond to equilibrium pore geometry, at which the interfacial energy is at a minimum, and indicate regions with non-equilibrium state where non-isotropic stress prevents the equilibrium pore geometry to be achieved. To specify a fine distribution of porosity and connectivity of micropore in the region, we carry out the joint analysis of the seismic velocities with the electrical resistivity data obtained by the magnetotelluric survey crossing the Nagamachi-Rifu fault. It is shown that the region at 10–17 km depths at about 20–40 km to the northwest from the hypocenter of the M5.0 earthquake occurred in 1998 exposes the highest connectivity among the adjacent areas, suggesting a strong deformation process.

A prototype of a geoscience information system was designed for the treatment of seismological data obtained by an integrated system of surface and borehole arrays at the vicinity of the Nojima fault, responsible to the Kobe earthquake in 1995 (Hyogo-ken Nanbu earthquake M7.2). The information system includes a database (DB) as the core of the system, provided by the graphic user interface (GUI) and allow visualization and treatment of spatial objects and seismic waveforms. Information about earthquake hypocenters and measuring techniques are stored in the relational database, which allows quick selection of the data respond to certain criteria. It is demonstrated that the SEISMO database system development reduces the time required for time-consuming phases of selection, treatment and visualization of the data in a scientific process.

Summary We have studied changes of static and dynamic elastic properties of the organic-rich Mancos shale caused by heating to the temperature within the range 380 °C-550°C. An increase of ultrasonic velocities and static Young’s moduli has been observed in the samples that experienced temperatures up to ~400°C. Further heating to temperatures up to 550°C has led to the decrease of elastic moduli. The samples subjected to temperatures above 430°C show an increase in stress sensitivity, which implies microcrack generation. Micro-CT images of small samples have been obtained to inspect the microstructural changes caused by this dry pyrolysis. Comparison of the registered micro-CT images of Mancos shale samples subjected to temperatures up to 470°C shows significant shrinkage of the kerogen that fills pore spaces resulted from bitumen expulsion into the pores.

Accurate overpressure detection and prediction is essential to the scientific understanding of the history and safe engineering exploitation of a sedimentary basin. We predict the velocity of shales using the Clay-Plus-Silt (CPS) model from logging measured parameters, the modeled velocity is then used as normal velocity trend to detect and predict overpressure using Eaton equation. The results show that the CPS model predicts accurate velocity in normal-pressure shales, and the modeled shale velocity can be used to detect and predict overpressure more accurately than the established normal velocity trend in overpressured wells.

Organic-rich shales, traditionally considered as source rocks, have recently become an ambitious goal for oil and gas industry as important unconventional reservoirs. Understanding of initiation and development of fractures in organic-rich shales is crucially important as they drastically increase permeability of these low permeable shales. Fracturing can be induced by rapid decomposition of organic matter caused by either natural heating, such as emplacement of magmatic bodies into sedimentary basins or thermal methods used for enhanced oil recovery. In this study we integrate laboratory experiment and numerical modeling to study fracture development in organic-rich shale. At the first step, we heat a cylindrical sample up to the temperature of 330 degrees Celsius. At the second step, we obtain high resolution microtomographic images of the sample. Large kerogen-filled pores and cracks initiated by the heating can be identified from these images. We repeat these steps for several temperatures in the range 330430 degrees. The microtomographic images are processed using AVIZO (Visualization Sciences Group) to estimate the dependency between the total area of fractures and the temperature experienced by the sample. Total organic carbon content is tested in the samples experienced the same temperatures. This approach enables a quantitative analysis of fracture initiation and development in organic-rich shales during heating.

Evaluation of the various rock properties of shales has become more prevalent in recent years although our understanding of these properties and the links between them is still relatively embryonic. While thick shale sequences can form sealing units above hydrocarbon traps, intra-reservoir shales can form baffles to flow in both petroleum and groundwater contexts. High field and low field nuclear magnetic resonance were used to evaluate wettability of shales. Preserved shales show mineral dependent variations in surface affinity for oil versus water. Hydrophilic shales have a higher cation exchange capacity (e.g. shales rich in illitic and/or smectite), whereas kaolinitic mudrocks are potentially hydrophobic and can be wetted preferentially by oil, sometimes retaining oil on the mineral surfaces after further exposure to brines. Porosity and cation exchange capacity correlate well with strength properties and dielectric constant measurements on intact shales and pastes made from powdered shales show strong relationships between high frequency electrical properties, mineralogy, cation exchange capacity and mechanical strength. Calibrating wireline logs with laboratory measurements and the development of physics-based models allows the prediction of rock properties and extrapolation to the borehole scale.

Summary Quantitative knowledge of the acoustic response of rock from an injection site on supercritical CO2 saturation is crucial for understanding the feasibility of time-lapse seismic monitoring of CO2 plum migration. A suite of shaley sandstones from the CRC-2 well, Otway Basin, Australia is tested to reveal the effects of supercritical CO2 injection on acoustic responses. CO2 is first injected into dry samples, flushed out with brine and then injected again into brine saturated samples. Such experimental protocol allows us to obtain acoustic velocities of the samples for the wide range of CO2 saturations from 0 to 100%. On injection of supercritical CO2 (scCO2) into brine-saturated samples, they exhibit observable perturbation of ~7% of compressional velocities with the increase of CO2 saturation form 0% to maximum (~50%). Changes of the dry samples before and after the CO2 injection (if any) are not traceable by acoustic methods.

Summary Elastic wave velocities in rocks vary with stress due to the presence of discontinuities and microcracks within the rock. In this paper, we analytically derive a model for seismic anisotropy caused by small triaxial stresses. We first consider a linearly isotropic elastic medium permeated by a distribution of cracks with random orientations. The geometry of cracks is not specified; instead, their behavior is defined by a ratio of their normal to tangential excess compliances. When this isotropic rock is subjected to small triaxial stresses, crack closure occurs perpendicular to the applied stresses. This effect is modeled using Sayers and Kachanaov (1995) non-interactive approximation. The model predicts ellipsoidal anisotropy and also expresses the ratios of Thomsen’s parameters ⁄ as a function of the compliance and Poisson’s ratios in the three orthogonal planes of symmetry. A reasonable agreement was obtained when comparing the model predictions to laboratory measurements for small stresses (up to 20 MPa). These results could be used to differentiate stress-induced anisotropy from that caused by aligned fractures. Besides, if the cause of anisotropy is known, then this model could enable one to fully estimate the elasticity tensor from log

The main causes of azimuthal anisotropy in hydrocarbon reservoirs are anisotropy of the horizontal stresses and presence of vertical or dipping fractures. The aim of this paper is to analyse and compare anisotropy patterns associated with these two causes of anisotropy. Stresses affect elastic properties of rocks due to presence of discontinuities such as cracks and compliant grain contacts.

Understanding seismic anisotropy in shales is important for quantitative interpretation of seismic data, 4D monitoring and pore pressure prediction. Along with intrinsic anisotropy caused by preferred mineral orientation that is common in shales, anisotropic stress is an important factor that affects shale elastic response. While variations of elastic coefficients with anisotropic stress have been the subject of experimental studies, theoretical insight is still largely lacking. Here we suggest a new model that allows parameterization of the stress dependency of elastic coefficients of shales under anisotropic stress conditions. We show that the parameterization requires four parameters, namely, specific tangential compliance of a single crack, the ratio of normal to tangential compliances, characteristic pressure and a crack orientation anisotropy parameter. These parameters can be estimated from experimentally measured stress sensitivity of elastic coefficients in shales to isotropic stress.

SUMMARY Exponential increase of seismic velocities with effective stress has usually been explained by presence of pores with a broad distribution of aspect ratios. More recently, a stress-related closure of soft pores with a narrow distribution of compliances (e.g. grain contacts) has been suggested to be sufficient to explain such exponential stress dependency. This theoretical interpretation has been verified here using laboratory measurements on dry sandstones. On the basis of these experimental data, linear dependency of elastic compressibility on soft porosity and exponential decay of soft porosity and elastic compressibility with effective stress up to 60 MPa is confirmed. Soft porosity, estimated from the fitting coefficients of elastic compressibilities, is on the same order of magnitude but slightly lower than obtained from strain measurements. The results confirm applicability of the previously proposed stress sensitivity model and provide justification for using this approach to model stress dependency of elastic properties.

Summary The technique of fluid substitution is widely used to model elastic properties of rocks saturated with different fluids The applicability of this technique to in-situ seismic and sonic measurements is a matter of frequent debate. Most of the analysis is based on laboratory measurements, with little or no constraints from field environments. In addition, until recently, most of the data were from sand reservoirs. Applicability of Gassmann fluid substitution to carbonates is even more uncertain. To analyze this problem, we compare elastic moduli obtained using fluid substitution against the moduli obtained from sonic and density logs The dry moduli for fluid substitution are obtained from ultrasonic measurements on 50 core samples from a cretaceous reservoir buried at 5000 meters depth in Santos Basin, offshore Brazil. The good agreement between the saturated moduli obtained from cores and logs is obtained. This shows that the Gassmann equations can be applied not only in siliciclastic reservoirs, but also deep and complex carbonates reservoirs.

One of the main causes of azimuthal anisotropy in sedimentary rocks is anisotropy of tectonic stresses in the earth’s crust. In this paper we analytically derive the pattern of seismic anisotropy caused by application of a small anisotropic stress. We first consider an isotropic elastic medium (porous or non-porous) permeated by a distribution of discontinuities with random (isotropic) orientation (such as randomly oriented compliant grain contacts or cracks). Geometry of individual discontinuities is not specified. Instead, their behaviour is defined by a ratio B of the normal to tangential excess compliances.

Understanding elastic properties of fluid saturated carbonates is a major challenge in quantitative interpretation of seismic data. In particular, the validity of Gassmann’s equations for carbonates is sometimes unclear. This complexity is belived to be caused by diversity of their pore geometries. One of the ways to study the impact of pore geometry is by measuring ultrasonic velocities. In this paper we compare the measured velocity as a function of pressure against a recently developed simple squirt flow model. This allows us to estimate effective aspect ratio of compliant pores. The resulting value of about 0.01 appears to be common for a number of samples examined.

Accurate numerical modeling tools are of practical importance for understanding of seismic wave propagation in reservoir rocks. 2D finite-difference (FD) poroelastic code is used for simulation of acoustic wave propagation through water saturated medium w

We develop a new simple model of squirt-flow model attenuation and dispersion, in which most parameters can be independently measured or estimated from measurements. The pore space of the rock is assumed to consist of stiff porosity and compliant (or soft

The exponential increase of seismic velocities with effective stress has usually been explained by the presence of pores with a broad distribution of aspect ratios. More recently, a stress-related closure of soft pores with a narrow distribution of compliances (e.g. grain contacts) has been suggested to be sufficient to explain such exponential stress dependency. This theoretical interpretation has been verified here using laboratory measurements on dry sandstones. On the basis of these experimental data, linear dependency of elastic compressibility on soft porosity and exponential decay of soft porosity and elastic compressibility with effective stress up to 60 MPa is confirmed. Soft porosity, estimated from the fitting coefficients of elastic compressibilities, is on the same order of magnitude but slightly lower than obtained from strain measurements. The results confirm applicability ofpreviously proposed stress sensitivity models and provide justification for using this approach to model stress dependency of elastic properties for isotropic and anisotropic rocks.

Ultrasonic velocities and fluid saturations are measured simultaneously during water injection into sandstone core samples. The experimental results obtained on lowpermeability samples show that at low saturation values the velocity-saturation dependence can be described by the Gassmann-Wood relationship. However, with increasing saturation a sharp increase of P-wave velocity is observed, eventually approaching the Gassmann-Hill relationship. We relate this transition behavior to the change of the fluid distribution characteristics inferred from CT scans. In particular, we show that for relatively large fluid injection rate this transition occurs at smaller degrees of saturation as compared with high injection rate.

Shales comprise a large proportion of the sedimentary pile in many hydrocarbon basins and impact significantly on exploration, development and production costs through the effect of seismic anisotropy on imaging and depth conversion. Shales play an important role in time-lapse seismic response and pore pressure prediction. To understand stress dependent anisotropy on shales, stress dependencies of the five elastic constants are measured for four sets of shales. These new measurements are used to test the method that calculates the shale elastic constants using the silt fraction, clay packing density and set of elastic constants derived on the basis of nano-indentation technique. The results of our analysis show considerable variability of clay properties for a given clay packing density. We also relate the variation of clay elastic constants with the amount of very small and compliant pores (soft porosity). For the first time, soft porosity of shales is estimated from the experimental data. The soft porosity correlates with stress dependent variations of elastic moduli. This suggests that stress dependency of shale properties results from the closing of the soft porosity. Variations of the compressibility depend on exponential functions of the effective confining stress and the soft porosity can be estimated from the fitting coefficients. The stress dependencies of the measured elastic compliances of wet shales and of the dry compressibilities estimated using Gassmann equation are approximated by exponential functions using a nonlinear fitting based on the Levenberg–Marquardt algorithm. The soft porosities estimated from the fitting coefficients for dry compressibilities are shown to be in a reasonable agreement with the measured values.

This special section is dedicated to seismic anisotropy, which is playing an increasingly more important role in our understanding and modelling of the subsurface. Most papers published in this section were presented at the 14th International Workshop on Seismic Anisotropy (IWSA), which was held on 12–16 April 2010 in Hillarys, Perth, Western Australia. The main organizers of the workshop were Andrej Bona and Boris Gurevich from Curtin University and Greg Ball from Chevron. This workshop continued the tradition of biennial meetings of anisotropists, which began in 1984. The workshop was dedicated to the memory of Mike Schoenberg, one of the most prominent and inspirational members of the anisotropic community, who died of cancer in 2008 shortly after attending the 13th IWSA in Colorado, U. S. A. Celebration of Mike's contribution to the geophysical community was particularly fitting for a workshop held in Perth, where Mike gave a lasting inspiration to colleagues and students during a number of his sabbatical visits to Curtin University, CSIRO, and University of Western Australia. The 14th IWSA was generously sponsored by CGGVeritas, Woodside Energy, WesternGeco, Curtin University, and Santos Energy. The sponsorship helped reduce the cost for the participants and offered travel grants for overseas students who otherwise would not have been able to attend the workshop. Seed funding from the organizers of the previous IWSA greatly eased the planning of the workshop; similar seed funding will be forwarded to organizers of a future IWSA. There were 71 overseas and local participants from academia, research organizations, and industry, 13 student participants and 31 day participants. The program consisted of 42 oral presentations, seven posters and concluded with a short course “Seismic …

A data-constrained modelling (DCM) approach has been developed at CSIRO, which enables 3D characterisation of pores and mineral phase distributions using quantitative multi-energy synchrotron CT. For a tight reservoir, such as a carbonate limestone or a shale rock, DCM can generate microscopic partial volume distributions of materials and pores which are the effects of the fine length scales below X-ray CT resolution. Using this information, a quantitative relation between recoverable reserve and pore-throat size can be established for a rock sample. The technique can also be used for characterisation of other unconventional reservoir rocks.

Organic-rich shales, traditionally considered as source rocks, have recently become an ambitious goal for the oil and gas industry as important unconventional reservoirs. Understanding of the initiation and development of fractures in organic-rich shales is crucially important as fractures could drastically increase the permeability of these otherwise low-permeable rocks. Fracturing can be induced by rapid decomposition of organic matter caused by either natural heating, such as emplacement of magmatic bodies into sedimentary basins, or thermal methods used for enhanced oil recovery. In this work the authors study fracture initiation and development caused by dry pyrolysis of Kimmeridge shale, which is characterised with a high total organic carbon content of more than 20%. X-ray diffraction (XRD) analysis exhibits high carbonate (both calcite and dolomite) and low clay (illite) content. Field emission gun scanning electron microscopy (FEG-SEM) shows that kerogen is presented either as a load-bearing matrix or as a filling of the primary porosity with pores being of micron size. Cylindrical samples of the Kimmeridge shale are heated up to temperatures in the range of 330–430°C. High-resolution X-ray microtomographic (micro-CT) images are obtained. The microtomographic images are processed using AVIZO (Visualization Sciences Group) to identify and statistically characterise large kerogen-filled pores and pre-existing and initiated cracks. The relationship between the total area of fractures and the temperature experienced by the sample has been obtained. Total organic carbon content is determined for samples subjected to heating experiments. This approach enables a quantitative analysis of fracture initiation and development in organic-rich shales during heating.

Microstructures are critical for defining material characteristics such as permeability, mechanical, electrical and other physical properties. However, the available techniques for determining compositional microstructures through segmentation of x-ray computed tomography (CT) images are inadequate when there are finer structures than the CT spatial resolution, i.e. when there is more than one material in each voxel. This is the case for CT imaging of geomaterials characterized with submicron porosity and clay coating that control petrophysical properties of rock. This note outlines our data-constrained modelling (DCM) approach for prediction of compositional microstructures, and our investigation of the feasibility of determining sandstone microstructures using multiple CT data sets with different x-ray beam energies. In the DCM approach, each voxel is assumed to contain a mixture of multiple materials, optionally including voids. Our preliminary comparisons using model samples indicate that the DCM-predicted compositional microstructure is consistent with the known original microstructure under low noise conditions. The approach is quite generic and is applicable to predictions of microstructure of various materials.

Maximising the recovery of known hydrocarbon reserves is one of the biggest challenges facing the petroleum industry today. Optimal production strategies require accurate monitoring of production-induced changes of reservoir saturation and pressure over the life of the field. Time-lapse seismic technology is increasingly used to map these changes in space and time. However until now, interpretation of time-lapse seismic data has been mostly qualitative. In order to allow accurate estimation of the saturation, it is necessary to know the quantitative relationship between fluid saturation and seismic characteristics (elastic moduli, velocity dispersion and attenuation). The problem of calculating acoustic properties of rocks saturated with a mixture of two fluids has attracted considerable interest (Gist, 1994; Mavko and Nolen-Hoeksema, 1994; Knight et al. 1998); for a comprehensive review of theoretical and experimental studies of the patchy saturation problem see Toms et al. (2006).

Anisotropic differential effective medium approach is used to simulate elastic properties of shales from elastic properties and volume fractions of clay and silt constituents. Anisotropic elastic coefficients of the wet clay pack are assumed to be independent of mineralogy and to be linearly dependent on clay packing density (CPD), a fraction of clay in an individual wet clay pack. Simulated compressional and shear velocities normal to the bedding plane and are shown to be in a good agreement with measured sonic velocities. Further, elastic coefficients of shales, and , calculated from the log sonic velocities, calibrated porosity and clay fraction obtained from the mineralogy tool are used to invert for elastic constants of clays, C33 and C44. The obtained elastic coefficients of clays show lower scatter than the original elastic coefficients of shales. The noticeable increase of the clay elastic coefficients with the depth increase is shown to result from the positive trend of the CPD with depth. Being interpolated to the same CPD = 0.8, elastic coefficients of clays show no depth dependency. Our findings show that the CPD and silt fraction are the key parameters that can be used for successful modelling of elastic properties of shales.

Stress dependency of full elastic tensor of shales is very important for seismic interpretation, overpressure prediction, 4D monitoring, etc. Using Sayers-Kachanov formalism, we develop a new stress sensitivity model for transversely isotropic (TI) media which predicts stress sensitivity behaviour of all five elastic constants. The model is used to parameterize elastic properties of about 20 shales obtained by laboratory measurements and from literature survey. The four fitting parameters (namely, tangential and normal compliances of a single crack, characteristic pressure, and pore orientation anisotropy parameter) show obvious correlations with the depth from which the shale was extracted. With increase of the depth, the tangential compliance decreases exponentially and the ratio of normal to tangential compliance increases linearly. The crack orientation anisotropy parameter exponentially increases with the depth for the most of the shales indicating that cracks are getting more aligned in the bedding plane. The characteristic pressure shows no visible correlation with the depth. The suggested model allows predicting of stress dependency of all five elastic constants if only two of them are known what can be useful, for instance, for the reconstruction of all five elastic constants of shale from log data.

We have studied changes of static and dynamic elastic properties of the organic-rich Mancos shale caused by heating to the temperature within the range 380 deg.C-550 deg.C. An increase of ultrasonic velocities and static Young’s moduli has been observed in the samples that experienced temperatures up to ~400 deg.C. Further heating to temperatures up to 550 deg.C has led to the decrease of elastic moduli. The samples subjected to temperatures above 430 deg.C show an increase in stress sensitivity, which implies microcrack generation. Micro-CT images of small samples have been obtained to inspect the microstructural changes caused by this dry pyrolysis. Comparison of the registered micro-CT images of Mancos shale samples subjected to temperatures up to 470 deg.C shows significant shrinkage of the kerogen that fills pore spaces resulted from bitumen expulsion into the pores.

Understanding seismic anisotropy in shales is important for quantitative interpretation of seismic data, 4D monitoring and pore pressure prediction. Along with intrinsic anisotropy caused by preferred mineral orientation that is common in shales, anisotropic stress is an important factor that affects shale elastic response. The effect of stress on elastic properties of shales is also important for understanding of depositional trends especially at the upper 2000-3000 meters where the compaction is mostly mechanical. Despite the importance of the effects of isotropic and especially anisotropic stress on elastic properties of shales, little work has been done on theoretical understanding and predicting such properties.

Elastic anisotropy is important for a number of geophysical applications, including seismic interpretation, fluid identification and 4D seismic monitoring. Due to their strongly anisotropic properties, the presence of shales in the subsurface may cause significant errors in depth obtained from surface seismic data, in normal and dip moveout correction, migration and amplitude versus offset analysis. Here we analyze Thomsen’s anisotropy parameters and anellipticities of 37 shales with different mineralogical compositions, maximum overburden stress experienced, silt and clay fractions, porosities and other petrophysical parameters. The shales extracted from depths below 4000 m exhibit the largest values of P- and S-wave anisotropies, while negative delta values are mostly observed for shales extracted from shallow depths. The observed diversity of Thomsen's anisotropy parameters and anellipticities can be explained with (1) intrinsic anisotropy of wet clay packs, (2) governed by compaction anisotropic distribution of discontinuities and (3) effects of aligned silt inclusions on shale elastic properties.

Understanding of physical rock properties is currently of great importance both for industry and fundamental science, for it allows improving interpretation and reducing risks. Digital rock physics provides us with a promising opportunity for rock analysis and quantification. Moreover it allows running simulations on rock samples unsuitable for laboratory experiments. A detailed computational rock physics workflow including 3D rock imaging, processing and simulations of physical experiments has been created and tested on different rock samples. Here we describe this workflow and demonstrate the results of elastic simulation in comparison with experimental data obtained in physical laboratory for a sandstone sample.

Summary We introduce an algorithm for simultaneous facies classification and fitting of rock physics models from multivariate well log data. Special features of the methodology are designed to render it resilient to data outliers. The algorithm is a robustified and globalized variety of the expectation-maximization algorithm, using reweighted robust nonlinear regression steps for the maximisation step, and heavy-tailed distributional models for the expectation step. Facies classifications are natural byproducts of the expectation step, and optimised rock physics models are produced by the maximisation step. The practical advantages of the approach are illustrated using data from the Satyr-5 well, located in the Northern Carnarvon Basin, North West Shelf of Australia. Outputs of the algorithm include facies labels and free parameters in the corresponding rock-physics models, which can be easily interpreted and directly used in downstream workflows such as facies-based seismic inversion.

Petrophysical properties of carbonate reservoirs are less predictable than the properties of silisiclastic reservoirs. The main reason for that is chemical interaction of carbonate rocks with percolating fluids, ion exchange and recrystallization in geological time. Quantification of the elastic variability of carbonate grains on the microscale is the first step to constrain models and to obtain more realistic predictions of practically important rock properties of carbonate reservoirs. In this study we present elastic moduli of an oolite sample from the Pleistocene Dampier Formation of Southern Carnarvon Basin, Western Australia obtained by the nanoindentation technique. Young moduli of this highly heterogeneous sample are measured at 49 points regularly distributed in a 70x70micrometer rectangular grid on the surface. The frequency diagram shows bimodal distribution of the Young moduli that correspond to dense calcite phase and rare (dissolved) calcite phase. These two solid phases are apparent in the high resolution scanning electron microscope images. We used the obtained moduli of the dense and rare phases for numerical modelling of elastic properties of the carbonate sample from micro-CT images. The results of the numerical modelling using finite element code are compared with the elastic moduli obtained from acoustic velocities measured by ultrasonic technique.

P- and S- velocities of clean quartz Lochaline sandstone are numerically simulated using microtomographic images with resolution of 2 micron and compared with the velocities measured at ultrasonic frequencies. The numerically simulated velocities are in a good agreement with velocities measured at confining pressure of about 30-40 MPa which is high enough to close soft pores but do not cause noticeable deformation of equant pores yet. The obtained results shows that while effects of soft porosity on elastic properties of sandstones cannot be directly simulated from microtomograms, such simulation give reliable results when soft pores are closed.

Nowadays the carbonate rocks are attracting growing interest as a possible site for CO2 geo-sequestration. Monitoring of the injected carbon dioxide is generally an important part of any sequestration project. CO2 is known to change the properties of the hosting rocks and to affect the reservoir characteristics - elastic moduli, reservoir capacity, etc. Thus, predicting changes in microstructures and elastic properties of carbonate rocks after the CO2 injection is of great importance. In the current work we study the effects of sc-CO2 injection on microstructure and elastic properties of carbonate rock. We use microtomographic images of two Savonniere limestone samples, one in its natural state and one after injecting sc-CO2. A statistical analysis exhibits that the injection of sc-CO2 has led to the increase in porosity and changes of the microstructure of the carbonate sample. It has resulted in the increase of the average volume of individual pores and the decrease of the total number of pores. The CO2 injection has increased the mean radii of pore throats, has raised length of pore network segments and makes orientation distribution of meso-pores more isotropic. These changes in microstructure have been reflected in lower numerically simulated elastic moduli of the sample.