Bedding Anisotropy and Effective Stress Law for the Permeability and Deformation of Clayey Sandstones (original) (raw)
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Effective Stress Law for the Permeability and Pore Volume Change of Clayey Sandstones
Journal of Geophysical Research: Solid Earth, 2020
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Effective stress law for the permeability of clay-rich sandstones
Journal of Geophysical Research, 2004
1] Two models of clay-rich sandstones are analyzed to explain the relative sensitivity of permeability to pore pressure and confining pressure. In one model the clay lines the entire pore wall in a layer of uniform thickness, and in the second model the clay is distributed in the form of particles that are only weakly coupled to the pore walls. Equations of elasticity and fluid flow are solved for both models, giving expressions for the effective stress coefficients in terms of clay content and the elastic moduli of the rock and clay. Both models predict that the permeability will be much more sensitive to changes in pore pressure than to changes in confining pressure. The clay particle model gives somewhat better agreement with data from the literature and with new data on a Stainton sandstone having a solid volume fraction of 8% clay.
The influence of hydrostatic and uniaxial stress states on the porosity and permeability of sandstones has been investigated. The experimental procedure uses a special triaxial cell which allows permeability measurements in the axial and radial directions. The core sleeve is equipped with two pressure samplers placed distant from the ends. They provide mid-length axial permeability measure as opposed to the overall permeability measure, which is based on the flow imposed through the pistons of the triaxial cell. The core sleeve is also equipped to perform flows in two directions transverse to the axis of the sample. Two independent measures of axial and complementary radial permeability are thus obtained. Both Fontainebleau sandstone specimens with a porosity of about 5.8 to 8% and low permeability ranging from 2.5 to 30mD and Bentheimer sandstone with a porosity of 24% and a high permeability of 3D have been tested. The initial axial permeability values obtained by each method are in good agreement for the Fontainebleau sandstone. The Bentheimer sandstone samples present an axial mid-length permeability 1.6 times higher than the overall permeability. Similar discrepancy is also observed in the radial direction, also it relates essentially to the shape of flow lines induced by the radial flow. All the tested samples have shown a higher stress dependency of overall and radial permeability than mid-length permeability. The effect of compaction damage at the pistons/sample and radial injectors/sample interfaces are discussed. The relevance of directional permeability measurements during continuous uniaxial compression loadings have been shown on the Bentheimer sandstone until the failure of the sample. We can efficiently measure the influence of brittle failure associated to dilatant regime on the permeability: it tends to increase in the failure propagation direction and to decrease strongly in the transverse direction.
Pure and Applied Geophysics, 2009
This study present the result of conventional triaxial tests conducted on samples of Rothbach sandstone cored parallel, oblique (at 45 degrees) and perpendicular to the bedding at effective pressures ranging from 5 to 250 MPa. Mechanical and microstructural data were used to determine the role of the bedding on mechanical strength and failure mode. We find that samples cored at 45 degrees to the bedding yield at intermediate level of differential stress between the ones for parallel and perpendicular samples at all effective pressures. Strain localization at high confining pressure (i.e., in the compactive domain) is observed in samples perpendicular and oblique to the bedding but not in samples cored parallel to the bedding. However, porosity reduction is comparable whether compactive shear bands, compaction bands or homogeneous cataclastic flow develop. Microstructural data suggest that (1) mechanical anisotropy is controlled by a preferred intergranular contact alignment parallel to the bedding and that (2) localization of compaction is controlled by bedding laminations and grain scale heterogeneity, which both prevent the development of well localized compaction features.
International Journal of Rock Mechanics and Mining Sciences, 2018
This study focuses on the poroelastic behaviour and permeability of tight sandstones, which are characterized by a low porosity, a low gas permeability and a strong sensitivity to in-situ stress. Experimentally the Biot coefficient takes a value close to 1 at low confinement and decreases with increasing confining pressure; the permeability shows an important reduction with increasing confining pressure. This behaviour can be attributed to pore entrapment and to the closure of cracks and joints. Micromechanical models, considering low permeable sandstones as made up of an assemblage of grains with interfaces and pores, reproduce well the observed trends of Biot coefficient and permeability.
Permeability evolution during progressive development of deformation bands in porous sandstones
2003
Triaxial deformation experiments were carried out on large (0.1 m) diameter cores of a porous sandstone in order to investigate the evolution of bulk sample permeability as a function of axial strain and effective confining pressure. The log permeability of each sample evolved via three stages: (1) a linear decrease prior to sample failure associated with poroelastic compaction, (2) a transient increase associated with dynamic stress drop, and (3) a systematic quasi-static decrease associated with progressive formation of new deformation bands with increasing inelastic axial strain. A quantitative model for permeability evolution with increasing inelastic axial strain is used to analyze the permeability data in the postfailure stage. The model explicitly accounts for the observed fault zone geometry, allowing the permeability of individual deformation bands to be estimated from measured bulk parameters. In a test of the model for Clashach sandstone, the parameters vary systematically with confining pressure and define a simple constitutive rule for bulk permeability of the sample as a function of inelastic axial strain and effective confining pressure. The parameters may thus be useful in predicting fault permeability and sealing potential as a function of burial depth and fault displacement.
Greenhouse Gases: Science and Technology, 2016
In this study, the effective stress law for the permeability of two core plugs selected from Berea (Cleveland Quarries, OH, USA) and Knorringfjellet (Longyearbyen, Svalbard, Norway) sandstones is studied experimentally by measuring the core permeability (k) under varying confining stress (σ c) and pore pressures (P p). The obtained results demonstrate that the permeabilities of the two core plugs decrease with increasing σ c or decreasing P p. The effective stress coefficient for the permeability (α k) values are more than 1.0 for both sandstone core plugs indicating higher sensitivity of the permeability with respect to the applied P p compared to the applied σ c. The previously presented models for calculating α k , such as the Clay Free, Clay Shell, and Clay Particle models, are discussed and a new modified Clay Shell model considering spherical geometry is presented to account for the considerable contrast between the elastic moduli of quartz and clay minerals. The discussed models strongly depend on the magnitude of the considered elastic moduli for the clay minerals. While the Clay Shell and Clay Particle models are capable of describing the observed α k values by considering extremely low elastic moduli for clays, the new modified Clay Shell model is capable of predicting α k values by considering moderate to low values of elastic moduli of clays. The increasing trend of α k values by increasing the σ c is discussed and a new correlation based on the observed k values for calculation of α k is presented.
Journal of Applied Geophysics, 2009
The dependence of permeability and complex electrical resistivity on direction was measured for lowpermeable sandstone samples from a tight gas reservoir. Both properties were measured in three directions at hydrostatic pressures up to 100 MPa. The decrease of permeability as a function of effective pressure (measured with a modified pressuretransient method) can be described by a power function. The pressure dependence is more controlled by the closure of thin aspect ratio pores and cracks than by the minor reduction of porosity. The anisotropy of permeability is also a function of pressure. For some samples the preferred direction of flow changes with increasing pressure. The Cole-Cole response-function can be fitted well to the complex resistivity spectra (kHz-MHz). Interfacial polarization is the dominant polarization effect in this frequency range. The relaxation time of the Cole-Cole model increases with increasing effective pressure, whereas the frequency exponent does not show any continuous behavior. According to the model of Lysne (1983) the geometrical distribution of pore shapes and their orientation can be derived from these quantities. The formation resistivity factor, taken from the real part of resistivity at 10 kHz, also increases with pressure. As the porosity does not change significantly, this increase means an increase of Archie's cementation exponent. Both, relaxation time and formation factor are also a function of the considered direction. But an overall relationship between these quantities and permeability could not be observed; neither for absolute values nor for their anisotropy.
Loading rate dependence of permeability evolution in porous aeolian sandstones
Journal of Geophysical Research, 2004
1] Mechanical properties of rocks are characterized by their notable dependence on the applied deformation rate. However, little is known about the strain rate dependence of fluid flow properties since most laboratory tests are conducted using a single, high strain rate. We have investigated the effect of loading rate on the permeability of porous sandstones by carrying out triaxial compression tests at four different temperatures and strain rates with continuous monitoring of permeability, acoustic emission (AE), and pore fluid chemistry. All tests are characterized by an initial permeability decrease due to inferred compaction of favorably oriented cracks. The amount of initial permeability reduction increases with decreasing strain rate, thus implying a more efficient initial compaction at slower strain rates. At a later stage of loading, permeability correlates with stress, ion concentration, or AE damage depending on the strain rate used. High strain rate tests are characterized by a positive power law or logarithmic correlation between permeability and AE damage. At slow strain rates, permeabilities decrease exponentially with mean effective stress and axial strain for the Locharbriggs sandstone. The Clashach sandstone exhibits a linear correlation between permeability and exit pore fluid concentrations (Si, Mg, Fe, Al) if a slow strain rate is used. These observations have important implications for the applicability of room temperature, high strain rate laboratory data to the conditions that prevail in the Earth's crust.
Change of bulk and shear moduli of dry sandstone with effective pressure and temperature
The bulk and shear moduli of dry sandstone increase with effective pressure and decrease with temperature and the rate of change varies with effective pressure and temperature. In order to calculate the effect of effective pressure, a rock physics model based on pore aspect ratio spectra (KT model) is adopted in computation of elastic moduli and velocities. The pore aspect ratio spectra for a set of water-saturated sandstone samples are first assumed to be proportional to that of the standard sample, and are then adjusted to fit velocity measurements. Dry bulk and shear moduli at different pressures are calculated with the optimized pore aspect ratio spectra by setting the bulk moduli of contained fluid equal zero. It is found that the exponential relationship exists between the rate of change of elastic moduli and effective pressure as follows: