Effective stress law for the permeability of clay-rich sandstones (original) (raw)
Related papers
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.
Effective Stress Law for the Permeability and Pore Volume Change of Clayey Sandstones
Journal of Geophysical Research: Solid Earth, 2020
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Effects of clay mineral and physico-chemical variables on sandstone rock permeability
Journal of Oil, Gas and Petrochemical Sciences, 2018
In this paper we highlighted the effect of clay mineralogy and the influence of physico-chemical factors on coagulation-dispersion processes and the rate of pore throat bridging by clays. In particular, we describe the effect of different pH (acidic 2.5 and alkaline 9.5) and the effect of salinity (NaCl) on the stability of clay particles in siliciclastic rocks and their influence on the rock permeability which are often ignored in reservoir engineering literature. We conducted the first experiments on powder (0.05-0.1 mm grain size) samples for tracking mineral reactions and core plugs for permeability. X-ray diffraction (XRD) was run for whole-rock and clay fraction by diagnostic treatments to specify types of clay minerals and to calculate the percentage of each type. The composition of starting material is quartz dominant, with carbonates (dolomite, calcite), feldspars (albite, microcline), chlorite, kaolinite, illite, muscovite, with traces of 14 Å smectite and illite/smectite randomly interstratified structure. In strongly acidic solution, according to XRD, smectite is altered (14 Å peak vanishing), chlorite being also partly reduced, while kaolinite is stable. Carbonates are not involved in reactions. In basic solution, smectite was less affected, but chlorite peaks indicate structural transformation. The shape and broadening of 10 Å peak shows changes in illite and illite/smectite interstratified structures. Altogether, mineralogical changes indicate reactions which lead to fines mobilization. Based on experimental work, in the case injection with pH 2.5 and pH 9.5, there has been a rapid and substantial reduction in permeability as an average 38.54% and 73.72% for acidic and basic solutions respectively. The results confirmed that the reason of the reduction in permeability is due to the non-swelling clay minerals which are dispersed and migrated during the changes in pH, salinity and time. Novelty of these experiments is the tracking of changes in mineralogy by Rietveld refinement on XRD, coupled with diagnostic clay mineral investigation.
Rock Mechanics and Rock Engineering, 2020
We performed a systematic investigation of the effective stress behaviors for permeability and deformation in relation to bedding anisotropy of two clayey sandstones. Permeability and deformation were measured in samples cored parallel and perpendicular to bedding over a broad range of hydrostatic pressures, covering 'stage I' for microcrack closure and 'stage II' for pore deformation. Our data show that bedding anisotropy has a significant influence on the effective stress coefficient for permeability, but little effect on the effective stress coefficient for pore volume change. The effective stress coefficient ⟂ of permeability for flow perpendicular to bedding was consistently larger than the corresponding || for parallel flow. The effective stress coefficient || for pore volume changes parallel to bedding and corresponding coefficient ⟂ values perpendicular to bedding coincided, because the scalar change of pore volume was not sensitive to the orientation of the samples. Furthermore, we confirmed that with the closure of preexisting microcracks, the effective stress coefficients for permeability in stages II were typically larger than the corresponding coefficients in stage I, and that the effective stress coefficients for axial strain and pore volume change decreased for samples both perpendicular and parallel to bedding. Our new results quantified the effect of bedding anisotropy and crack closure on the effective stress behavior of clayey sandstones.
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.
International Journal of Rock Mechanics and Mining Sciences, 2017
Tight sandstones samples from an Ordovician gas field in Algeria studied in this work are characterized by low connected porosity (below 10%) and low gas permeability (below 0.1 mD under ambient condition). Under confining pressure (up to 40 MPa) the permeability has decreased by more than 80% while the porosity goes down between 10% and 25%. Regarding the porous structure which is constituted of large angular pores connected by micro-cracks, the high stress-sensitivity of permeability is mainly the result of micro-crack closure. In addition, the decrease of porosity potentially involves porosity trapping up to a confining pressure of 20 MPa caused by the closure of certain cracks. This hypothesis is further supported by the pore volume variation test and poro-mechanical test. The resulting improvement in our understanding of these physical phenomena will be very useful in the forthcoming analysis of the combined impact of water saturation and confinement on the effective gas permeability of this type of sandstone.
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.
Swelling of Clay Minerals and Its Impact on Permeability
2014
Formation damage has been described as any irreversible alteration, i.e. decrease of permeability, of the hydrocarbon reservoir rock after wellbore operations, which may have a serious economic impact upon the productivity of the reservoir. The presence of clays minerals is often considered as a major cause of formation damage. Thus, clay behavior in porous media needs to be taken into account when evaluating the potential productive capacity of hydrocarbon reservoir sandstones. Since the main mechanisms of permeability reduction, such as fine migration and clay swelling, occur at the pore-scale, in this study we couple permeability experiments with X-ray μ-computed tomography (μ-CT) to visualize and quantify clay behavior in porous media. The selected porous media were packed columns (diameter, d = 1.27 cm, average length, L=6.4 cm) of soda lime beads and quartz grains. Various amounts of swelling (montmorillonite) and non-swelling (kaolinite) clays were added as coatings on beads and quartz. The amount of clay in the samples varied from 1.38 to 5.5wt.% in montmorillonite-coated samples and from 2.0 to 6.84wt.% in kaolinite-coated samples. Permeability measurements were performed on each sample to investigate the impact of clay content and grain size on permeability reduction as a result of clay swelling. Permeability changes were monitored as a function of time. Visualization of coated bead and grains columns by μ-CT provided quantitative information on morphological changes of clay grains/coatings among dry and water-saturated samples. All claycoated samples showed 10-40% decrease in permeability as compared to uncoated samples (K/K o = 0.6-0.9). In general, the higher the clay content was, the larger permeability reduction was observed. Permeability remained constant within an error after 4 hours of flow experiment in montmorillonite samples. In contrast, permeability of kaolinite samples oscillated with time, possibly due to fine migration. A 39% volume increase of montmorillonite particles was observed by μ-CT immediately after the sample was saturated with water, i.e. swelling occurred almost instantaneously after water-clay contact. However, no further changes in volume were observed after 4 hours of flow. Kaolinite-coated samples showed a 15% volume increase of clay particles, which was attributed to the hydration of clay pellets by water. The calculated porosity reduction associated with clay swelling ranged from 0.4-1.7% including both montmorillonite-and kaolinite-coated samples. This decrease in porosity was estimated to cause only a 2-5% reduction in permeability, primarily due to the high initial porosity and permeability of the selected samples. This study presents a base line to estimate changes in permeability as a result of clay swelling for variable clay content, grain size, and porosity. v
Observations of heterogeneous pore pressure distributions in clay-rich materials
Mineralogical Magazine, 2012
The concept of effective stress is one of the basic tenets of rock mechanics where the stress acting on a rock can be viewed as the total stress minus the pore water pressure. In many materials, including clayrich rocks, this relationship has been seen to be imperfect and a coefficient (w) is added to account for the mechanical properties of the clay matrix. Recent experimental results during the flow testing (both gas and water) of several rocks (Callovo-Oxfordian claystone, Opalinus Clay, Boom Clay) and geomaterials (bentonite, kaolinite) has given evidence for stable high pressure differentials. The design of the experiments allows multiple measurements of pore pressure, which commonly shows a complex distribution for several different experimental geometries. The observed stable high pressure differentials and heterogeneous pore pressure distribution makes the describing of stress states in terms of effective stress complex. Highly localized pore pressures can be sustained by argillaceous materials and concepts of evenly distributed pore pressures throughout the sample (i.e. conventional effective stress) do not fit many clay-rich rocks if the complexities observed on the micro-scale are not incorporated, especially when considering the case of gas flow.