Experimental study and micromechanical interpretation of the poroelastic behaviour and permeability of a tight sandstone (original) (raw)
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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.
Understanding rock behaviour as a function of pore pressure and confining pressure is crucial for petroleum and geomechanical analysis. Indeed, deformation and local stress variations within hydrocarbon reservoirs and their surroundings occur due to pore pressure changes. Theoretically, pore pressure changes coupled with stress variations in hydrocarbon reservoirs are a function of the Biot's coefficient, the elastic properties of the rock and the reservoir shape. 5 Thus, in this study, the Biot's coefficient was measured as a function of porosity, permeability, and volumetric strain for five Gosford sandstone samples. A triaxial loading system was used to measure rock volumetric strain while pore pressure and confining pressure were varied. The constant deformation technique was employed for these experiments; i.e. the variation of pore pressure created a volumetric strain, and the confining pressure required to restore the original volumetric strain was measured to calculate Biot's coefficient. For the investigated samples, measured liquid permeabilities were in the 10 range of 7-10 mD and Biot's coefficients were 0.84-0.91. This is consistent with similar investigations by other researchers in which measured Biot's coefficients were in the range of 0.65-0.90. This study thus illustrates how liquid permeability and the Biot's coefficient decrease as a function of confining pressure.
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.
Mechanical Compaction of Porous Sandstone
Oil & Gas Science and Technology
Compaction mécanique des grès poreux -Pour de nombreux problèmes de tectonique et d'ingénierie de réservoir, la capacité à prévoir à la fois la fréquence, l'ampleur de la déformation inélastique et les ruptures repose sur une compréhension fondamentale de la phénoménologie et de la micromécanique de compaction dans les roches-réservoirs. Cet article présente les résultats de recherches récentes sur la compaction mécanique des grès poreux. On insiste plus particulièrement sur la synthèse des données de laboratoire, la caractérisation microstructurale quantitative de l'endommagement, ainsi que sur les modèles théoriques basés sur un contact élastique et sur la mécanique de la rupture. Les attributs mécaniques de la compaction sur des échantillons initialement secs et saturés ont été étudiés sous des chargements hydrostatiques et non hydrostatiques dans une large gamme de pression. Les sujets spécifiques étudiés ici incluent : la comparaison des données d'émission acoustique et mécanique avec une théorie de la plasticité ; le contrôle microstructural du début et du développement de la compaction ; l'écrouissage et l'évolution spatiale de l'endommagement lors de la compaction ; enfin, l'effet affaiblissant de l'eau sur le seuil de compaction et l'évolution de la porosité.
Micromechanical models of the strength of a sandstone
International Journal for Numerical and Analytical Methods in Geomechanics, 2009
In this paper, the strength of a sandstone is determined from a micromechanical approach. The microstructure of the rock is described as a porous polycrystal. A von Mises criterion is used for the strength of the solid grains. The grains are surrounded by Mohr-Coulomb interfaces describing the cement phase. The macroscopic strength of the polycrystal is determined by means of nonlinear homogenization techniques. The failure mechanism of the grains is assumed to be ductile. It is combined with a failure mechanism of the interfaces, which is successively assumed to be ductile and brittle. The theoretical predictions are then compared with the experimental data.
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.
Effect of Hydrostatic Loading on the Pore Structure and Transfer Properties of a Tight Gas Sandstone
2015
Our study focuses on tight gas sandstones from a field located in North Africa and explored by GDFSUEZ E&P. This rock is characterized by its low porosity (below 10%) and its low absolute permeability (below 0.1mD i.e. 10-16m2 under ambient conditions). A dedicated experimental setup has been designed to allow simultaneous measurements of gas permeability, connected porosity and poro-elastic properties, under given hydrostatic loading (up to 60 MPa). In the dry state, the decrease in accessible porosity is demonstrated experimentally: for one sample, accessible porosity is reduced by more than 10% (relative value) under 40 MPa hydrostatic loading. Simultaneously, gas permeability reduction is observed of a factor 8. In the partial water saturation state, the decrease in accessible porosity and effective gas permeability is enhanced. This shows an important stress sensitivity of the petrophysical properties of the sandstone: it is interpreted as pore entrapment under hydrostatic load...
Rock Mechanics and Rock Engineering, 2014
The experimental device previously used to study the hydromechanical behaviour of individual fractures on a laboratory scale, was adapted to make it possible to measure flow through porous rock mass samples in addition to fracture flows. A first series of tests was performed to characterize the hydromechanical behaviour of the fracture individually as well as the porous matrix (sandstone) comprising the fracture walls. A third test in this series was used to validate the experimental approach. These tests showed non-linear evolution of the contact area on the fracture walls with respect to effective normal stress. Consequently, a non-linear relationship was noted between the hydraulic aperture on the one hand, and the effective normal stress and mechanical opening on the other hand. The results of the three tests were then analysed by numerical modelling. The VIPLEF/HYDREF numerical codes used take into account the dual-porosity of the sample (fracture ? rock matrix) and can be used to reproduce hydromechanical loading accurately. The analyses show that the relationship between the hydraulic aperture of the fracture and the mechanical closure has a significant effect on fracture flow rate predictions. By taking simultaneous measurements of flow in both fracture and rock matrix, we were able to carry out a global evaluation of the conceptual approach used.
Effective Stress Law for the Permeability and Pore Volume Change of Clayey Sandstones
Journal of Geophysical Research: Solid Earth, 2020
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