Isotropic Effective Stress Research Articles

Relative permeability evolution with thermo-poromechanical process during N2 and scCO2 injection in brine saturated Deadwood sandstone from Aquistore

This study delves into the intrinsic and multiphase flow properties, specifically steady-state drainage relative permeability, of a subsurface Deadwood sandstone from the Aquistore CO2 storage site in Canada. Consecutive core-flooding experiments were conducted utilizing N2- and scCO2-brine pairs across a broad range of temperatures (20–70 °C) and isotropic effective stress (0–30 MPa). Moreover, we monitored crack initiation and propagation of the sandstone during uniaxial loading at an elevated temperature using an integrated approach that combines microCT scanning with an in-situ heating/loading test. Our findings reveal a 54 % decrease and a 3 % increase in the absolute permeability of the sandstone through isothermal compaction followed by thermal expansion processes, respectively. Elevating the temperature from 20 °C to 70 °C results in a systematic 24 % increase in irreducible brine saturation and nearly doubles the end-point N2 mobility, indicating an increased tendency of the rock surface towards the brine phase with temperature. Substituting N2 with scCO2 demonstrates a leftward shift in relative permeability and a decrease in irreducible brine saturation (from 0.36 to 0.31), consistent with low interfacial tension and the de-wetting effect during cyclic scCO2-brine injections. Micro-CT image analysis reveals micro-crack initiation at 10 MPa stress and 70 °C temperature, suggesting that a mixed impact of induced cracks, dynamic wettability, and thermo-mechanical deformation is responsible for the substantial increase in well injectivity over time in Aquistore. This novel experimental program provides indispensable insight into thermo-poromechanical and wettability controls on multiphase flow at the Aquistore injection site in Canada, with potential applicability to similar scenarios globally.

Influence of plastic fines content on the liquefaction susceptibility of sands: cyclic loading

The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand-fines mixtures under cyclic loading. The results of undrained cyclic triaxial tests conducted on mixtures of Hostun sand with varying amounts (0–20%) and types (kaolin and calcigel bentonite) of plastic fines are presented. The specimens were prepared with different initial densities using the moist tamping method, consolidated at the same isotropic effective stress of 100 kPa and subjected to different deviatoric stress amplitudes. From the experimental observations, it was found that sand-clay mixtures with 10% or 20% clay content showed a lower cyclic liquefaction resistance than pure sand. Furthermore, the reduction in the cyclic stress ratio resulting in liquefaction after twenty cycles was found larger for sand-kaolin mixtures than for the sand-calcigel ones. Possible explanations are provided.

Poromechanical controls on spontaneous imbibition in earth materials

Over the last century, the state of stress in the earth’s upper crust has undergone rapid changes because of human activities associated with fluid withdrawal and injection in subsurface formations. The stress dependency of multiphase flow mechanisms in earth materials is a substantial challenge to understand, quantify, and model for many applications in groundwater hydrology, applied geophysics, CO2 subsurface storage, and the wider geoenergy field (e.g., geothermal energy, hydrogen storage, hydrocarbon recovery). Here, we conduct core-scale experiments using N2/water phases to study primary drainage followed by spontaneous imbibition in a carbonate specimen under increasing isotropic effective stress and isothermal conditions. Using X-ray computed micro-tomography images of the unconfined specimen, we introduce a novel coupling approach to reconstruct pore-deformation and simulate multiphase flow inside the deformed pore-space followed by a semi-analytical calculation of spontaneous imbibition. We show that the irreducible water saturation increases while the normalized volume of spontaneously imbibed water into the specimen decreases (46–25%) in response to an increase in effective stress (0–30 MPa), leading to higher residual gas saturations. Furthermore, the imbibition rate decreases with effective stress, which is also predicted by a numerical model, due to a decrease in water relative permeability as the pore-space becomes more confined and tortuous. This fundamental study provides new insights into the physics of multiphase fluid transport, CO2 storage capacity, and recovery of subsurface resources incorporating the impact of poromechanics.

Elastic shear modulus variations during undrained cyclic loading and subsequent reconsolidation of saturated sandy soil

To evaluate the variations in the elastic shear modulus G0 of saturated sandy soil during consolidation, undrained cyclic loading and reconsolidation, we performed a series of multiple-phased triaxial tests with measurements of shear wave velocity on specimens having different dry densities. At the same isotropic effective stress, the G0 value during undrained cyclic loading became smaller than that during initial consolidation. For the same effective stress parameter that was found to be relevant to the isotropic and anisotropic stress states, the G0 value at an identical effective stress parameter at isotropic and anisotropic stress states during undrained cyclic loading decreases similarly with an increase in the previous strain amplitude. By reconsolidation to the original effective stress state, the G0 value increased partly due to recovery from the shear-induced damage and partly due to a decrease in the void ratio. The effects of the former factor decreased as the specimen became denser, while the effects of the latter factor increased as the specimen became looser. As a result, compared with the initial value at the end of initial consolidation, the G0 value after reconsolidation of the same specimen became significantly smaller with dense specimens while it became significantly larger with loose specimens.

Improved subloading thermo-viscoplastic model for soil under strictly isotropic conditions

This paper presents a thermo-viscoplastic subloading soil model with a mobile centre of homothety. The model is formulated to describe the influence of non-isothermal conditions on the stress–strain-time behaviour of soils and is restricted to isotropic stress and strain conditions. Numerical simulations of three isotropic drained heating tests, two of which followed by a cooling stage, at constant isotropic effective stress for different overconsolidation ratios were performed. The model was able to accurately reproduce the experimental results, including the cooling behaviour, mainly due to the introduction of a direct influence of temperature on the mobile centre of homothety hardening law and also rate dependency.

Comparison of Coupled and Uncoupled Consolidation Equations Using Finite Element Method in Plane-Strain Condition

In the current paper, the consolidation settlement of a strip footing over a finite layer of saturated soil has been studied using the finite element method. In Biot’s coupled consolidation equations, the soil deformation and excess pore pressure are determined simultaneously in every time step which refers to the hydro-mechanical coupling. By considering a constant total stress throughout the time and by assuming that volume strain is a function of isotropic effective stress, uncoupled consolidation equations can be obtained using coupled consolidation equations. In these uncoupled equations, excess pore pressure and deformation are determined separately. In this approach, the excess pore pressure can be identified in the first stage. Using the calculated excess pore pressure, the soil deformation is determined through effective stress-strain analyses. A computer code was developed based on coupled and uncoupled equations that are capable of performing consolidation analyses. To verify the accuracy of these analyses, the obtained results have been compared with the precise solution of Terzaghi’s one-dimensional consolidation theory. The capability of these two approaches in estimation of pore water pressure and settlement and to show Mandel-Crayer’s effect in soil consolidation is discussed. Then, the necessity of utilizing coupled analyses for evaluating soil consolidation analysis was investigated by comparing the coupled and uncoupled analyses results.

Dynamics of potential fill–backfill material at very small strains

The paper presents a synthesis of past and recently acquired laboratory test results on granular soils using wave propagation techniques at very small strain amplitudes. Resonant column tests on uniform to well-graded coarse sands and gravels of angular and low sphericity grains were analyzed. Empirical-type equations were developed for the prediction of the elastic modulus and material damping at small strains considering the effects of the grading characteristics, the isotropic effective stress and the void ratio. The G0–p' relationship, expressed through exponent nG, was affected by the sample preparation method. For the narrow range in relatively low pressures adopted in the study, it was observed that nG decreased slightly with an increase in relative density. Due to the limited initial void ratios of those tests, the effect of the preparation method was not incorporated into the proposed formulae for the nG prediction. In this direction, additional experiments from the literature, which adopted the resonant column and bender element methods, were further analyzed, including soils of variable types tested with a wider range in relative densities. By employing typical formulae from the theory of elasticity, the bulk modulus and the changes in void ratio were estimated based on the change in isotropic effective stress in the literature data. Considering the recent micromechanical experimental findings associated with the nature of the contact response of soil particles, the importance of soil type and particle-contact behavior in the constant-state response of soils was demonstrated and quantified. Material damping values ranged from about 1.10% to about 0.45% for p' from 25 to 200kPa with a slight decrease in Ds0 with an increase in pressure.

The effects of disseminated methane hydrate on the dynamic stiffness and damping of a sand

Methane hydrates are ice-like compounds that can exist only under restricted thermobaric conditions, at low temperatures or under high ambient pressure. They are important because of their potential contributions as a future source of energy, to global warming, and as a possible trigger for long run-out submarine slope instability. This paper describes laboratory experiments to synthesise disseminated methane hydrates and to characterise them under small-strain dynamic loading in the resonant column apparatus. The effects of depositing varying quantities of methane hydrate within a sand are investigated by reference to their shear and bulk modulus, and damping, over a range of isotropic effective stress. Results are compared with those obtained on the same sand without hydrate bonding and after dissociation.

Volume Change Characteristics of Undisturbed Fibrous Peat

By using vertical and horizontal samples of undisturbed fibrous peat containing the amount of organic matters of 20%-80%, the volume change behavior during compression-swelling- recompression was investigated under an isotropic stress condition, and the change of pore diameter distribution due to compression was measured with a porosimeter apparatus. Moreover, in order to investigate the volume change behavior during drained shear on the isotropically normally consolidated specimens, the two series of triaxial compression tests, in which axial stress was increased while lateral stress or effective mean stress was maintained constant, were performed. Test results indicate that the large majority of openings of undisturbed samples of peat by has the pore diameters in the ranges of 100 μm-0.1 μm, and by compressing, the number of openings with the pore diameter of 0.1 μm-0.1 μm increases. The volume change of void space due to isotropic effective stress increment can be quantitatively estimated with the two inclinations Cc and Cs of compression-swelling line plotted on In e vs. In p' plane. Also, it was found that the amount of dilatancy of normally consolidated peat showed the linear relation against shear stress ratio and the rate of that to total volume change of peat element was roughly 0.3 over a wide range of shear stress ratio. Furthermore, it was verified that the total volume change could be represented by the sum of change due to the increment of effective mean stress and that due to the increment of deviatoric stress.

Comparison of the effects of soil matric potential and isotropic effective stress on the germination of Lactuca sativa

The matric potential contributes positively to the isotropic effective stress operating in the solid framework of a soil system. Collis-George and Hector (1966) suggested that the matric potential may influence seed germination through its contribution to the effective stress in the solid framework surrounding the seed. Experiments are described which separate the effects of matric potential on seed germination into those that can be attributed to the free energy of the soil water and those that can be attributed to the effective stress in the soil system. The results clearly indicate that (1) seed germination is influenced by the isotropic effective stress in the solid framework of the soil system, and (2) the influence of matric potential on seed germination in the range 0 to - 400cm of water can be wholly attributed to the isotropic effective stress in the solid framework and not to the free energy of the soil water as defined by the matric potential. These conclusions are discussed in terms of the behaviour of other organisms in response to an applied matric potential.