Abstract: The results of a macro-scale experimental study of the effect of heating on a fluid-saturated hardened cement paste are analysed using a multi-scale homogenization model. The analysis of the experimental results revealed that the thermal expansion coefficient of the cement paste pore fluid is anomalously higher than the one of pure bulk water. The micromechanics model is calibrated using the results of drained and undrained heating tests and permits the extrapolation of the experimentally evaluated thermal expansion and thermal pressurization parameters to cement pastes with different water-to-cement ratios. It permits also to calculate the pore volume thermal expansion coefficient [alpha][phi] which is difficult to evaluate experimentally. The anomalous pore fluid thermal expansion is also analysed using the micromechanics model.
Abstract: The effect of the variations of clinker composition on the poroelastic properties of class G oil-well cement pastes is studied using a multiscale homogenization model. The model has been calibrated in a previous work based on the results of a laboratory study. Various compositions of class G cements from literature are used in a hydration model to evaluate the volume fractions of the microstructure constituents of hardened cement paste. The poroelastic parameters such as drained bulk modulus, Biot coefficient, and Skempton coefficient are evaluated using the homogenization model. The results show that the variations in chemical composition of class G cements have no important effect on the variations of the poroelastic properties.
Abstract: Evaluation of the poroelastic properties of oil-well cement paste is essential for prediction of the performance of the cement sheath during the life of a well. A multiscale homoge-nization model is used to evaluate the poroelastic properties of different classes of oil-well cement paste. The model has been calibrated in a previous work based on the results of a laboratory study on a hardened class G cement paste. A hydration model is used to evaluate the volume fractions of different microstructure phases of cement paste based on the chemical composition of clinker and the water-to-cement ratio. Typical chemical compositions of API class A to class H oil-well cements with their corresponding water-to-cement ratios are used to evaluate the poroelastic parameters such as drained bulk modulus, Biot coefficient, Skempton coefficient, etc. The results show that the difference in the chemical compositions of these cements has not an important effect on the variations of the poroelastic properties. Contrarily, the water-to-cement ratio has an important effect on the variations of these parameters.
Abstract: The equations governing plane steady-state flow in heterogeneous porous body containing cracks are presented first. Then, a general transformation lemma is presented which allows extending a particular solution obtained for a given flow problem to another configuration with different geometry, behaviour and boundary conditions. An existing potential solution in terms of discharges along the cracks, established by Liolios and Exadaktylos (J Solids Struct 43:3960-3982, 2006) for non-intersecting cracks in isotropic matrix, is extended to intersecting cracks in anisotropic matrix. The basic problem of a single straight crack in an infinite body submitted to a pressure gradient at infinity is then investigated and a closed-form solution is presented for the case of void cracks (infinite conductivity), as well as a semi-analytical solution for the case of cracks with Poiseuille type conductivity. These solutions, derived first for an isotropic matrix, are then extended to anisotropic matrices using the general transformation lemma. Finally, using the solution obtained for a single crack, a closed-form estimation of the effective permeability of micro-cracked porous materials with weak crack density is derived from a self-consistent upscaling scheme.
Abstract: The theoretical basis of the thermal response of the fluid-saturated porous materials in undrained condition is presented. It has been demonstrated that the thermal pressurization phenomenon is controlled by the discrepancy between the thermal expansion of the pore fluid and of the solid phase, the stress-dependency of the compressibility and the non-elastic volume changes of the porous material. For evaluation of the undrained thermo-poro-elastic properties of saturated porous materials in conventional triaxial cells, it is important to take into account the effect of the dead volume of the drainage system. A simple correction method is presented to correct the measured pore pressure change and also the measured volumetric strain during an undrained heating test. It is shown that the porosity of the tested material, its drained compressibility and the ratio of the volume of the drainage system to the one of the tested sample, are the key parameters which influence the most the error induced on the measurements by the drainage system. An example of the experimental evaluation of undrained thermoelastic parameters is presented for an undrained heating test performed on a fluid-saturated granular rock.
Abstract: The results of a macro-scale experimental study performed on a hardened class G cement paste [Ghabezloo et al. (2008) Cem. Con. Res. (38) 1424-1437] are used in association with the micromechanics modelling and homogenization technique for evaluation of the complete set of poroelastic parameters of the material. The experimental study consisted in drained, undrained and unjacketed isotropic compression tests. Analysis of the experimental results revealed that the active porosity of the studied cement paste is smaller than its total porosity. A multi-scale homogenization model, calibrated on the experimental results, is used to extrapolate the poroelastic parameters to cement pastes prepared with different water-to-cement ratio. The notion of cement paste active porosity is discussed and the poroelastic parameters of hardened cement paste for an ideal, perfectly drained condition are evaluated using the homogenization model. (C) 2010 Elsevier Ltd. All rights reserved.
Abstract: A discussion is presented on the effect of porosity change on the thermal expansion coefficient of porous materials. It is shown that both drained and undrained thermal expansion coefficients normally increase with porosity increase. Consequently the reduction of the thermal expansion coefficient with the porosity increase, as obtained in the experimental results of Shui et al.[1], cannot be considered as a general trend for the behaviour of porous materials. (C) 2010 Elsevier Ltd. All rights reserved.
Abstract: For evaluation of the undrained thermo-poro-elastic properties of saturated porous materials in conventional triaxial cells, it is important to take into account the effect of the dead volume of the drainage system. The compressibility and the thermal expansion of the drainage system, along with the dead volume of the fluid filling this system, influence the measured pore pressure and volumetric strain during an undrained thermal or mechanical loading in a triaxial cell. The correction methods previously presented by Wissa [Pore pressure measurement in saturated stiff soils. ASCE J Soil Mech Found Div 1969; 95(SM 4):1063-73], Bishop [Influence of system compressibility on observed pore pressure response to an undrained change in stress in saturated rock. Geotechnique 1976; 26(2): 371-5] and Ghabezloo and Sulem [Stress dependent thermal pressurization of a fluid-saturated rock. Rock Mech Rock Eng 2009; 42(1):1-24], only permit one to correct the measured pore pressures during an undrained isotropic compression test or an undrained heating test. An extension of these methods is presented in this paper to correct the measured volumetric strain, and consequently the measured undrained bulk compressibility and undrained thermal expansion coefficients, during these tests. Two examples of application of the proposed correction method are presented for an undrained isotropic compression test and an undrained heating test performed on a fluid-saturated granular rock. A parametric study has demonstrated that the porosity and the drained compressibility of the tested material, and the ratio of the volume of the drainage system to the one of the tested sample, are the key parameters that most influence the error induced by the drainage system. (C) 2009 Elsevier Ltd. All rights reserved.
Abstract: The effect of undrained heating on Volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6 MPa/degrees C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement paste pore fluid is back analysed front the results of the undrained heating test. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: A method is presented for the evaluation of the permeability-porosity relationship in a low-permeability porous material using the results of a single transient test. This method accounts for both elastic and non-elastic deformations of the sample during the test, and is applied to a hardened class G oil well cement paste. An initial hydrostatic undrained loading is applied to the sample. The generated excess pore pressure is then released at one end of the sample while monitoring the pore pressure at the other end and the radial strain in the middle of the sample during the dissipation of the pore pressure. These measurements are back-analysed to evaluate the permeability and its evolution with porosity change. The effect of creep of the sample during the test on the measured pore pressure and volume change is taken into account in the analysis. This approach permits to calibrate a power law permeability-porosity relationship for the tested hardened cement paste. The porosity sensitivity exponent of the power-law is evaluated equal to 11, and is shown to be mostly independent of the stress level and of the creep strains. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: The effective stress law for the permeability of a limestone is studied experimentally by performing constant-head permeability tests in a triaxial cell with different conditions of con. ning pressure sigma and pore pressure p(f). Test results show that a pore pressure increase and a con. ning pressure decrease both result in an increase of the permeability, and that the effect of the pore pressure change on the variation of the permeability is more important than the effect of a change of the con. ning pressure. A power law is proposed for the variation of the permeability with the effective stress (sigma' = sigma-n(k)p(f)). The permeability-effective stress coefficient n(k) increases linearly with the differential pressure and is greater than unity as soon as the differential pressure exceeds few bars. The test results are well reproduced using the proposed permeability-effective stress law. A conceptual pore-shell model based on a detailed observation of the microstructure of the studied limestone is proposed. This model is able to explain the experimental observations on the effect of the total stress and of the pore pressure on the permeability of the limestone. Effective stress coefficients for the stress-dependent permeability which are greater than one are obtained. It is shown that the controlling factor is the ratio of the different bulk moduli of the various constituents of the rock. This ratio is studied experimentally by performing microhardness tests. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: Temperature increase in saturated porous materials under undrained conditions leads to thermal pressurization of the pore fluid due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. This increase in the pore fluid pressure induces a reduction of the effective mean stress and can lead to shear failure or hydraulic fracturing. The equations governing the phenomenon of thermal pressurization are presented and this phenomenon is studied experimentally for a saturated granular rock in an undrained heating test under constant isotropic stress. Careful analysis of the effect of mechanical and thermal deformations of the drainage and pressure measurement system is performed and a correction of the measured pore pressure is introduced. The test results are modelled using a non-linear thermo-poro-elastic constitutive model of the granular rock with emphasis on the stress-dependent character of the rock compressibility. The effects of stress and temperature on thermal pressurization observed in the test are correctly reproduced by the model.
Abstract: The poromechanical behaviour of hardened cement paste under isotropic loading is studied on the basis of an experimental testing program of drained, undrained and unjacketed compression tests. The macroscopic behaviour of the material is described in the framework of the mechanics of porous media. The poroelastic parameters of the material are determined and the effect of stress and pore pressure on them is evaluated. Appropriate effective stress laws which control the evolution of total volume, pore volume, solid volume, porosity and drained bulk modulus are discussed. A phenomenon of degradation of elastic properties is observed in the test results. The microscopic observations showed that this degradation is caused by the microcracking of the material under isotropic loading. The good compatibility and the consistency of the obtained poromechanical parameters demonstrate that the behaviour of the hardened cement paste can be indeed described within the framework of the theory of porous media. (C) 2008 Elsevier Ltd. All rights reserved.
