Induced Pore Pressure Research Articles

Convective cooling/heating induced thermal stresses in a fluid saturated porous medium undergoing local thermal non-equilibrium

Local thermal non-equilibrium (LTNE) may have profound effects on the pore pressure and thermal stresses in fluid saturated porous media under transient thermal loads. This work investigates the temperature, pore pressure, and thermal stress distributions in a porous medium subjected to convective cooling/heating on its boundary. The LTNE thermo-poroelasticity equations are solved by means of Laplace transform for two fundamental problems in petroleum engineering and nuclear waste storage applications, i.e., an infinite porous medium containing a cylindrical hole or a spherical cavity subjected to symmetrical thermo-mechanical loads on the cavity boundary. Numerical examples are presented to examine the effects of LTNE under convective cooling/heating conditions on the temperature, pore pressure and thermal stresses around the cavities. The results show that the LTNE effects become more pronounced when the convective heat transfer boundary conditions are employed. For the cylindrical hole problem of a sandstone formation, the thermally induced pore pressure and the magnitude of thermal stresses are significantly higher than the corresponding values in the classical poroelasticity, which is particularly true under convective cooling with moderate Biot numbers. For the spherical cavity problem of a clay medium, the LTNE effect may become significant depending on the boundary conditions employed in the classical theory.

Effect of excavation stages on stress and pore pressure changes for an underground nuclear repository

This paper provides a brief overview of evolution of deformations, stresses, and pore pressures due to different excavation stages during construction of an URL. Excavation of a rockmass develops damage zone of finite width around the excavated zone which is associated with changes in permeability and affect the fluid flow mechanism. In this study, a coupled hydromechanical (HM) analysis for a water saturated porous rock mass has been done which is caused by sequential excavation and backfill of vertical disposal pit of an URL. For this purpose, an Atomic Energy of Canada Limited’s in-floor concept for a deep geologic repository suitable for Indian condition has been used. Changes in rock deformations, stresses, strains and mechanically induced pore pressure in an undrained condition, during excavation, as well as those caused by mechanically induced rock deformations after backfill of the disposal pit have been modeled. A three-dimensional finite difference tool FLAC3D (Itasca Consulting Group Inc. FLAC-3D 1997) has been used for the analysis.

Cyclic Large Strain and Induced Pore Pressure Models for Saturated Clean Sands

Semiempirical probabilistic models are described to assess cyclic large strain and induced excess pore-water pressure responses of fully saturated clean sands. For this purpose, available cyclic simple shear and triaxial tests were compiled and studied. The resulting ru versus γ, and γ versus N databases are composed of 101 and 84 cyclic test data, respectively. Key parameters of the proposed ru and γ models are defined as critical shear strain, relative density, effective confining stress, and equivalent number of loading cycles. Consistent with the maximum likelihood methodology, model coefficients were estimated by maximizing the likelihood function. For comparison purposes, the compiled database was again used to evaluate the performance of existing ru models. Both for comparison and calibration purposes, for each framework, two separate sets of limit-state models were used: model implemented with (1) the original and (2) the updated model coefficients. The model performances are assessed by simple st...

A Laboratory Investigation on Thermal Properties of the Opalinus Claystone

Some aspects of the thermal behavior of the Opalinus claystone are investigated through laboratory tests conducted on a new hollow cylinder triaxial apparatus specially designed for studying the thermo-hydro-mechanical behavior of very low permeable materials. Two hollow cylinder samples are first resaturated under isotropic stress state equal to the mean effective in situ one in order to minimize swelling and induced damage during the resaturation phase. Two drained heating–cooling cycles are performed on the first sample of Opalinus claystone. During the first cycle, a thermo-elasto-plastic response similar to that of plastic clays with low overconsolidation ratio is obtained. The thermal hardening of the sample is demonstrated by the quasi-reversible behavior of the sample during the second heating–cooling cycle. An undrained heating test performed on the second sample of Opalinus claystone induces an excess pore pressure in this sample. This induced pore pressure is attributed to the higher thermal expansion coefficient of pore water compared to that of the solid phase. It is shown that the excess pore pressure generated in the sample by undrained heating cannot be modeled by considering the free water thermal expansion coefficient. The thermal expansion coefficient of the Opalinus claystone water is back-analyzed from the experimental results which show a higher value than free water.

Electrically induced pore pressures in high salt content clay slurry

The development of stagnant pore water pressures due to non-uniform and non-steady distribution of voltage gradients were investigated in high water and salt content clay slurry using a floor-scale apparatus. The experiment was intended to simulate electrically enhanced dewatering of sea harbor sediments by horizontal drainage towards vertically installed electrodes. The scale of the test apparatus allowed for accurate measurements of the spatial and temporal variations of water pressure head, temperature, electrical potential and current throughout the test duration of 5 weeks. The pore water pressure dissipation measured at one-third the depth of the clay bed from mud-line appeared to alternate in rate and magnitude between the equally separated adjacent probe-stations, before it ceased completely. The voltage gradients between these stations fluctuated in the similar alternating manner indicative of significant variations in the local electro-osmotic water mass transport rates among the regions. It is postulated that the non-uniform voltage gradients give rise to competing pressure gradients; and the electrical potential applied orthogonal to the settling direction polarize clay particles hence disrupting the dewatering process by pore water pressure dissipation and compaction in high salt content clay sediments.

Analytical solution for the dynamic response of a saturated poroelastic half-space to harmonic stress loading

A theoretical analysis for the dynamic response of a semi-infinite fluid-bearing porous medium to external harmonic loading is presented in this study based on the decoupled poroelasticity equations of Biot (1962). A corresponding initial and boundary value problem is formulated and the analytical solution for the induced pore pressure and total dilatational stress is determined using the technique of Laplace transforms. To investigate the quantitative impact of inertial effect on the poroelastic response, the problem is also solved analytically in the diffusive model (i.e. inertial terms are ignored). Comparison of the analytical solutions obtained from two different models shows that as inertial effect is accounted for, the response undergoes in a dynamic manner but lags behind the loading by a physical factor equal to the travel time necessary for pressure wave to reach the location that is prescribed. A numerical study is then conducted for water-containing Columbia fine sandy loam at lower excitation frequencies as a representative example. Our numerical results reveal that the dynamic model yields a cyclic response for the induced pore pressure and total dilatational stress with respect to depth, but the diffusive model fails to predict this attribute. Lastly, we find in the dynamic model that effective stress may take on a positive value at some depths due to the existence of time lag in the response of pore fluid to external loading so that the solid skeleton needs to sustain excess fluid pressure. This positive value is crucial for the phenomenon of liquefaction if the loading is substantial enough.

Analytic solutions with ionic flow for a pressure transmission test on shale

Ion/water transport between drilling fluid and shale formations can result in altered pore pressure in the shale. This chemically induced pore pressure plays an important role in controlling shale stability during drilling operations. In this paper analytic solutions are derived for a transient pressure transmission test on shale by solving the previously derived partially-coupled diffusivity equations taking into account chemical ionic flow effects. The diffusivity equations were solved under a no-flow boundary condition which is prescribed in a particular kind of transient pressure transmission test. By determining the three chemo-poroelastic coefficients K I (permeability coefficient), K II (membrane efficiency coefficient), and D eff (ion diffusivity) experimentally, the analytic solutions are compared with the actual experimental data. In general, a good agreement between the analytic solutions and the actual pressure measurements in the time-dependent pressure transmission tests has been found and is presented. The analytic solutions are provided for both the linear and cylindrical pressure transmission geometries. The pressure transmission experimental data discussed in this paper are from linear-geometry tests. Results presented in this paper can be used to study the transient pore pressure and stresses in the near-wellbore region, and to optimize the drilling fluid design (e.g., optimized salt type, salt concentration, optimized membrane efficiency, fluid pressure etc.).

Experimental techniques for the investigation of coupled phenomena in geomaterials

The paper describes different experimental setups and techniques used to investigate coupled stress, fluid (water and air) and temperature effects on geomaterials. Two temperature controlled cells are described: a) a constant volume cell in which thermal pulses can be performed under controlled hydraulic conditions to induce pore pressure build-up during quasi-undrained heating and later dissipation; and b) an axisymmetric triaxial cell with controlled suction and temperature to perform drained heating and cooling paths under partially saturated states. The paper also presents an experimental setup to perform controlled flow-rate gas injection experiments on argillaceous rocks using a high-pressure triaxial cell. This cell is used to study gas migration phenomena and the conditions under which gas breakthrough processes occur. Selected test results are presented, which show the capabilities of the different experimental setups described to capture main behavioural features.

Cyclic Pore Pressure Generation, Dissipation and Densification in Granular Mixes

Knowledge of cyclic load induced pore pressure generation, post-liquefaction dissipation and volumetric densification characteristics of sands, silty sands, and silts are important for the analysis of performance of loose saturated granular deposits in seismic areas. This article presents results from an experimental study of these characteristics for such soils containing 0 to 100% non-plastic silt. Pore pressure generation characteristics are studied using undrained cyclic triaxial tests. Pre- and post-liquefaction compressibility and coefficient of consolidation, and post-liquefaction volumetric densification characteristics are determined from consolidation data prior to cyclic tests and pore pressure dissipation tests following undrained cyclic tests. Effects of fines content on these characteristics compared to those of clean sands are examined in the context of intergranular void ratio and intergranular contact density concepts.

Natural and man-made induced hydrological signals, detected by high resolution tilt observations at the Geodynamic Observatory Moxa/Germany

It is well known, that high resolution borehole tiltmeters are able to observe deformations, caused by hydrological variations. The quantitative coherence is often unexplained, especially if the sources of deformation can be based on both natural as well as man-made hydrological variations. Since 1999 tilt observations have been taken at the Geodynamic Observatory Moxa in Thuringia/Germany. In two 50 m and one 100 m deep boreholes the ASKANIA tiltmeters are installed. The high quality of the recorded tilt data can be proved by the analysis of well known geodynamic signals like the tides of the solid Earth and the free modes of the Earth. Here we focus on investigations of induced tilt signals caused by pore pressure changes due to precipitation and/or ground water level changes and, in addition, on man-made induced pore pressure variations. The correlation of natural ground water level changes with the observed tilt data can be shown by different events of precipitation and snow melting. However, also the load effect of a big truck yields a small elastic deformation which is clearly detectable in the ground water level recording. The correlated tilt effect is discussed regarding changes of the tilt amplitude and the orientation of the induced pendulum tip movement during the load phase.

Engineering of Ground for Liquefaction Mitigation Using Granular Columnar Inclusions: Recent Developments

Problem statement: Liquefaction was the most hazardous damage during an earthquake. Ground improvement techniques were employed to mitigate liquefaction hazards. Most common methods to improve engineering properties of soils are densification, reinforcement, grouting/mixing and drainage. Among various remedial measures available, installation of columnar granular inclusions is the most widely adopted method for liquefaction mitigation. Approach: Columnar granular inclusions function as drains and permit rapid dissipation of earthquake induced pore pressures by virtue of their high permeability. Results: One of the chief benefits of ground treatment with granular piles is the densification of in situ ground by which the in-situ properties of the ground get modified to mitigate liquefaction potential. Further, the very high deformation modulus and stiffness of the granular pile material provide reinforcement for the in situ soil and offer another mechanism to mitigate liquefaction. The study described briefly the phenomenon of liquefaction and the associated features. A short discussion on various ground improvement methods available for liquefaction mitigation was presented highlighting the importance of columnar inclusions. Construction methods of different granular columnar inclusions like sand compaction piles/ granular piles were discussed briefly. Recent developments in the research of columnar granular inclusions as liquefaction counter measures were presented in relation to physical, numerical and analytical model studies. Conclusion/Recommendations: Columnar granular inclusions were demonstrated to be very effective for liquefaction mitigation in different case studies and research investigations.

Effect of Fire Induced Spalling on the Response of Reinforced Concrete Beams

A macroscopic finite element model is applied to investigate the effect of fire induced spalling on the response of reinforced concrete (RC) beams. Spalling is accounted for in the model through pore pressure calculations in concrete. The principles of mechanics and thermodynamics are applied to compute the temperature induced pore pressure in the concrete structures as a function of fire exposure time. The computed pore pressure is checked against the temperature dependent tensile strength of concrete to determine the extent of spalling. Using the model, case studies are conducted to investigate the influence of concrete permeability, fire scenario and axial restraint on the fire induced spalling and also on the response of RC beams. Results from the analysis indicate that the fire induced spalling, fire scenario, and axial restraint have significant influence on the fire response of RC beams. It is also shown that concrete permeability has substantial effect on the fire induced spalling and thus on the fire response of concrete beams. The fire resistance of high strength concrete beams can be lower that that of normal strength concrete beams due to fire induced spalling resulting from low permeability in high strength concrete.

The effect of undrained heating on a fluid-saturated hardened cement paste

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/°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 from the results of the undrained heating test.

Application of Strain-Induced Permeability Model in a Coupled Geomechanics-Reservoir Simulator

Abstract A fully coupled geomechanics-reservoir model is developed to simulate the directional changes in permeability due to fluid injection and production in a reservoir. The model is implemented numerically by fully coupling a geomechanics model with a single-phase reservoir flow model using the finite element method. A strain-induced permeability model is developed based on the analysis of the grain fabric of intact and sheared oil sand specimens using a thin section imaging method. It can quantify the changes in permeability when material experiences shear deformation. In addition, the directions of the principal values of permeability are not restricted to some arbitrary axes, but governed by the induced strains. Thus, the effects of stress path and stress level are implicitly considered through effective stress-strain constitutive laws. The transient pressure response of a water injection test in a horizontal well is analyzed. Parametric studies are also conducted to investigate the effects of permeability, deformability and initial stress conditions on the injection pressure. It is found that the changes in permeability resulting from dilations of the oil sands cannot be captured using the conventional permeability model as a function of volumetric strain because the volumetric strain is small. However, the strain-induced permeability model can accurately reflect the directional increase in permeability during water injection. It is shown that the induced pore pressure is relatively insensitive to the deformation modulus of the reservoir, as compared to the permeability. The initial stress condition dominates the propensity of hydraulic fracturing. Introduction Geomechanics plays a key role in accounting for rock deformations due to pore pressure and temperature changes resulting from fluid injection and production in a thermal reservoir. During fluid injection and production, absolute permeability at any given location may change in response to localized changes in stress within the rock pore system owing to changes in reservoir pressure. Pore pressure changes can, in turn, influence the effective reservoir stress (overburden pressure minus pore pressure) which can alter the pore geometry of the reservoir rock; especially the shape and dimension of the pore and the pore throats(1–7). The geomechanical behaviour of porous media has become increasingly important in deformable and fractured reservoirs. In current geomechanics-reservoir coupled simulators, the permeability of the rock is assumed to be transversely or orthogonally isotropic(8,9). In other words, the direction of flow is aligned with the pressure gradient. This simplifying assumption is not valid in many cases. For example, the permeability of fractured rock is substantially dependent on microfractures. Therefore, the dominating orientation of the fractures can make the direction of flow different from that of the pressure gradient(10). Another example of permeability anisotropy is demonstrated in thermal recovery processes, such as cyclic steam stimulation (CSS) and steam assisted gravity drainage (SAGD). In these thermal recovery processes, shear dilation deformation takes place in local areas due to the increase of pore pressure, thereby enhancing permeability in these areas(11,12). Clearly, an understanding of permeability anisotropy is important. However, in the petroleum literature, the permeability change of a reservoir formation subjected to deformation changes is usually determined as a function of volumetric strain or porosity, which relates to the mean or minimum effective stress.

Role of flexure in earthquake triggering along the Western Ghat escarpment, India

The occurrence of earthquakes near reservoirs in the region of the Western Ghat escarpment far exceeds the reported rate in other areas of peninsular India. Reservoir loading and induced pore pressure are generally thought to be triggering earthquakes, but in order to explain the anomalous rates of occurrence in the Western Ghat, we invoke the additional effects of elastic plate flexure. The height of the escarpment is usually considered to be maintained by flexure in the east–west direction. Intense erosion and sediment loading further control its evolution. We propose that this promotes failure at shallow depth by a reduction of the normal stress on escarpment-parallel sub-vertical planes in the region. The same mechanism has been proposed to operate globally in several other escarpment regions along passive margins.

Wave-Induced Pore Pressures—Air/Gas Content

Wave induced pore pressures in bottom soils have an influence on the geotechnical stability of structures and may also have an effect on the transport of sand. Wave induced pore pressures in a seabed of sand or clay has been an item for extensive research. One of the major difficulties in analyzing and interpreting both laboratory and field data has been related to the question of air/gas content in the pores of fine soil. Only a small fraction of air/gas significantly changes the bulk modulus of the water/gas mixture and also the attenuation of the wave induced pressures in the soil with depth. The reason for the difficulty of analyzing data has been the lack of good instrumentation for measuring the gas content in the seabed soil.

Effective normal stress alteration due to pore pressure changes induced by dynamic slip propagation on a plane between dissimilar materials

Recent, detailed examinations of fault zones show that walls of faults are often bordered by materials that are different from each other and from the more uniform material farther away. In addition, they show that the ultracataclastic core of mature fault zones, where slip is concentrated, is less permeable to flow across it than the adjoining material of the damage zone. Inhomogeneous slip at the interface between materials with different poroelastic properties and permeabilities causes a change in pore pressure there. Because slip causes compression on one side of the fault wall and extension on the other, the pore pressure on the fault increases substantially when the compressed side is significantly more permeable and decreases when, instead, the extended side is more permeable. This change in pore pressure alters the effective normal stress on the slip plane in a way that is analogous to the normal stress alteration in sliding between elastically dissimilar solids. The magnitude of the effect due to induced pore pressure can be comparable to or larger than that induced by sliding between elastic solids with a dissimilarity of properties consistent with seismic observations. The induced pore pressure effect is increased by increasing contrast in permeability, but the normal stress alteration due to elastic contrast increases rapidly as the rupture velocity approaches the generalized Rayleigh velocity. Because the alteration in effective normal stress due to either effect can be positive or negative, depending on the contrast in properties, the two effects can augment or offset each other.

A review of the state of the art in modelling progressive mechanical breakdown and associated fluid flow in intact heterogeneous rocks

The purpose of this review is to present techniques, advances, problems, and new developments in modelling the progressive mechanical breakdown of, and associated fluid flow in, intact heterogeneous rock. In general, the theoretical approach to this physical process can be classified into the three categories of discrete models based on fracture mechanics, the continuum damage mechanics approach, and statistical approaches. This categorisation forms the skeleton of this article. Recognising that intact rock contains ubiquitous cracks and flaws between grains and particles of various shapes, fracture mechanics has been widely used to study the mechanical breakdown process in terms of the growth of these discrete defects. Two types of fracture mechanics models, namely open crack and sliding crack models, are used to simulate the progressive microfracturing of rock upon loading, and the application of these to modelling mechanical breakdown is discussed in Section 2. As an alternative to the explicit treatment of cracking given by fracture mechanics, continuum damage mechanics (CDM) takes a phenomenological route that considers the averaged effect of microstructural changes, and has found widespread use in simulating macroscopic stress–strain responses. Through the introduction of damage variables as state operators that quantify change in mechanical properties such as stiffness and strength with respect to damage, CDM models are capable of reproducing realistic hydro-mechanical responses during rock disintegration. This method is discussed in Section 3. Recognising that intact rock is never strictly an isotropic and homogeneous material, statistical approaches use, in general terms, statistical distributions as a means of describing the variation of material properties in natural rock. These approaches have enjoyed a great deal of attention in past decades. By employing different numerical schemes, three major statistical models have been developed: continuum-based damage mechanics models, particle models, and network/lattice models. The merits and drawbacks of these models are discussed in Section 4. Coupled deformation and pore fluid diffusion can be important in the process of progressive breakdown. This poromechanical effect involves the interaction between the solid constituents of, and the interstitial fluids in, heterogeneous rocks under those circumstances when mechanical perturbation occurs sufficiently rapidly that induced pore pressure changes cannot fully dissipate. The mechanisms and analytical approaches, and their development relevant to progressive mechanical breakdown and fluid-flow modelling, are outlined in Section 5. Finally, based on this review, remarks are made summarising the current progress of, and fundamental problems with, these developments. Ideas for further improvements towards more comprehensive and robust techniques are suggested.

Spatiotemporal pore pressure evolution due to fluid-mass point sources in dynamic poroelasticity

SUMMARY The spatiotemporal behaviours of pore pressure induced by a fluid mass point source in quasi-static and dynamic poroelasticity are compared and contrasted. Explicit results for the pore pressure in time domain are derived using the known fundamental solution of dynamic poroelasticity in frequency domain. Depending on the frequency content of the source function, different regimes can be identified. First, the diffusive regime where only the diffusive slow wave exists; the latter is equivalent to the process of pore pressure diffusion. Second, there is the propagation–diffusion regime where slow and fast wave modes coexist. It is shown that the amount of induced pore pressure due to the diffusive slow wave is different for quasi-static loadings (e.g. fluid injection experiments) and transient loads (e.g. propagating low-frequency seismic waves). The results are discussed in the context of fluid injection experiments where short-term fluctuations of the injection rate can create both, the fast compressional wave and the slow diffusive wave. The results can be used in order to generate synthetic seismograms and validate numerical schemes for pressure disturbances in dynamic poroelasticity.

Fully coupled thermohydraulic modelling of the sealing bulkheads and adjacent rock in a full-scale underground test

The Tunnel Sealing Experiment (TSX) was a two-phase international project funded by Canada, Japan, France, and the United States. The first phase was pressurizing the TSX chamber to 4 MPa to investigate the ability of clay and concrete bulkheads to reduce hydraulic flows. The second phase involved circulating heated water through the chamber to evaluate the influence of elevated temperature on the performance of the bulkheads and adjacent rock. A numerical analysis to simulate thermohydraulic evolution of the bulkheads and surrounding rock of the TSX was conducted to help in understanding the physical test process and the interaction between heat and pore pressure evolutions. The simulated rock temperature matched the measured data quite well; however the simulated bulkhead temperatures were greater than the measured temperatures. The difference may have been caused by entrapped air or formation of microchannels in the chamber sand, which would decrease the amount of heat reaching the bulkheads. The simulated thermally induced pore pressure increase in the clay bulkhead reasonably matched the measured data for the saturated portion. The difference in magnitude between simulated and measured rock pore pressures indicates that thermo hy draulic simulation should be coupled with a mechanical component when the stiffness of the media is large and hydraulic conductivity is low.Key words: numerical modelling, Tunnel Sealing Experiment, nuclear waste management, hydraulic head, thermal conduction, thermal convection.