Increase In Pore Pressure Research Articles

3D coupled hydro-mechanical modeling of multi-decadal multi-zone saltwater disposal in layered and faulted poroelastic rocks and implications for seismicity: An example from the Midland Basin

Knowledge of subsurface coupled pore pressure and stress perturbations due to saltwater disposal (SWD) is crucial for understanding and managing induced seismicity. Current studies choose Biot poroelasticity to implement fluid-rock coupling and perform numerical modeling in relatively simple settings. In this study, we first introduce a general poromechanical framework to capture a broader range of hydro-mechanical coupling and then apply it to model real-world SWD problems with geological and operational complexities. We demonstrate a viable workflow through the case study of a 2500 km2, layered, faulted, and recently seismically awaken area that has undergone multi-decadal multi-zone injection by nearly 200 SWD wells in the Midland basin. The example provides a unique opportunity to study the respective impact of shallow and deep injections across depths and the role of inter-zone interactions, among others. Our modeling suggests over 500 and 250 psi increases in pore pressure and Coulomb stress, respectively, in the deep disposal zone in the last decade, which were 5 to 10 times greater than those accumulated over the past four decades in the two shallower disposal zones. Vertically, shallower injections impacted mostly shallower zones, whereas the deep injection produced changes across depths and dominated especially in the deep zone and the upper segment of the basement. Towards the mid-segment of the basement, changes due to shallow and deep injections were comparable, with the latter 2 to 5 times higher. Laterally, in-zone changes were dominated by pore pressure diffusion in the near field and poroelastic stressing in the far field in shallower zones but were dominated by pore pressure diffusion everywhere in the deep zone, owing to their different hydraulic diffusivities. Faults acted as pore pressure and Coulomb stress sinks for disposal zones and sources for non-disposal zones, and fault-zone permeability structure had a minor impact. Finally, seismological modeling suggests that seismicity in the area was most likely driven by the deep injection.

Research on dynamic p-y curves in liquefied sand under cyclic loading conditions

Under dynamic cyclic loads, the pore water pressure in saturated sand rises continuously, which leads to the reduction of horizontal bearing capacity or the damage of the pile foundation. Under dynamic cyclic loads, the existing calculations methods of horizontal bearing capacity of pile foundations were obtained by proposing a reduction factor for the static p-y curve model mostly. Compared with the test results, the soil resistance around the pile obtained by this method is often overvalued and the pile-soil relative displacement has a relative underestimation. As a result, this method is not suitable for pile foundation design. Therefore, the variations of soil resistance around the pile and the pile-soil relative displacement are obtained by analyzing the effect of pore pressure ratio on soil strength around the pile in saturated sand in this paper. The theoretical formulas related to the cyclic vibration times and pore pressure ratio are proposed. The proposed formulas are introduced into the dynamic p-y curve model to obtain the dynamic p-y backbone curve in the process of sand liquefaction. The research results show that the variations of the soil resistance around the pile and the pile-soil relative displacement are related to the pore pressure ratio and the cyclic vibration times during the sand liquefaction. As the cyclic vibration times increases, the pore pressure increases, the soil resistance around the pile decreases and the pile-soil relative displacement increases.

Poroelastic response of inclined wellbore geometry in anisotropic dual-medium/media

Shale gas formations are typically characterized by anisotropic and fracture properties. Existing shale wellbore stability models have only considered the anisotropy of shale rocks and have ignored the effect of fractures. However, the mechanism of wellbore instability under the combined effects of shale anisotropy and dual-media (DM) remains unknown. Therefore, based on anisotropic poroelasticity, the DM theory, and the assumption of generalized plane strain, this study established a pseudo-3D hydraulic-mechanical coupling finite element model considering the DM and anisotropy of shale in wellbores. The effects of elastic anisotropy, fracture volume fraction, fracture permeability, mass transfer coefficient, and fluid pressure on wellbore stress were analyzed parametrically. The results showed that an anisotropic single-medium predicts that the wellbore will fail when the formation is drilled, whereas an anisotropic DM predicts periodic wellbore instability. The poroelastic effect of the matrix pore pressure is much greater than that of the fracture pore pressure. The matrix elastic anisotropy parameters control the distribution of mechanical parameters throughout the DM, further affecting the poroelastic response of the entire DM. When the drilling direction was mostly parallel to the bedding plane, the anisotropy of the matrix and fracture system of the elastic parameters were larger, and the wellbore was more stable. A larger fracture permeability leads to a larger fracture volume fraction and mass transfer coefficient, the matrix pore pressure and fracture pore pressure increase, and the instability of the wellbore also increases.

A small amount of water reduces the friction of the preferentially oriented montmorillonite gouge

The presence of smectite may control the updip seismogenic boundary of a subducting plate. The smectite near the boundary can be dehydrated and highly preferentially oriented in the smectite-to-illite transition zone. Herein, the frictional properties of preferentially oriented (PO) montmorillonite gouge were examined under dry conditions. Highly PO montmorillonites were prepared as self-supporting sheets, which were dried at 70–200 °C for 12 h before conducting shear experiments. The double direct-shear tests on these PO sheets and nominally randomly oriented (NRO) montmorillonite were conducted at room temperature and dry conditions under applied normal stress from 5 to 40 MPa. The friction coefficients of the NRO gouges (0.42–0.45) were not affected by drying temperature, while those of the PO sheets increased with increasing drying temperature from 0.32 to 0.52 at normal stress of 5 MPa. The estimated water contents for both samples depended on the drying temperature, but it was significantly lower (<1 mass%) than the upper limit of the water content of 4.5 mass% for dry friction as estimated from previous studies. A small amount of water (<1 mass%) might become vapor by frictional heating in the low porosity, low permeability PO smectite gouge and increased pore pressure resulting in the decrease of the effective normal stress and shear stress. Although there is much to consider under the natural conditions of smectite-to-illite transition, it was found that preferentially oriented smectite showed a low friction coefficient, even in a dehydrated state.

Effects of water injection on the stress field in the changning shale gas field, Sichuan Province, southwest China

It is generally considered that injection-induced seismicity is related to injection duration, injection volume, geological settings, and spatial relationships between the wells and preexisting faults. The seismicity in the Changning shale gas field, which is the largest shale gas field in China, has increased significantly since 2012. A 3D finite element model based on this area is established in our study for calculating the effect of injection on the regional stress field. The modelling results show that the permeability of sedimentary rock is a key factor for determining the fluid diffusion range and affects the stress field in deeper strata by controlling the gravity load. This effect is sufficient to induce earthquakes under critical conditions. Under a reasonable permeability (1.45 × 10−15 m2), injection may induce shallow-focus earthquakes (<3 km) by pore pressure increase and deep-focus earthquakes by increasing the gravity load. In addition, the low-permeability detachment surface below the injection well may buffer the perturbation of the stress field beneath it during injection.

Relationship between structural style and the petroleum system in the siba gas field, southern Iraq

The Siba gas field is the only non-associated gas field in southern Iraq, located in an area primarily for oil production. The study aimed to identify the reasons for gas generation and accumulation in the field's Lower Cretaceous Yamama reservoir and the source of the gas. The study focuses on seven wells from the field, namely Siba-1, 4, 5, 6, 7, 8, and 9, and the structural and tectonic settings of the field were studied by interpreting seismic data. Geometric analysis showed that the Siba field is a non-cylindrical, asymmetrical, open, anticlinal fold. The anticline is northeast-southwest-trending with two culminations, of which the northeastern one is the higher by about 100 m. The Siba field overlies deepest parts of the Mesopotamian foreland basin, almost adjacent to the Zagros orogenic front, where burial depth and thermal conditions were ideal for organic-matter maturation and increased pore pressures that lead to hydrocarbon migration from Middle Jurassic source rocks to the Lower Cretaceous Yamama Formation reservoir. The geochemical parameters of oils from the Yamama Formation are like those of oils from Sargelu Formation in adjacent fields, indicating that the Middle Jurassic Sargelu Formation is the most likely source of Siba hydrocarbons. Vitrinite reflectance and thermal analyses of oils from the Yamama reservoir (0.87% R0; 128.1 °C) and from the Sargelu Formation (1.36% R0; 156.96 °C) show that only the Sargelu Formation had the right burial depths and temperatures in this area during Miocene deformation to source Yamama hydrocarbons in the Siba field. Although the anhydrites of the Upper Jurassic Gotnia Formation form a prominent regional seal separating Jurassic source rocks from Cretaceous reservoirs, structure-contour maps and seismic analyses indicate that many faults and fractures, mostly related to Miocene deformation, penetrate Gotnia anhydrites. Hence, the study interprets that these faults, as well as facies changes on the margin of the Gotnia basin, seriously impaired the Gotnia seal, allowing the vertical migration of hydrocarbons from the Middle Jurassic Sargelu Formation into the Lower Cretaceous Yamama reservoir in the Siba field.

2000 yrs of earthquakes inferred from subsidence events on the Imperial fault, California: Effect of lake-level changes and implication for variable slip rates

The Salton Trough has filled repeatedly with freshwater Lake Cahuilla from avulsions of the Colorado River. The relationship between lake fillings and large earthquakes is a topic of debate. From a new study on the Imperial fault applying cone penetrometer soundings, we show that the long hiatus in lake fillings between about ca 100 BCE and 950 CE resulted in accelerated slip in the few hundred years after re-inundation, an observation that is also seen on the San Andreas and southern San Jacinto faults. A regional, basin-wide signal of transient accelerated slip is interpreted to be the result of the effects of increased pore pressure on fault strength resulting from the 100 m of water load during full lake inundations. These results demonstrate the effects of dramatically increasing pore pressure, termed the reservoir induced seismicity effect, on fault activity and explain the current “earthquake drought” on the southernmost section of the San Andreas fault.

Isotropic compression behavior for methane hydrate-bearing soil with varying temperature and pore pressure

Hydrate exploitation may trigger large-scale seafloor subsidence, especially when the depressurization method is used. The isotropic compression characteristics for methane hydrate-bearing soil (MHBS) need to be investigated to evaluate this deformation failure. Change in temperature and pore pressure (TP) condition can generate a considerable influence on the mechanical deformation and strength of MHBS. However, few studies focus on the isotropic compression deformation of MHBS. This paper carried out a series of isotropic compression tests on MHBS with different hydrate saturations and TP conditions. Experimental results show that the MHBS specimens exhibit a completely different isotropic compression curve at different hydrate saturations and TP conditions. The compressibility of MHBS is reduced remarkably with increased hydrate saturation, raised pore pressure, and decreased temperature. Hydrate saturation does not affect the compression index, but it explicitly reduces the swelling index. The yield stress is also enhanced with increasing hydrate saturation. The compression index of MHBS is independent of TP condition, while the swelling index is weakened and the yield stress is enhanced as temperature decreases and/or pore pressure increases. Through employing the phase state parameter as an indicator for TP condition, the influence of TP condition on the isotropic compression characteristics of MHBS is analyzed. Besides, the isotropic compression mechanism of MHBS at different hydrate saturation and TP condition is illustrated.

Pore pressure effects on the anisotropic joint elastic-electrical properties of artificial porous sandstones with aligned fractures

Fractures widely exist in rocks and joint interpretation of elastic and electrical survey data can improve the characterization of fractured rocks. However, the cross-property relationship between the elastic and electrical rock properties, which is the key to the successful joint elastic-electrical interpretation, remains poorly understood in fractured rocks with varying pore pressure. This knowledge gap was by developing a method for making calcite-cemented sandstones with penny-shaped fractures and by simultaneously measuring the anisotropic elastic and electrical properties of the artificial sandstones with and without fractures as a function of pore pressure through the development of an anisotropic joint elastic-electrical measurement system. We find that elastic velocities and electrical resistivities of the intact and fractured samples reduce with increasing pore pressure, and the reduction in the physical properties of the fractured rock is less significant than that of the intact sample. We also find that elastic and electrical anisotropic parameters decrease in the sample without fractures but decrease first and then increase with increasing pore pressure in the sample with fractures. Most interestingly, the anisotropic elastic and electrical properties are found to exhibit a strong linear correlation in the intact and fractured samples with varying pore pressure and a difference exists in the slopes between the samples with and without fractures. The experimental data are explained and interpreted through the contrary contributions of the dilation of the random microcracks and the aligned fractures with increasing pore pressure to the anisotropic elastic and electrical properties. The results reveal the pore pressure effects on the anisotropic joint elastic-electrical properties of sandstones with fractures and provide a potential for the detection of overpressure in fractured rocks through integrated elastic and electrical surveys.

Numerical simulation of three-dimensional fracturing fracture propagation in radial wells

A fracture propagation model of radial well fracturing is established based on the finite element-meshless method. The model considers the coupling effect of fracturing fluid flow and rock matrix deformation. The fracture geometries of radial well fracturing are simulated, the induction effect of radial well on the fracture is quantitatively characterized, and the influences of azimuth, horizontal principle stress difference, and reservoir matrix permeability on the fracture geometries are revealed. The radial wells can induce the fractures to extend parallel to their axes when two radial wells in the same layer are fractured. When the radial wells are symmetrically distributed along the direction of the minimum horizontal principle stress with the azimuth greater than 15°, the extrusion effect reduces the fracture length of radial wells. When the radial wells are symmetrically distributed along the direction of the maximum horizontal principal stress, the extrusion increases the fracture length of the radial wells. The fracture geometries are controlled by the rectification of radial borehole, the extrusion between radial wells in the same layer, and the deflection of the maximum horizontal principal stress. When the radial wells are distributed along the minimum horizontal principal stress symmetrically, the fracture length induced by the radial well decreases with the increase of azimuth; in contrast, when the radial wells are distributed along the maximum horizontal principal stress symmetrically, the fracture length induced by the radial well first decreases and then increases with the increase of azimuth. The fracture length induced by the radial well decreases with the increase of horizontal principal stress difference. The increase of rock matrix permeability and pore pressure of the matrix around radial wells makes the inducing effect of the radial well on fractures increase.

En echelon faults reactivated by wastewater disposal near Musreau Lake, Alberta

SUMMARY We use machine-learning and cross-correlation techniques to enhance earthquake detectability by two magnitude units for the earthquake sequence near Musreau Lake, Alberta, which is induced by wastewater disposal. This deep catalogue reveals a series of en echelon ∼N–S oriented strike-slip faults that are favourably oriented for reactivation. These faults require only ∼0.6 MPa overpressure for triggering to occur. Earthquake activity occurs in bursts, or episodes; episodes restricted to the largest fault tend to have earthquakes starting near the southern end (distant from injectors) and progressing northwards (towards the injectors). While most events are concentrated along these ∼N–S oriented faults, we also delineate smaller faults. Together, these findings suggest pore pressure as the triggering mechanism, where a time-dependent increase in pore pressure likely caused these faults to progressively reawaken. Analysis of the ‘next record-breaking event’, a statistical model that forecasts the sequencing of earthquake magnitudes, suggests that the next largest event would be ML ∼4.3. The seismically illuminated length of the largest fault indicates potential magnitudes as large as Mw 5.3.

Separating Primary and Secondary Mechanisms for Seismicity Induced by Hydraulic Fracturing

ABSTRACT Hydraulic fracturing operations are generally associated with induced seismic events with magnitudes that are typically lower than those of tectonic-based earthquakes. Although fluid migration and the resulting pore-pressure increase appear to be the primary driving mechanisms, one must also understand fault activation and the secondary interevent triggering process(es) leading to the generation of aftershock sequences. This requires distinguishing between seismic events that are a direct consequence of fluid injections and those that predominantly arise because of secondary mechanisms such as static or dynamic stress triggering caused by preceding earthquakes. Focusing on a case study in the Montney area in western Alberta, Canada, where both surface array and borehole geophone data are available, we (1) establish the importance of secondary mechanisms and aftershock sequences in this area and (2) show that the statistical properties of the aftershock sequences can allow one to establish whether significant fault activation is at play and whether the activated fault zone was critically prestressed. Specifically, we find evidence for aftershock sequences using different statistical methodologies, although the presence of aftershocks in the studied hydraulic-fracturing-induced context is significantly less dominant compared with tectonic seismicity. The identified aftershock sequences are characterized by long-range spatial aftershock zones, setting them apart from other cases in fluid-driven settings but comparable to tectonic aftershock sequences and indicative of fault activation. Despite this observation, we find overall a strong dominance of smaller triggers for aftershock sequences and an aftershock production associated with large-magnitude events that is significantly lower than what has been observed in tectonic settings. This suggests that the activated fault zone as a whole was not critically prestressed, and the associated initial seismic hazard was significantly lower compared with tectonic cases.

Tracking supercritical geothermal fluid distribution from continuous seismic monitoring

Continuous seismic monitoring could play a pivotal role in deep geothermal energy exploration. We monitored seismicity near geothermal production areas of the Kuju volcanic complex with a dense seismic network and automated event detection. Most events were shallow (less than 3 km below sea level) and distributed along a boundary between regions of high and low resistivity and S-wave velocity, interpreted as a lithological boundary or related fracture zone. Deeper events located on top of subvertical conductors may reflect fracturing associated with magmatic fluid intrusion. A correlation may exist between seismicity and heavy rainfall three days prior to increased pore pressure in pre-existing fractures. Our findings support the presence of supercritical geothermal fluids and demonstrate the importance of continuous seismic monitoring in supercritical geothermal energy exploration.

Impact of Adsorption Swelling and Slippage Effects on Changes in Coal’s Permeability at Different Temperatures

As a critical parameter, coal’s permeability influences the coalbed’s methane (CBM) production efficiency, which, in turn, is affected by a slippage effect, adsorption swelling, and effective stress in the CBM exploitation process. In this paper, an isothermal adsorption experiment under various temperatures, including seepage tests comprising CH4 and He in coal at different temperatures under constant effective stress, was carried out separately. We established a modified Shi&Duruca (S&D) model that considered constant effective stress and a modified adsorption model, in which the effect of excess adsorption capacity and temperature was considered. The results revealed that swelling deformation occurred when coal adsorbed CH4, which would result in the permeability of CH4 being lower than that of He under similar conditions. With an increase in pore pressure, coal’s permeability decreased gradually under the combined effects of slippage and adsorption swelling. Temperature is an important factor that affects the movement and storage of gas. Any change in temperature and pore pressure would result in coal’s permeability being comprehensively affected by thermal expansion, thermal cracking, and adsorption swelling, including the effects of slippage. Thermal expansion and thermal cracking increase coal’s permeability under low pressure and decrease coal’s permeability under high pressure. The modified adsorption model and permeability model could better characterize the adsorption and permeability properties of coal. The calculation results illustrated that the slippage effect and adsorption swelling are two important factors affecting permeability, with its permeability increment sharing a similar change trend with changes in permeability. Under the condition of low pressure, the adsorption swelling effect was dominant, and the contributary rate of the slippage effect on permeability would increase, obviously, with an increase in temperature.

Rate and State Simulation of Two Experiments With Pore Fluid Injection Under Creep Conditions

AbstractThis study uses a spring‐block model and rate and state friction to simulate experiments conducted in a double direct‐shear apparatus on carbonate fault gouge (Scuderi et al., 2017, https://doi.org/10.1016/j.epsl.2017.08.009) and on shale bearing rock (Scuderi &amp; Collettini, 2018, https://doi.org/10.1029/2018jb016084). Both sets of experiments used the same loading protocol and injected pore fluid under creep conditions. When velocity strengthening rate and state friction is used to simulate the experiments on the carbonate fault gouge the results agree well with the observed onset of tertiary creep in the experiment. Thus, the simulation reinforces the observation that pore fluid injection can induce rapid slip even when the friction relation is velocity strengthening. The rate and state framework provides an interpretation alternative to the standard one of the Mohr's circle moving to the left as pressure increases. In the rate and state framework, the friction coefficient must increase with pore pressure increase. The shale has a low nominal friction coefficient (0.28) and is much more velocity strengthening than the carbonate. The simulation agrees with the observations that increases in pore pressure induce an increase in slip velocity but the magnitudes reach only about 100 microns/s by the end of the experiment. The simulation for the shale also agrees well with the magnitude of the observed displacement at the end of the experiment but observed displacement is increasing much more rapidly the calculated. Although the calculations agree well with features of the observations near failure, the overall curves of displacement and velocity are significantly different.

Thermal Expansion and Adsorption Characteristics of Coal Based on Temperature Change and Its Influence on the Slippage Effect

The slippage effect is an important phenomenon of coalbed methane (CBM) migration. As the exploitation of CBM, the slippage effect becomes more and more obvious. For exploring the evolution of the slippage effect as CBM extraction by heat injection, the thermal expansion test and the temperature increasing desorption test under different stress states, as well as the seepage test under variable temperature conditions, were conducted by operating the high-temperature multifunctional triaxial test system. The effect rules for temperature and pore pressure on the slippage effect were analyzed, and then the evolution mechanism of the slippage factor at the united action of pore pressure and temperature was comprehensively analyzed based on thermal expansion deformation and adsorption characteristics. The results showed that: first, the influence of pore pressure on the slippage effect is mainly caused by the molecular mean free path and effective stress when pore pressure is greater than 0.8 MPa in the range of 30–150 °C. With the pore pressure increase, the slippage effect decreases. Second, the temperature influence on the slippage effect is mostly induced by the variation of cracks in coal due to thermal expansion deformation created by temperature. With the increase in temperature, the slippage factor average increases first from 30 to 60 °C is 19 times and then decreases from 60 to 90 °C is 36%. Finally, at the combined influence of temperature and pore pressure, the slippage effect is mainly influenced by thermal expansion deformation, desorption deformation, and effective stress. At 30–60 °C, the slippage factor is principally affected by thermal expansion deformation. At 60–90 °C, the slippage factor is mostly influenced by desorption deformation and effective stress. The research results provide a foundation for the technology of CBM exploitation by heat injection.

Validity of Juang's Method to Predict the Liquefaction Potential of the Soils of Sablette Algiers

This work presents a deterministic and probabilistic analysis of the liquefaction potential of soils. Soil liquefaction is a process leading to the total loss of shear strength of loose sands under seismic stresses by increasing pore pressure. It is accompanied by deformations. Certain factors related to the characteristics of the ground and the seismic excitation can significantly influence the appearance or not of the phenomenon. Empirical methods of liquefaction potential assessment are widely used. The probabilistic method has been widely used for estimating the probability of liquefaction. Initially, the uncertain parameters are modeled by normal random variables. A parametric study of the coefficients of variation of the random variables showed that the number of strokes (N1)60 of the SPT test is the parameter that has the greatest influence on the probability of liquefaction. This study focuses on the liquefaction potential by methods based on the results of the in-situ "Standard Penetration Test" Through the results obtained in this study of the site of Sablettes in Algiers, in general, there is a risk of liquefaction of the soil in the sandy layers, and we can say that the site studied has an average liquefaction potential. According to the classification of liquefaction potential proposed by Juang et al., (2012).

Effect of Pore Pressure on Strain Rate-Dependency of Coal

Viscoelastic strain rate-dependent behaviour of coal is critical in several subsurface engineering applications especially coal seams gas production. Such rate dependency is controlled by the interaction between coal bulk and gas sorption (a sorbing gas) or gas pressure (a non-sorbing gas). Despite the research conducted to date, the gas pressure effect (non-sorbing) on the viscous behaviour of sediments in particular coal remains unexplored. We, therefore, investigate the strain rate-dependent mechanical behaviour of coal under isotropic loading to specifically explore the effect of gas pressure (Helium) on its rate dependency eliminating the sorption effect. We perform a set of triaxial experiments on coal specimens at dry and pressurised gas (Helium) conditions under different strain rates under isotropic loading. The experimental results show that all coal specimens have viscoelastic strain rate dependency at a dry condition where viscous effect increases with strain rate. As a result, the bulk modulus of the specimens increases with the increase in strain rates. This strain rate dependency response, however, reduces with an increase in pore pressure and vanishes at a certain pore pressure under the same effective stress to that of dry specimens. We further employ X-ray micro-Computed Tomography (XRCT) to 3D scan a coal specimen saturated with Krypton gas undergoing different loading rates to shed light on the micro-mechanisms of gas pressure effect on specimens’ rate dependency. The XRCT results show that gas can be trapped in small-scale fractures and pores during the loading process leading to a localised undrained response that can stiffen the specimen and reduce its ability to show viscous rate dependency. The obtained results are significant in optimizing coal seam gas production and coal seam gas drainage applications.

Numerical investigation into wave-induced progressive liquefaction based on a two-layer viscous fluid system

Based on a two-layered viscous fluid theory, a numerical model to capture the progressive nature of wave-induced liquefaction has been developed. Unlike the existing models, this model can consider the cyclic shear stress associated with vibrating liquefied soil layers and its effect on sub-liquefied soils during the wave-induced liquefaction. The reliability of this model is validated by simulating wave flume tests, which show a promising prediction when compared to analytical solutions, particularly after the onset of liquefaction where an increased amplitude in the oscillatory pore water pressure can be observed. The numerical results show that the moving characteristics of liquefied soil are similar to those of a water particle in the presence of surface water waves with the horizontal velocity being much greater than the vertical velocity. Unlike the model without considering the liquefied soil-induced cyclic shear stress, the proposed model predicts a prompt increase in residual pore pressure associated with the onset of liquefaction at shallow soil layers, which may change the curvature of residual pore pressure versus time in sub-liquefied soils from concave downwards to concave upwards at a certain depth. This phenomenon is consistent with many of the existing experiments for wave-induced seabed response and becomes more pronounced as the kinematic viscous characteristics of liquefied soil become more apparent, i.e., with a larger kinematic viscosity coefficient. Correspondingly, the cyclic shear stress induced by the vibrations of liquefied soil can accelerate the downward advancement of the liquefaction front and result in a larger depth of liquefaction.

Thermally Induced Pore Pressure Build Up in Saturated Porous Low Permeable Formation

Heating a saturated low permeable geological formation induces pore pressure build-up, so-called thermal pressurization, due to the difference in thermal dilation of the solid skeleton and the pore water. The excess pore pressure generally leads to a decrease in the effective stress and can provoke thermally hydraulic fracturing or shear failure. This paper aims at deriving an explicit solution of the pore pressure increase as a linear function of the temperature variation and total mean stress variation in the framework of linear poroelasticity under undrained conditions. The thermal pressurization coefficient is deduced and then under the condition o constant total stress, which is a function of the difference between the thermal expansion coefficients of the pore water and the porous medium, porosity, bulk moduli of water, porous medium and solid phase. The finite element solution is used to show the accuracy of mathematical development. The presence of this solution allows evaluation the effect of temperature dependent thermal expansion coefficient and temperature and pressure dependent bulk modulus of water. An equivalence between thermo-poro-elastic and thermo-elastic problems are established. This equivalence allows determining the pore pressure from a thermo-mechanical simulation. Moreover, the undrained dilation coefficient and undrained bulk modulus are also derived from this analysis.