Pore Pressure Changes Research Articles

Effect of spatially variable saturated hydraulic conductivity with non-stationary characteristics on the stability of reservoir landslides

Conventional landslide analyses, based on deterministic methods or simple random variable methods, cannot consider the spatial variability and the depth-dependent nature of saturated hydraulic conductivity (Ks) in accumulative landslides. However, studies show that these conditions exist objectively and have a large impact on the seepage and deformation of landslides. Thus, the non-stationary characteristics of Ks need to be properly accounted for. In this paper, the Baishuihe landslide is taken as a case study. First, based on surface nuclear magnetic resonance (surface-NMR) technology, spatial variability and the depth-dependent nature of the Ks are obtained. Then, a non-stationary random field model is established to model the trend and fluctuating components of the Ks individually, and a deterministic model is established to model the trend of the Ks with depth. Finally, numerical simulations of the stochastic model and deterministic model under reservoir water fluctuation conditions are carried out by the non-intrusive stochastic finite element method, and the seepage-deformation and stability characteristics of the landslide are analyzed. The results show that compared with that in the deterministic model, the change in pore pressure in the stochastic model obviously lags, during reservoir water drawdown, the displacement of the trailing edge in the stochastic model is larger than that in the deterministic model, with increasing reservoir water deceleration, the hysteresis of the pore pressure in the stochastic model is more notable, and the deformation is larger and the stability is worse in the stochastic model. It is demonstrated that ignoring the spatial variability of the Ks will overestimate the stability of the landslide.

A parametric theory of isotropic consolidation for saturated geomaterials with compressible phases

Abstract Theories of isotropic consolidation for geomaterials with compressible phases require two loading tests: a full-drainage test at constant pore-pressure and a mixture test in which pore-pressure varies. In a no-drainage mixture test, the pore-pressure change falls short of the confining pressure change. In the unjacketed mixture test, the pore-pressure and confining pressure rates are equal. This paper develops a parametric theory that supports extrapolation of unjacketed bulk compressibility from the compressibilities measured in the full-drainage and mixture tests, elaborates and clarifies the isotropic version of the Biot and Willis theory of consolidation and expresses the unjacketed pore compressibilities of several sandstones in a combination of macroscopic and mesoscopic terms. The mesoscopic model includes solid and fluid phase compressibilities and phase-pressure distribution ratios. The coefficient of fluid content is identified as the product of porosity and the pore flux coefficient. The compressibility-induced component of the strain-rate in an unjacketed test is postulated to be an objective measure. This solution provides a framework for geotechnical modelling software that simulate the effect of porosity variation due to consolidation. With this theory, engineers can infer the phase-pressure distribution ratios from the measured Brown-Korringa compressibilities and represent the effects of both meso-heterogeneity and solid-grain rearrangement in constitutive models.

New insights into the mechanisms of seismicity in the Azle area, North Texas

We have performed a site-specific study of the mechanics of induced seismicity in the Azle area, North Texas, using a coupled 3D fluid flow and poroelastic simulation model, extending from the overburden into the crystalline basement. The distinguishing feature of our study is that we account for the combined impact of water disposal injection and gas and water production on the pore pressure and stress distribution in this area. The model is calibrated using observed injection wellhead pressures and the location, timing, and magnitude of seismic events. We used a stochastic multiobjective optimization approach to obtain estimated ranges of fluid flow and poroelastic parameters, calibrated to the pressure, rate, and seismic event data. Mechanisms for induced seismicity were examined using these calibrated models. The calibrated models indicate no fluid movement or pressure increase in the crystalline basement, although there is plastic strain accumulation for the weaker elements along the fault in the basement. The accumulation of strain change appears to be caused by the unbalanced loading on different sides of the fault due to the differential in fluid injection and production. Previous studies ignored the produced gas volume, which is almost an order of magnitude larger than the produced water volume under reservoir conditions and which significantly impacts the pore pressure in the sedimentary formations and the stress distribution in the basement. A quantitative analysis indicates that the poroelastic stress changes dominate in the basement with no noticeable change in pore pressure. Even though the low-permeability faults in the basement are not in pressure communication with the Ellenburger formation, the poroelastic stresses transmitted to the basement can trigger seismicity without elevated pore pressure.

Effects of patch size changes on the time-lapse seismic reflections of highly attenuating reservoirs

Reservoir fluid depletion and injection inevitably induce changes in pore pressure and saturation, leading to a varying fluid patch size and hence perturbations in seismic velocities and observable time-lapse seismic attributes. The patch size, which may exhibit spatial variations, can possess a characteristic size at each scenario, and therefore another variable impacting time-lapse seismic signatures. In this study, we extend previous research efforts of time-lapse seismic interpretation by considering the fluid patch size effect on time-lapse seismic signals to investigate uncertainties in time-lapse seismic observations. We predicted and analyzed the influence of reservoir permeability and gas saturation on seismic acoustic properties, i.e., attenuation and velocity dispersion, reflectivity and phase, on the basis of a generalized White's spherical model for varied patch sizes on millimeters to centimeters. A stratified model consisting of a fluid-saturated sandstone reservoir showing viscoelastic properties sandwiched between two elastic half-spaces is employed, so as to give insights into the impacts of fluid extraction induced changes of velocity dispersion and attenuation, as well as acoustic impedance contrast, on time-lapse seismic reflections. Analysis of time-lapse seismic travel-time and reflection amplitude differences indicates that ignoring the time-dependent patch size behaviour can give rise to underestimation or overestimation of the reservoir fluid saturation and permeability change.

Resolution limits of fluid overpressures from mineralogy, porosity, and sonic velocity variations in North Sea mudrocks

Pore pressure identification in mudrocks largely relies on indirect methods, including analyses of sonic velocity data. The magnitude of pore pressure changes that can be resolved from such data depends on the impact of fluid pressures on well log responses and on the velocity responses to lithological variations that could be mistaken for indications of pore pressure changes. Analysis of relationships between sonic velocity, mineralogy, and porosity from 11 North Sea wells demonstrated porosity variations of up to 5% from the mean value in mudrocks at any given depth between 1 and 3.5 km (3300–11,500 ft). Comparisons of these wells with seven wells from the Norwegian Sea demonstrated that the sonic velocity and density variations are influenced by the content of silica with a biogenic origin, but that they can otherwise not, in general, be related to mineralogy and pore pressure variations. The sonic velocity deviations from the mean value are of the same magnitude as those that would be caused by porosity changes of ±8% in mudrocks of uniform lithology. Furthermore, the deviations are not significantly reduced when the velocity data are compensated for changes in density (porosity). Detection limits for mechanically compacting mudrocks that are at their maximum effective stress are equivalent to static mud density changes of 0.1–0.15 g/cm3. These numbers apply to mudrocks with similar lithological variability to those analyzed in our data set but should be much higher when significant amounts of silica with a biogenic origin are present. The impact of effective stress changes on the sonic velocities of chemically compacting as well as of mechanically unloading rocks is much less because small or no porosity changes follow from the effective stress changes in such cases. The velocity responses for mechanically unloading and stress-insensitive chemical compaction can only be identified from four-dimensional (time-lapse) seismic data unless the rocks are close to the fracturing limit. These observations imply that reasonably accurate velocity-based pore pressure identification in North Sea mudrocks requires a knowledge of local mudrock heterogeneity and compaction that cannot easily be obtained. As a result, sonic velocity data should be treated with considerable caution in pore pressure evaluation.

An effective stress theoretical model for shear resistance and adhesion factor of dynamically installed anchors

This paper proposes a theoretical effective stress model for estimating the shear resistance and the time-dependent adhesion factor of dynamically installed anchors. The anchor installation process is approximated by a cylindrical cavity expansion process and the residual friction at the soil–anchor interface is assumed to be fully developed. The shear resistance and adhesion factor just after installation are first determined by consideration of the short-term changes in total stresses and excess pore pressure around the anchor. This allows the short-term effective stress state of the soil around the shaft to be evaluated. For a finned anchor, the circumferential stresses on the fins are considered explicitly. The time-dependent adhesion factor is then evaluated by consideration of the changes in total stress and pore pressure in the vicinity of the anchor during post-installation consolidation. Reasonably good agreement is obtained between the predicted results and the results of centrifuge model tests, as well as commonly recommended methods. Finally, a simplified procedure for estimating the total shear resistance and adhesion factor approximately is proposed, which simplifies the evaluation of the mathematically complex functions and allows a quicker estimation of the adhesion factor and shear resistance. An important finding in the study is that the fins are less efficient in mobilising interface friction than the shaft because the effective circumferential stresses are invariably lower than the effective radial stresses. As a result, the shear resistance that can be mobilised by the fins is correspondingly lower.

Deformation‐Based Monitoring of Water Migration in Rocks Using Distributed Fiber Optic Strain Sensing: A Laboratory Study

AbstractRock deformation induced by pore‐fluid pressure carries useful information about fluid flow owing to hydromechanical coupling. Thus, obtaining spatiotemporal changes in rock deformation could provide improved understanding of the fluid and pressure migration in aquifers or the role of fluid in the evolution of rainfall‐induced landslide. Here we deployed high‐resolution Rayleigh‐scattering‐type distributed fiber optic strain sensing (DFOSS) to measure rock deformation while injecting water into low‐permeability dry sandstone. X‐ray computed tomography imaging was simultaneously used to visualize the water migration. DFOSS measurements showed the rock developed a dilation deformation that grew during water saturating process. The movement of water wetting front can be revealed by the changes in the measured distributed strain. Strain changes were shaped by poroelastic changes due to the fluid pressure buildup and swelling by the water–clay reaction (i.e., adsorption). The latter mechanism caused increase in the strain when water first entered the dry pore spaces and the change in pore pressure was slight. The mechanism continued contributed to the overall deformation to peak magnitude of ~600 μϵ together with poroelastic mechanism. However, after the rock was fully saturated, further deformation during the flow test can be explained by the poroelastic mechanism alone. Our study suggests that the two factors can be employed as signatures for effectively monitoring fluid behavior in natural sediments using DFOSS. Moreover, we obtained the spatial hydromechanical properties, permeability and stiffness, from the distributed strain measurement and pressure responses. Using DFOSS in the field may substantially improve our ability to monitor and model fluid activity related to rock deformations in reservoirs and rainfall‐induced landslides that would help in warning people of risks and preventing disasters.

Stratigraphic architecture and petrophysical characterization of formations for deep disposal in the Fort Worth Basin, Texas

Disposal of hydraulic fracturing flowback and produced water into Ordovician and Cambrian formations of the Fort Worth Basin (FWB), coupled with an increase in observed seismicity in the Dallas-Fort Worth area, necessitates an understanding of the geologic character of these disposal targets. More than 2 billion barrels (Bbbls) of wastewater have been disposed into the Ordovician Ellenburger Group of the FWB over the past 35 years. Since the implementation of the TexNet Earthquake Catalog (1 January 2017), more than 20 earthquakes of local magnitude ML2.0 or greater have been detected in the area, with depths ranging from 2 to 10 km (approximately 6500–33,000 ft). The cited mechanism for inducement of these earthquakes is reactivation of basement faults due to pore pressure changes, either directly related to proximal disposal or due to disposal volume buildup over time. Here, we present a stratigraphic and petrophysical analysis of FWB disposal targets and their relation to basement rocks. The Ellenburger consists of alternating layers of limestone and dolomite, with minor siliciclastics above the basement toward the Llano Uplift. Matrix porosity averages <5 porosity units (p.u.), with higher porosity in dolomitic layers than in limestone. Dolomite dominates at the top of the Ellenburger, which was exposed at the end of both the Lower and Upper Ordovician. Where crystalline basement rocks are penetrated, the composition ranges from granitic to chlorite-bearing metamorphosed lithology. The basement-sediment interface is frequently marked by increased porosity. An updated map of structure on top of basement indicates elevations ranging from outcrop at the Llano Uplift to more than [Formula: see text] ([Formula: see text]) subsea toward the northeast. The disposal zone pore volume is estimated from thickness and porosity maps and ranges from [Formula: see text] to [Formula: see text] billion barrels per square mile ([Formula: see text]).

Delayed Weakening and Reactivation of Rate‐and‐State Faults Driven by Pressure Changes Due to Fluid Injection

AbstractFault reactivation induced by pore pressure changes involves complex frictional phenomena because effective normal stresses vary in space and time due to fluid flow and rock deformation. The impact of time‐varying normal stresses on fault friction has been characterized in stress‐step laboratory experiments and modeled through extended rate‐and‐state laws that incorporate a stressing‐rate dependence of the state variable. Building on these rate‐and‐state models, we use 2‐D poroelastic simulations to understand how the evolution of pore pressures due to fluid injection affects fault strength and reactivation. A sharp increase in pore pressure, associated with fluid injection, leads to an increase in friction coefficient and to a delayed weakening of the fault. Conversely, a sharp pressure decrease during extraction leads to a delayed strengthening. Stressing‐rate effects emerge as a purely frictional mechanism that delays or accelerates the onset of fault slip, suggesting that reactivation under fluid injection may occur long after flow rates have decreased and pore pressures have stabilized at the fault. Hence, earthquakes induced by injection may ensue as a deferred process triggered several days later than predicted by simple estimates based on constant friction. The duration of the delay depends on the type of rate‐and‐state law used in calculations and on a characteristic relaxation time: The memory time over which the state variable evolves under stationary contact. Our results help understand the connection between injection protocols and frictional weakening mechanisms, suggesting that injection processes can be engineered to minimize the risk of induced seismicity for a given injected volume.

Analysis of instability mechanisms of natural fractures during the approach of a hydraulic fracture

A fluid-driven fracture propagation model dominated by viscosity-leak-off regime was presented to investigate the mechanical stability of natural fracture (NF) when hydraulic fracture (HF) approaches. The induced stresses generated by the approaching HF were calculated by a fully coupled integro-differential equation. The pore pressure effect caused by fracturing fluid leak-off was simulated based on dual porosity medium theory. The critical failure conditions of NF were determined based on maximum tensile criterion and Barton-Bandis criterion. The sensitivity of the stability of NF to in-situ stress anisotropy, approaching angle and distance, injection rate, fracturing fluid viscosity, and NF properties was analyzed in detail. Simulation results reveal that the shearing-mode failure of NF is obviously easier than the opening-mode failure, and the shear slip zone is much larger than the tensile failure zone. As HF approaches NF non-orthogonally, the induced debonding process is unstable, and the debonding zone is asymmetric with respect to the center of NF. The tensile-induced expansion zone is primarily located in the portion of NF ahead of the HF tip, however the shear-induced slip zone can even occur on NF behind of the HF tip. Induced stresses alone have negligible effect on the stability of the NF that is unconnected to the HF. The change in pore pressure due to leak-off effect dominates the final stimulated reservoir volume during large-scale fracturing. The instability of the NF dominates the propagation trajectory of the subsequent HF. A more suitable arrested/crossing condition by extending Gu-Weng criterion was established to predict the path of HF propagation.

Capillary pore pressure effects on acoustic to seismic coupling detection of buried containers

The effects of rainfall on one type of buried container has been investigated at a test site near the University of Mississippi. Simulated and natural rainfall experiments were carried out over a three-year time period. The measurements allow for weathering effects on the buried container seismic to acoustic ratio detection contrast. The container was a filled 20 l plastic fuel container buried 30 cm in the natural loess soil. Six containers were buried, two were controls, two were exposed to the natural weather and two were used for simulated rainfall experiments. A loudspeaker was used to excite vibrations in the ground and a co-located vertical component geophone and a microphone measured the out of plane seismic/acoustic ratio in the vicinity of the containers. The rain induced wetting phase causes the container’s acoustic-to-seismic coupling contrast to increase and resonant frequency to decrease. The water drying phase restores the original frequency responses of smaller amplitude and higher resonant frequency. The phenomenon is cyclical. The capillary pressure in the pore spaces of soil grains reduces with wetting and increases with drying. These pore pressure changes are controlling the soil stiffness and acoustic response of the buried container.The effects of rainfall on one type of buried container has been investigated at a test site near the University of Mississippi. Simulated and natural rainfall experiments were carried out over a three-year time period. The measurements allow for weathering effects on the buried container seismic to acoustic ratio detection contrast. The container was a filled 20 l plastic fuel container buried 30 cm in the natural loess soil. Six containers were buried, two were controls, two were exposed to the natural weather and two were used for simulated rainfall experiments. A loudspeaker was used to excite vibrations in the ground and a co-located vertical component geophone and a microphone measured the out of plane seismic/acoustic ratio in the vicinity of the containers. The rain induced wetting phase causes the container’s acoustic-to-seismic coupling contrast to increase and resonant frequency to decrease. The water drying phase restores the original frequency responses of smaller amplitude and higher resonan...

Natural fractures within Knox reservoirs in the Appalachian Basin: characterization and impact on poroelastic response of injection

AbstractUnderstanding the distribution and orientation of natural fractures within Knox Groups is of significance in seeking potential CO2 storage zones with high practical storage capacity. Over 700 observations of natural fractures were interpreted on acquired resistivity and acoustic image logs collected at multiple well locations ranging in depth from 730 to 3900 m in the Knox Group interval on the western flank of Appalachian Basin. We evaluated the structural parameters of the fractures using statistical analysis. Natural fracture intensity was observed to increase up‐dip within the studied area. The present day maximum horizontal stress direction was derived using the interpretation of wellbore breakouts and drilling‐induced tensile fractures in image logs. Overall, a high percentage of fractures with varying dip directions were observed to strike subparallel to the contemporary maximum horizontal stress direction. Multiphase flow–geomechanics coupled numerical simulations and poromechanics analytical solutions were then used to study pressure and stress response of CO2 injection into the fractured Knox reservoirs. In addition, we applied a dual permeability model combined with a fracture activation model to study the permeability enhancement and its effect on injection mass increase. We also showed the line source injection solution can reasonably predict stress changes of CO2 injection into the deep saline formations. Results were analyzed to understand the potential effect of natural fractures in sandstone formations and fractured layers in thick carbonate formations on CO2‐injected mass, time‐dependent stress evolution, and the ratio of stress to pore pressure changes. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.

Fracture and pore structure dynamic evolution of coals during hydraulic fracturing

Hydraulic fracturing is an effective method for producing coalbed methane. To better understand the evolution of the fracture and pore structure of coals that contain macro-fractures during hydraulic fracturing, we have performed a series of in-situ compression experiments under continuous injection liquid conditions and dynamically monitored the samples using nuclear magnetic resonance methods. The mechanism of fracture and pore structure dynamic evolution is identified in terms of spherical expansion theory. The total porosity increases dramatically within 10–120 min of the initial injection stage, and pore channels form. The number of micro-pores (<10 nm) and transition pores (10–100 nm) continues to increase during the injection pressure reduction stage or increasing confining pressure stage. Total porosity tends to initially decrease and then increase when the confining pressure remains either constant or increases. Continuous liquid injection leads to a constant change of coal pore pressure and effective stress, such that pores break, close, and continuously reorganize. The results presented here provide a reference for permeability calculations, coalbed methane productivity predictions, and improved hydraulic fracturing techniques.

Nanoscale confined multicomponent hydrocarbon thermodynamic phase behavior and multiphase transport ability in nanoporous material

Abstracts Multicomponent hydrocarbon transport in nanoporous material is influenced by the nanoscale thermodynamic phase behavior, heterogeneous pore structure and hydrocarbon-molecules-pore wall interaction. Little is known about the multicomponent hydrocarbon transport mechanisms in realistic 3D nanoporous material. We here propose a nanoscale pore network multiphase multicomponent hydrocarbon transport model to study multicomponent hydrocarbon thermodynamic phase behavior and transport ability across a wide range of pressure and temperature. A novel thermodynamic phase equilibrium calculation model is proposed considering the multiphase interface curvature change and phase saturation in irregular pore cross-sections. The multicomponent gas slippage effect and liquid phase boundary slip length variation with pore size are incorporated into the mode. Study results find that for low liquid phase saturations with high capillary pressure, the liquid mainly resides in the pore corners, and for high saturations with low capillary pressure it forms liquid bridges. The formation of liquid bridges, and thus ultimately decrease in gas phase transport ability, depends on the interplay of multicomponent hydrocarbon composition, pressure, temperature, the local pore shape, pore size as well as three-phase contact angle. The multicomponent hydrocarbon gas phase permeability through 3D pore network is influenced by the composition and temperature when the pore pressure is less than 15 MPa, while multicomponent gas phase permeability is unaffected by the pore pressure and temperature change at relatively high pressure (>15 MPa). The liquid phase permeability increases with the increase of dense phase composition and the decrease of temperature at pressure less than 15 MPa. This work provides a new multicomponent hydrocarbon thermodynamic phase equilibrium model and a novel work flow of assessing multicomponent hydrocarbon multiphase transport ability in heterogeneous irregular nanoporous material.

Experimental Study on the Distribution of Wave-Induced Excess Pore Pressure in a Sandy Seabed around a Mat Foundation

Mat foundations are widely used in jack-up offshore platforms to support and transfer loads. Regarding mat foundations working on the seabed, the excess wave-induced pore pressure is critical to seabed stability, which may finally cause structural failure. Therefore, it is important to investigate the distribution of the excess pore pressure in the seabed around the mat foundation. In this study, experiments were performed to study the excess pore pressure distribution around a mat foundation in scale considering the true load state by recording wave profiles and pore pressures inside a sandy seabed. To guarantee the reliability of experiments, a numerical study was conducted and compared with the experimental results. Experimental results indicate that with the existence of the mat foundation, the excess pore pressure is higher at the region, the range of which is the width of the model mat (Wm) before the structure. The maximum pore pressure appears at 0.55 Wm in front of the center of the mat foundation. In addition, the current significantly increases the range of high pore pressure area and the amplitude of the excess pore pressure. As the mat orientation changes, the position of the maximum pore pressure changes from the front to the edge of the mat.

Study on the Influence of Pore Water Pressure on the Stability of Slope

The influence of pore water pressure on slope stability under water drop and seismic action is studied in this paper, taking an embankment as the research object. The temporal changes of seepage field are analyzed first, and then the stability of dam slope is investigated by coupling the stress field with the seepage field. During the process of water drop, the minimum safety factor of slope stability appears at the moment when it stops falling. The greater the falling speed is, the faster the pore water pressure dissipates, and the smaller the safety factor is. Seismic action stimulates the increase of excess pore pressure, and whether to consider the coupling effect of seismic load and pore pressure change will make the results of stability analysis quite different. The cumulative effect of pore pressure may be the crucial factor of the dynamic instability of two-phase media slopes.

닫힌 시스템에서의 CO2 주입에 따른 저장층 내 공극압력 변화와 그 기작

닫힌 시스템에서의 CO2 주입에 따른 저장층 내 공극압력 변화와 그 기작

The effects of mineral distribution, pore geometry, and pore density on permeability evolution in gas shales

We explored the ensemble effects of pore density, pore geometry, and pore stiffness on the permeability evolution of an ellipsoidal pore under both applied uniaxial stress and relative pore pressure change. We found that rocks undergoing identical compressional strain and pore pressure can undergo significantly different magnitudes of pore closure or dilation based on the concomitant influence of these three variables. This is especially important in gas shales, where nano-porosity is challenging to characterize and heterogeneity at all scales has led to disparate permeability responses in both the field and laboratory. Simulations were carried out using a finite element solver, a method that allowed each variable to be studied in isolation and concomitantly. We found that the aspect ratio is the most sensitive parameter influencing pore compressibility. The pore density becomes important when external stress is applied, but it has no significant effect when pore pressure is varied in the absence of external stress. To capture the effects of mineral precipitation and transformation in pore walls, we simulated mismatches between the mineral stiffness of the pore and the surrounding matrix. We determined that for a given strain, mineralogically soft pores (soft relative to the bulk material) experience higher increase in permeability than pores that are mineralogically stiff relative to the surrounding matrix. While soft pores experience greater closure than stiff pores for a given applied stress, they also experience a greater amount of dilation when pore pressure increases.

Pore pressure profiling in Siercza landslide colluvium in the Carpathian flysch using a Cone Penetration Test (CPTU)

This study identifies zones with significant changes in pore water pressure influenced by landslide processes. Measurements were taken in the near-surface strata of the Carpathian flysch, in colluvium of the Siercza landslide, near Krakow. Measurement of pore water pressure in flysch deposits is complicated due to the strongly heterogeneous properties of the medium and by variable water conditions, which are strongly influenced by rainfall intensity. Pore pressure profiling was performed in six series using a cone penetration test with a NOVA Acoustic cone. The tests were carried out in the colluvium to a depth of ~6.0 m under varying water conditions. The cone pore pressure results were compared to results of inclinometer measurements in the research area. Five zones with significant differences in pore pressure have been identified. Changes in both cone pore pressure and inclinometer displacement are evident at a depth range from 1.5 to 2.5 m. Two slip surfaces are likely present in this section. Such information can be used in engineering practice for more reliable assessment of slope stability in the Carpathian flysch

Stress heterogeneities and failure mechanisms induced by temperature and pore-pressure increase in volcanic regions

We study the strain and stress fields produced by temperature and pore pressure increases within and outside a Thermo-Poro-Elastic (TPE) inclusion (the source region), embedded within a medium (the matrix) in isothermal drained conditions. This model is suitable to describe a crustal region in a volcanic environment pervaded by hot pressurized fluids released by an underlying magma chamber. After introducing the pertinent constitutive relations, a formal solution for the displacement field is provided in terms of the Green's function for an elastic medium with drained isothermal elastic moduli, employing a generalization of Eshelby (1961) procedure. If an unbounded medium is considered, a displacement potential can be introduced, obeying the Laplace equation within the source region and the Poisson equation within the matrix. If a spherically symmetric source region is considered, simple analytical solutions are obtained for the displacement, the strain and the stress fields, showing that thrust faulting mechanisms are promoted within the source region while normal faulting mechanisms prevail in the embedding matrix. Employing reasonable numerical values for the thermo-poro-elastic parameters, suitable to describe highly porous sedimentary rock, strain and stress variations are found to be significant even for moderate changes of temperature and pore pressure. Variations of the Coulomb failure function are high in the TPE region and are strongly dependent on the friction coefficient and pore pressure. Application of these results to the 1982-84 and 2011-13 unrest episodes at Campi Flegrei caldera (Italy) suggests that an oblique dike intrusion across a previously unfaulted TPE region took place with a mixed tensile-thrust dislocation mechanism in both events, as previously inferred from accurate inversion of geodetic data.