Fluid Pressure Variations Research Articles

Relating mass movement with electrical self-potential signals

SUMMARY Landslides present a latent danger to lives and infrastructure worldwide. Often such mass movements are caused by increasing pore pressure. The electrical self-potential (SP) method has been applied in a broad range of monitoring studies. When fluid flow is involved the most relevant source of SP is the streaming potential, caused by the flow of an electrolyte through porous media with electrically charged internal surfaces. We experimentally investigated the SP signal associated with deformation of partially saturated soil. For partly saturated scenarios, we observed an SP signature correlated with the mass movement. In dry experiments, we did not observe any significant change in the electric signal. Results of numerical simulations match with the experimental observations when assuming a local and temporary alteration of the hydraulic pressure due to the sliding mass. Our findings suggest that SP measurements can be used to observe mass movement triggered by fluid pressure variations through the streaming potential.

Subsurface Fluid Pressure and Rock Deformation Monitoring Using Seismic Velocity Observations

AbstractFluid pressure within the Earth's crust is a key driver for triggering natural and human‐induced seismicity. Measuring fluid pressure evolution would be highly beneficial for understanding the underlying driving mechanisms and supporting seismic hazard assessment. Here we show that seismic velocities monitored on the 20‐m scale respond directly to changes in fluid pressure. Our data show that volumetric strain resulting from effective stress changes is sensed by seismic velocity, while shear dislocation is not. We are able to calibrate seismic velocity evolution against fluid pressure and strain with in situ measurements during a decameter‐scale fluid injection experiment in crystalline rock. Thus, our 4‐D seismic tomograms enable tracking of fluid pressure and strain evolution. Our findings demonstrate a strong potential toward monitoring transient fluid pressure variations and stress changes for well‐instrumented field sites and could be extended to monitoring hydraulic stimulations in deep reservoirs.

Frictional Properties of Subduction Input Sediments at an Erosive Convergent Continental Margin and Related Controls on Décollement Slip Modes: The Costa Rica Seismogenesis Project

AbstractThe spectrum of slip modes occurring along shallow portions of the plate boundary décollement in subduction zones includes aseismic slip, slow slip, and seismogenic slip. The factors that control slip modes directly influence the hazard potential of subduction zones for generating large‐magnitude earthquakes and tsunamis. We conducted an experimental study of the frictional behavior of subduction input sediments, recovered from two Integrated Ocean Drilling Program expeditions to the erosive subduction margin offshore Costa Rica (Expeditions 334 and 344), employing rotary shear under hydrothermal conditions. The velocity dependence of friction was explored, using simulated gouges prepared from all major lithologies, covering a wide range of conditions representative for the initial stages of subduction. Temperature, effective normal stress, and pore fluid pressure were varied systematically up to 140 °C, 110 MPa, and 120 MPa, respectively. Sliding velocities up to 100μm/s, relevant for earthquake rupture nucleation and slow slip, were investigated. The only sediment type that produced frictional instabilities (i.e., laboratory earthquakes) was the calcareous ooze carried by the incoming Cocos Plate, which by virtue of its slip‐weakening behavior is also a likely candidate for triggering slow slip events. We evaluate this mechanism of producing unstable slip and consider alternatives. Therefore, locking and unlocking of plate boundary megathrusts are not only related to variations in pore fluid pressure but may also depend on the presence of pelagic carbonate‐rich lithologies. Subduction systems containing such input are likely low latitude, where extensive deposition of carbonates takes place above the carbonate compensation depth.

Temporal changes in the internal stresses and pore pressures in a large-scale submarine mass transport deposit

We examined the temporal changes in the internal stresses and pore fluid pressures of a submarine mass transport deposit (MTD) in the Akkeshi Formation of the Upper Cretaceous–Paleocene Nemuro Group, eastern Hokkaido Island, Japan. We first analyzed previous paleostress field results from meso-scale faults in the MTD blocks, which indicated two phases during the evolution of the debris flow: phase I, radial spreading of the flow body during downslope movement; phase II, the flow body underwent compression during deposition on the basin plain. We also estimated the pore fluid pressure ratio from the fault orientation distribution. There was a large increase in the pore fluid pressure ratio during the transition from phase I to phase II that continued to rise during the initial stage of phase II and then decreased in its latter stages, whereas the maximum horizontal compressive stress increased throughout phase II. This variation in pore fluid pressure relates to the dynamics and evolution of the debris flow, where the clasts in the central part of the flow were supported by the excess pore pressure due to the compression of the debris flow as the flow head decelerated. Although pore fluid pressure plays a critical role in the dynamics of debris flows, there was no previous methodology to quantify both the stress fields and pore fluid pressures in large debris flows and their resultant MTDs. Our results implemented for outcrop studies imply that meso-scale faults in MTDs can provide clues to better understand these paleoflow mechanisms.

Microseismic Emissions During Pneumatic Fracturing: A Numerical Model to Explain the Experiments

AbstractModeling of fluid injection processes into a deformable porous medium is a challenging area of physics that has a wide range of applications like the food, construction, and petroleum industries. In this research, we investigate pneumatic fracturing of a porous medium experimentally and numerically in a Hele‐Shaw cell. In the experiments, we inject air into the porous medium (initially random loose packed) to create compaction, channeling, and fracturing while monitoring the cell with accelerometers and a high‐speed camera. Furthermore, we develop a numerical model in two steps: (1) a poroelastoplasticity‐based model to explain dynamic fluid pressure variations and (2) a solid stress model based on Janssen's theory. The contributions of the different pressure sources air in channels and solid stress in the experiments, and the simulations are compared with respect to amplitude and frequency. Afterward, the variations of the normal stress exerting on the plates are convolved with a Lamb Wave green function to generate acoustic emissions numerically. The physics behind the evolution of the experimentally recorded power spectrum of the out‐of‐plane plate vibrations are explained using numerical models. The frequency bands (in the simulated power spectra) are influenced by the size of the opened channels and the Hele‐Shaw cell and are in the same range with the experimentally measured peaks of the acoustic emissions.

Rapid tremor migration and pore-pressure waves in subduction zones

Rapid tremor migration (RTM) in subduction zones is a manifestation of complex fault-zone processes on the plate interface. Recent observations have revealed a large diversity of RTM patterns that are always associated with aseismic, shear strain at the interface. Small unstable asperities embedded in the stable shear zone are thus believed to originate tremor radiation during migration. Tectonic tremors have been recognized to occur where overpressured fluids exist. Spatial variations of fluid pressure may lead to non-linear diffusion processes with potentially large implications in tremor generation. Here, we show that pore-pressure waves are likely to exist in the plate interface, propagating with speeds and pathways similar to RTMs observed in different subduction zones including Guerrero, Mexico, where we introduce new high-resolution tremor locations and a RTM source physical model. These waves may explain the whole hierarchy of RTM patterns by producing transient reductions of the fault strength and thus secondary slip fronts triggering tremor during slow earthquakes.

A multi-layered poroelastic slab model under cyclic loading for a single osteon

BackgroundAn osteon consists of a multi-layered bone matrix and interstitial fluid flow in the lacunar–canalicular system. Loading-induced interstitial fluid flow in the lacunar–canalicular system is critical for osteocyte mechanotransduction and bone remodelling.MethodsTo investigate the effects of the lamellar structure and heterogeneous material properties of the osteon on the distributions of interstitial fluid flow and seepage velocity, an osteon is idealized as a hollow two-dimensional poroelastic multi-layered slab model subjected to cyclic loading. Based on poroelastic theory, the analytical solutions of interstitial fluid pressure and seepage velocity in lacunar–canalicular pores were obtained.ResultsThe results show that strain magnitude has a greater influence on interstitial fluid pressure than loading frequency. Interestingly, the heterogeneous distribution of permeability produces remarkable variations in interstitial fluid pressure and seepage velocity in the cross-section of cortical bone. In addition, interstitial fluid flow stimuli to osteocytes are mostly controlled by the value of permeability at the surface of the osteon rather than at the inner wall of the osteon.ConclusionInterstitial fluid flow induced by cycling loading stimuli to an osteocyte housed in a lacunar–canalicular pore is not only correlated with strain amplitude and loading frequency, but also closely correlated with the spatial gradient distribution of permeability. This model can help us better understand the fluid flow stimuli to osteocytes during bone remodelling.

Episodic Tremor and Slip Explained by Fluid‐Enhanced Microfracturing and Sealing

AbstractEpisodic tremor and slow‐slip events at the deep extension of plate boundary faults illuminate seismic to aseismic processes around the brittle‐ductile transition. These events occur in volumes characterized by overpressurized fluids and by near failure shear stress conditions. We present a new modeling approach based on a ductile grain size‐sensitive rheology with microfracturing and sealing, which provides a mechanical and field‐based explanation of such phenomena. We also model pore fluid pressure variation as a function of changes in porosity/permeability and strain rate‐dependent fluid pumping. The fluid‐enhanced dynamic evolution of microstructures defines cycles of ductile strain localization and implies increase in pore fluid pressure. We propose that slow‐slip events are ductile processes related to transient strain localization, while nonvolcanic tremor corresponds to fracturing of the whole rock at the peak of pore fluid pressure. Our model shows that the availability of fluids and the efficiency of fluid pumping control the occurrence and the P‐T conditions of episodic tremor and slip.

Chilean megathrust earthquake recurrence linked to frictional contrast at depth

Fundamental processes of the seismic cycle in subduction zones, including those controlling the recurrence and size of great earthquakes, are still poorly understood. Here, by studying the 2016 earthquake in southern Chile—the first large event within the rupture zone of the 1960 earthquake (moment magnitude (Mw) = 9.5)—we show that the frictional zonation of the plate interface fault at depth mechanically controls the timing of more frequent, moderate-size deep events (Mw 8.5). We model the evolution of stress build-up for a seismogenic zone with heterogeneous friction to examine the link between the 2016 and 1960 earthquakes. Our results suggest that the deeper segments of the seismogenic megathrust are weaker and interseismically loaded by a more strongly coupled, shallower asperity. Deeper segments fail earlier (~60 yr recurrence), producing moderate-size events that precede the failure of the shallower region, which fails in a great earthquake (recurrence >110 yr). We interpret the contrasting frictional strength and lag time between deeper and shallower earthquakes to be controlled by variations in pore fluid pressure. Our integrated analysis strengthens understanding of the mechanics and timing of great megathrust earthquakes, and therefore could aid in the seismic hazard assessment of other subduction zones. The recurrence time of megathrust earthquakes in Chile may be controlled by frictional contrasts at depth, according to analyses of stress build-up and release related to the December 2016 southern Chile earthquake.

Subduction Thermal Regime, Slab Dehydration, and Seismicity Distribution Beneath Hikurangi Based on 3‐D Simulations

AbstractThe downdip limit of seismogenic interfaces inferred from the subduction thermal regime by thermal models has been suggested to relate to the faulting instability caused by the brittle failure regime in various plate convergent systems. However, the featured three‐dimensional thermal state, especially along the horizontal (trench‐parallel) direction of a subducted oceanic plate, remains poorly constrained. To robustly investigate and further map the horizontal (trench‐parallel) distribution of the subduction regime and subsequently induced slab dewatering in a descending plate beneath a convergent margin, we construct a regional thermal model that incorporates an up‐to‐date three‐dimensional slab geometry and the MORVEL plate velocity to simulate the plate subduction history in Hikurangi. Our calculations suggest an identified thrust zone featuring remarkable slab dehydration near the Taupo volcanic arc in the North Island distributed in the Kapiti, Manawatu, and Raukumara region. The calculated average subduction‐associated slab dehydration of 0.09 to 0.12 wt%/km is greater than the dehydration in other portions of the descending slab and possibly contributes to an along‐arc variation in the interplate pore fluid pressure. A large‐scale slab dehydration (>0.05 wt%/km) and a high thermal gradient (>4 °C/km) are also identified in the Kapiti, Manawatu, and Raukumara region and are associated with frequent deep slow slip events. An intraslab dehydration that exceeds 0.2 wt%/km beneath Manawatu near the source region of tectonic tremors suggests an unknown relationship in the genesis of slow earthquakes.

Influence of stress and formation water properties on velocity sensitivity of lignite reservoir using simulation experiment

To study the velocity sensitivity of a lignite reservoir under different stresses and formation water conditions, orthogonal experiments are performed with pH, salinity, and effective stress as the main factors, and evaluation indexes are extracted at different stages. The effects of the main factors on each index are analyzed by means of range analysis, variance analysis, and effect diagram to study the mechanism of velocity sensitivity in a lignite reservoir. Under a given effective stress condition, the change in lignite permeability with the variation of fluid pressure undergoes three evolutionary stages (stable-rise, decline, and fluctuation) and two effects (sensitization and velocity sensitivity effects) exist. On the basis of the seven evaluation indexes, an evaluation index (Dm) suitable for describing velocity sensitivity damage in the presence of sensitization stage is proposed. Extreme and variance analyses show that salinity is the main factor that controls the lignite sensitization (rise) stage. In the velocity sensitivity (decline) stage, the permeability attenuation rate is controlled by pH and salinity, and the sustained range of velocity sensitivity is affected by pH and effective stress. The fluctuation stage is mainly affected by effective stress. It is analyzed that pH mainly controls the migration and agglomeration of pulverized coal particles through hydrophilicity, and salinity controls the dispersion and agglomeration of pulverized coal particles through the zeta potential generated by an electric double layer. Effective stress influences the sensitivity of lignite reservoir by effecting pore structure.

3D multi-scale multi-physics modelling of hot cracking in welding

A 3D multi-scale and multi-physics numerical model has been developed and validated to predict the occurrence of hot cracking during fusion welding of Al-Mg-Si alloys. The new model consists of four modules: (I) a welding solidification module that creates the desired weld microstructure consisting of both columnar and equiaxed grains and varies as a function of welding conditions; (II) a thermo-mechanical analysis module that predicts the deformation of the weld mushy zone due to solidification contraction and the response of the base metal; (III) a fluid flow module that calculates the variation in fluid velocity and pressure within the micro liquid channels of the semisolid; and (IV) a crack initiation module that applies Kou's hot cracking criterion [1] to identify cracked liquid channels based on inputs from the solidification, thermo-mechanical and fluid-flow modules. The results identify the underlying mechanisms by which welding process parameters (current and travel speed) and external restraining conditions influence hot cracking susceptibility during Gas Tungsten Arc welding. Interestingly, micro hot cracks seem to initiate near the fusion zone but then localize and form a macroscopic hot crack at the core of the weld.

Numerical Investigation of Gas–Liquid Flow in a Newly Developed Carbonation Reactor

This paper presents a numerical study of the gas–liquid flow inside a novel Venturi carbonation reactor for red mud processing. The model is based on the Eulerian–Eulerian model, with population balance method being used to describe bubble size distribution. The validity of the model is confirmed through different applications. It is then used to analyze the flow and gas dispersion inside the reactor at different liquid flow rates. The numerical results reveal that the new reactor can produce periodic variations of fluid pressure and gas holdup with time. Correspondingly, the ascending liquid flow presents a vortex column, swinging periodically in the radial direction. This flow oscillation becomes more obvious at an increased liquid flow rate, promoting bubble refining and uniform distribution. The results of this study should be useful not only for better understanding the gas–liquid flow but also for designing and controlling this new reactor.

Slow slip rate and excitation efficiency of deep low-frequency tremors beneath southwest Japan

We estimated the long-term average slip rate on the plate interface across the Nankai subduction zone during 2002–2013 using deep low-frequency tremors as a proxy for short-term slow slip events based on empirical relations between the seismic moment of short-term slow slip events and tremor activities. The slip rate in each region is likely to compensate for differences between the convergence rate and the slip deficit rate of the subducting Philippine Sea plate estimated geodetically, although the uncertainty is large. This implies that the strain because of the subduction of the plate is partially stored as the slip deficit and partially released by slow slip events during the interseismic period. The excitation efficiency of the tremors for the slow slip events differs among regions: it is high in the northern Kii region. Some events in the western Shikoku region show a somewhat large value. Antigorite serpentinite of two types exists in the mantle wedge beneath southwest Japan. Slips with more effective excitation of tremors presumably occur in high-temperature conditions in the antigorite+olivine stability field. Other slip events with low excitation efficiency are distributed in the antigorite+brucite stability field. Considering the formation reactions of these minerals and their characteristic structures, events with high excitation efficiency can be correlated with a high pore fluid pressure condition. This result suggests that variation in pore fluid pressure on the plate interface affects the magnitude of tremors excited by slow slip events.

In situ measurement of velocity-stress sensitivity using crosswell continuous active-source seismic monitoring

The ability to characterize time-varying reservoir properties, such as the state of stress, has fundamental implications in subsurface engineering, relevant to geologic sequestration of [Formula: see text]. Stress variation, here in the form of changes in pore fluid pressure, is one factor known to affect seismic velocity. Induced variations in velocity have been used in seismic studies to determine and monitor changes in the stress state. Previous studies conducted to determine velocity-stress sensitivity at reservoir conditions rely primarily on laboratory measurements of core samples or theoretical relationships. We have developed a novel field-scale experiment designed to study the in situ relationship between pore-fluid pressure and seismic velocity using a crosswell continuous active-source seismic monitoring (CASSM) system. At the Cranfield, Mississippi, [Formula: see text] sequestration field site, we actively monitored seismic response for five days with a temporal resolution of 5 min; the target was a 26 m thick injection zone at approximately 3.2 km depth in a fluvial sandstone formation (lower Tuscaloosa Formation). The variation of pore fluid pressure was obtained during discrete events of fluid withdrawal from one of the two wells and monitored with downhole pressure sensors. The results indicate a correlation between decreasing CASSM time delay (i.e., velocity change for a raypath in the reservoir) and periods of reduced fluid pore pressure. The correlation is interpreted as the velocity-stress sensitivity measured in the reservoir. This observation is consistent with published laboratory studies documenting a velocity ([Formula: see text]) increase with an effective stress increase. A traveltime change ([Formula: see text]) of 0.036 ms is measured as the consequence of a change in pressure of approximately 2.55 MPa ([Formula: see text]). For [Formula: see text] total traveltime, the velocity-stress sensitivity is [Formula: see text]. The overall results suggest that CASSM measurements represent a valid technique for in situ determination of velocity-stress sensitivity in field-scale monitoring studies.

O-34 南海付加体デコルマ帯における変形と間隙水圧変動

O-34 南海付加体デコルマ帯における変形と間隙水圧変動

Frictional and viscous effects on the nucleation phase of an earthquake

We study the frictional and viscous effects on earthquake nucleation, especially for the nucleation phase, based on a one-degree-of-freedom spring-slider model with friction and viscosity. The frictional and viscous effects are specified by the characteristic displacement, U c, and viscosity coefficient, η, respectively. Simulation results show that friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. Higher viscosity causes a smaller amplitude of lower velocity motion than lower viscosity. A change of either U c (under large η) or η (under large U c) from a large value (U ch for U c and η h for η) to a small one (U cl for U c and η l for η) in two stages during sliding can result in a clear nucleation phase prior to the P-wave. The differences δU c = U ch − U cl and δη = η h − η l are two important factors in producing a nucleation phase. The difference between the nucleation phase and the P-wave increases with either δU c or δη. Like seismic observations, the peak amplitude of P-wave, which is associated with the earthquake magnitude, is independent upon the duration time of nucleation phase. A mechanism specified with a change of either η or U c from a larger value to a smaller one due to temporal variations in pore fluid pressure and temperature in the fault zone based on radiation efficiency is proposed to explain the simulation results and observations.

Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle–ductile transition and seismogenesis. In a subduction environment, switching in deformation mode and mechanisms and scale of localization may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). However, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in some cases for the localization of deformation and shear zone development, thus representing a case in which switching deformation mechanisms and scale and style of localization (deformation mode) interact and relate to each other. This contribution analyses an example of a millimetre-scale shear zone localized by brittle precursor formed within a host granitic protomylonite. The studied structures, developed in ambient pressure–temperature (P–T) conditions of low-grade blueschist facies (temperature T of ca. 300 °C and pressure P ≥ 0. 70 GPa) during involvement of Corsican continental crust in the Alpine subduction. We used a multidisciplinary approach by combining detailed microstructural and petrographic analyses, crystallographic preferred orientation by electron backscatter diffraction (EBSD), and palaeopiezometric studies on a selected sample to support an evolutionary model and deformation path for subducted continental crust. We infer that the studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations, may be considered as a small-scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow-slip structures. Our case study represents, therefore, a fossil example of association of fault structures related to stick-slip strain accommodation during subduction of continental crust.

Analysis of pressure variation of fluid in bounded circular reservoirs under the constant pressure outer boundary condition

In this work, we have investigated the well pressure distribution in a bounded circular reservoir under the condition of constant pressure outer boundaries. The diffusivity equation was used in the analysis. The finite element technique, using Lagrange quadratic shape elements was employed to carry out the analysis over the cross-section of the reservoir which involves discretizing the domain into finite element, analysing these finite element, assembling the results from the analysis of the analysed finite element, imposing the boundary conditions and finally, getting the results that represent the entire domain. The results obtained where shown in a log log plot (dimensionless pressure against dimensionless time) for dimensionless radii of 1 to 1,000,000 in log cycles. It was shown that the relationship between dimensionless pressure and dimensionless time was linear whose slope was zero. The result obtained at the wellbore was compared with the results obtained by Van Everdigen and Hurst. It was shown that there was a strong positive correlation between the results. The result obtained from the analysis also shows the pressure variation outside wellbore of the same reservoir. It is important to note that solutions from existing literature only state the pressure at the wellbore at a particular time but this work predicts the pressure variation in the entire reservoir from the wellbore to the external boundary at the same time. http://dx.doi.org/10.4314/njt.v36i1.20

Analysis of pressure variation of fluid in bounded circular reservoirs under the “No Flow” outer boundary condition

Predicting the pressure at the wellbore in bounded circular reservoir has been the hub in the study of Reservoir Engineering. Predicting the pressures outside the reservoir was not an easy task. The Ei function method has been the only method for determining the pressure outside the wellbore of a bounded circular reservoir. The disadvantage of the Ei function method is that it involves rigorous iterations. This study sorts to use another approach to determine the pressure within and outside the wellbore at a glance. The finite element method was used in this study. The reservoir domain was divided into smaller subdomains and analysed. The results from these subdomains were assembled to represent pressure in the entire reservoir. The results obtained where shown tabular form (dimensionless pressure and dimensionless time) for dimensionless radii of 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7, 8, 9, and 10. It was shown that the relationship between dimensionless pressure and dimensionless time was linear. The result obtained at the wellbore was compared with the results obtained by Van Everdigen and Hurst. It was shown that there was a strong positive correlation between the results.Keywords: Bounded circular reservoir, constant terminal rate, dimensionless variables, diffusivity equation, and Crank-Nicholson scheme. Nomenclature