Pore Fluid Pressure Conditions Research Articles

Hydro-mechanical earthquake cycles in a poro-visco-elasto-plastic fluid-bearing fault structure

A major goal in earthquake physics is to derive a constitutive framework for fault slip that captures the dependence of shear strength on fault rheology, sliding velocity, and pore-fluid pressure. In this study, we present H-MEC (Hydro-Mechanical Earthquake Cycles), a newly-developed two-phase flow numerical framework — which couples solid rock deformation and pervasive fluid flow — to simulate how crustal stress and fluid pressure evolve during the earthquake cycle. This unified, continuum-based model, incorporates a staggered finite difference–marker-in-cell method and accounts for inertial wave-mediated dynamics and fluid flow in poro-visco-elasto-plastic compressible medium. Global Picard-iterations and an adaptive time stepping allow the correct resolution of both long- and short-time scales, ranging from years to milliseconds. We present a comprehensive in-plane strike-slip setup in which we test analytical poroelastic benchmarks of pore-fluid pressure diffusion from an injection point along a finite fault width. We then investigate how pore-fluid pressure evolution and solid–fluid compressibility control sequences of seismic and aseismic fault slip. While the weakening phase is controlled by localized compaction of pores and dynamic self-pressurization of fluids inside the undrained fault zone, the subsequent propagation of dynamic ruptures is driven by pore-pressure waves. Furthermore, pore-fluid pressure conditions on the fault and shear strength weakening associated with rapid self-pressurization of fluids control the characteristic slip-weakening distance, the final size of seismic events, and the scaling between slip and fracture energy observed for large earthquakes. Our modeling results demonstrate that fault failure can occur due to poroelastic coupling on a finite-width shear zone, thus highlighting the importance of considering the realistic hydro-mechanical structure of faults to investigate fluid-driven seismic and aseismic slip, either as a natural process or induced by human activities.

Blueschist-facies paleo-earthquakes in a serpentinite channel (Zagros suture, Iran) enlighten seismogenesis in Mariana-type subduction margins

The architecture and pressure-temperature conditions reached by a Cretaceous block-in-matrix serpentinite mélange exposed in the Zagros suture resemble those imaged in the active Mariana subduction zone. There, large magnitude-earthquakes (M>w9) have never been recorded but smaller events – of poorly-constrained physical origin – in the range M∼w3-6 are widespread. Field and petro-structural constraints led to a first report of blueschist-facies seismic fault-related rocks in the Zagros serpentinite melange, including breccias, foliated cataclasites and ultracataclasites; all observed within a foliated mafic metatuffaceous block embedded in serpentinite schists. Fine-scale petrological characterization of ultrafine-grained, fluidized cataclastic material reveals the presence of newly-formed glaucophane, lawsonite, phengite, albite and pumpellyite, an assemblage inferred (based on thermodynamic modelling) to have crystallized in the lower lawsonite-blueschist facies at ∼0.6-1.0 GPa and 230-300°C. Extensional veins containing similar mineral assemblages are observed crosscutting the aforementioned rocks but are also identified as comminuted fragments in all fault-related lithologies. Crosscutting relationships among the multiple generations of fluidized ultracataclasites and brecciated blueschists suggest that episodic faulting and hydrofracturing were contemporaneous processes at ∼20-35 km depth, i.e., at similar conditions as reported for metabasalts expelled by Mariana serpentinite mud volcanoes. Mechanical modelling confirms that the studied fault-related features can only have formed under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promoted recurrent seismic faulting, as monitored in the Mariana seismogenic zone. These fluids are likely associated with externally and deeply-generated fluid pulses that may have reached the seismogenic window, imprinting a Ta-Th-Nb-HREEs-enriched trace element signature. This new faulted blueschist occurrence highlights the physical nature and the mechanical processes operating within fluid-saturated fault zones in the serpentinized subduction channel.

Time‐Dependent Deformations of Sandstone During Pore Fluid Pressure Oscillations: Implications for Natural and Induced Seismicity

AbstractIn tectonically active regions, natural seismicity is often correlated with the seasonal hydrology, suggesting that cyclic loading variations might trigger seismicity. Moreover, recent field observations suggest that cyclic fluid injection strategies into geological reservoirs could produce less seismicity than monotonic injections. Here we present 10 brittle creep laboratory triaxial experiments that bring new constraints on fluid‐rock interactions during cyclic pore fluid variations. The experiments were performed on Fontainebleau sandstone with various pore fluid pressure conditions: (i) with constant pore fluid pressure levels from 1 to 10 MPa, at constant Terzaghi effective pressure (Pc − Pf = 30 MPa); and (ii) with cyclic (sinusoidal) pore fluid pressure oscillations of varying amplitudes (from 0 to 8 MPa) and periods (from 30 to 3,000 s) around a mean value of 5 MPa. During deformation, the rock's mechanical properties and the high‐frequency acoustic emission signals were monitored to investigate the physics underlying the rupture processes. Under macroscopically drained conditions, rather than their amplitude, the period of the oscillations appeared to strongly affect the rock sample strength, time‐to‐failure, and dilatancy behavior. Moreover, even for small variations of pore fluid amplitude, and at all pore fluid pressure period, pore fluid pressure and acoustic emissions were strongly correlated. Our experiments demonstrate that pore fluid pressure oscillations may strongly affect rocks mechanical behavior and associated seismic activity.

The Role of Pore Fluid Pressure on the Failure of Magma Reservoirs: Insights From Indonesian and Aleutian Arc Volcanoes

AbstractWe use numerical models to study the mechanical stability of magma reservoirs embedded in elastic host rock. We quantify the overpressure required to open tensile fractures (the failure overpressure), as a function of the depth and the size of the reservoir, the loading by the volcanic edifice, and the pore fluid pressure in the crust. We show that the pore fluid pressure is the most important parameter controlling the magnitude of the failure overpressure rather than the reservoir depth and the edifice load. Under lithostatic pore fluid pressure conditions, the failure overpressure is on the order of the rock tensile strength (a few tens of megapascals). Under zero pore fluid pressure conditions, the failure overpressure increases linearly with depth (a few hundreds of megapascals at 5 km depth). We use our models to forecast the failure displacement (the cumulative surface displacement just before an eruption) on volcanoes showing unrest: Sinabung and Agung (Indonesia) and Okmok and Westdahl (Aleutian). By comparison between our forecast and the observation, we provide valuable constraint on the pore fluid pressure conditions on the volcanic system. At Okmok, the occurrence of the 2008 eruption can be explained with a 1,000 m reservoir embedded in high pore fluid pressure, whereas the absence of eruption at Westdahl better suggests that the pore fluid pressure is much lower than lithostatic. Our finding suggests that the pore fluid pressure conditions around the reservoir may play an important role in the triggering of an eruption by encouraging or discouraging the failure of the reservoir.

Slow fault propagation in serpentinite under conditions of high pore fluid pressure

The rupture, localization, and slip of faults in serpentinite were studied under varying pore fluid pressure conditions to understand deformation mechanisms potentially responsible for slow slip in fault zones. Experiments were conducted at a constant effective confining pressure of 10 MPa and under pore fluid pressures from 0 to 120 MPa and at temperatures from 23 to 110 °C. With no fluid pressure, faulting occurs rapidly and audibly, and the duration of failure increases monotonically with increasing fluid pressure and temperature. Although non-dilatant during initial strain hardening, the serpentinite dilates during strain weakening concomitant with fault rupture and slip. Non-dilatant strain hardening occurs by microcracking along serpentine basal planes and grain boundaries and rarely in mode I orientations, consistent with previous studies. Dilatant fault rupture produces a network of transgranular shear fractures in conjugate orientations, generally with one dominant fracture. Structural observations show that as fluid pressure increases, the number of transgranular fractures increases. We propose that when faulting occurs over a distributed zone rather than a pre-existing principal slip surface, dilatant hardening causes deformation to migrate. This process causes an increase in slip weakening distance and fracture energy at elevated fluid pressures that can lead to more stable failure. Further, thermally-activated processes caused deformation at propagating crack tips, which also increases the slip weakening distance and the effective fracture energy with increasing temperature. Given the geologic settings for slow slip, our results indicate that high fluid pressure, distributed deformation, and thermally-activated processes may all contribute to slow fault rupture and slip.

Mechanical behaviour of the Rotokawa Andesites (New Zealand): Insight into permeability evolution and stress-induced behaviour in an actively utilised geothermal reservoir

High-enthalpy geothermal systems, such as that of Rotokawa in the central Taupo Volcanic Zone (TVZ) of New Zealand, commonly host fracture systems that control reservoir permeability. The loss of porosity and permeability through compaction and mineral precipitation, and the opening of permeable fracture zones, occurred in tandem as the geothermal system evolved. In order to better understand how porosity can evolve in an active reservoir, a systematic study of triaxial deformation experiments has been undertaken to investigate the mechanical behaviour and failure mode of coherent andesite lavas and andesite breccias (the predominant reservoir lithologies) with varying porosity and alteration intensity from the Rotokawa Geothermal Field under high confining pressure and high pore fluid pressure conditions. These experiments provide insight into the formation of permeable fracture zones and low-permeability zones within the reservoir. Although we observed brittle failure mode in all our experiments (covering a range of rock attributes and relevant pressure conditions), we find that the amount of dilatancy during deformation, inferred to assist in the creation of a permeable network, is considerably reduced at high effective pressures and for high-porosity and highly altered samples. Importantly, some high-porosity and/or highly altered samples experienced a net compaction at the end of the experiments. Microstructural observations suggest that, although the failure mode remains ultimately brittle, microcracking and pore collapse can operate in concert at high effective pressure, porosity, and/or alteration, potentially explaining the reduction in dilatancy. We infer, therefore, that the deformation of the rocks within the reservoir could result in either a bulk permeability increase or a permeability decrease, depending on the physical characteristics of the rocks (e.g. porosity and alteration intensity) and the pressure conditions. Using our new experimental data we provide a Hoek-Brown failure criterion for an intact Rotokawa andesite with a representative porosity and show the distribution of stresses around a vertical and deviated borehole during drilling using a finite element model. Finite element modelling shows that brittle failure of intact rock (and therefore off-fault seismicity) during production and injection is unlikely in RKA containing the average porosity assumed for the reservoir. Production-induced compaction and injection-induced tensile failure could occur however in high-porosity and/or highly altered intact rock (which could lead to instability at the well boundary), particularly in deviated wells. The experiments and modelling presented herein provide insight into the evolution of an actively utilised geothermal reservoir.

A 48 m.y. history of fracture opening, temperature, and fluid pressure: Cretaceous Travis Peak Formation, East Texas basin

Research Article| July 01, 2010 A 48 m.y. history of fracture opening, temperature, and fluid pressure: Cretaceous Travis Peak Formation, East Texas basin S.P. Becker; S.P. Becker * 1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924, USA *Current address: ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252-2189, USA. Search for other works by this author on: GSW Google Scholar P. Eichhubl; P. Eichhubl † 1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924, USA †E-mail: peter.eichhubl@beg.utexas.edu Search for other works by this author on: GSW Google Scholar S.E. Laubach; S.E. Laubach 1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924, USA Search for other works by this author on: GSW Google Scholar R.M. Reed; R.M. Reed 1Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924, USA Search for other works by this author on: GSW Google Scholar R.H. Lander; R.H. Lander 2Geocosm LLC, 3311 San Mateo Drive, Austin, Texas 78738, USA Search for other works by this author on: GSW Google Scholar R.J. Bodnar R.J. Bodnar 3Department of Geosciences, 4044 Derring Hall, Virginia Tech, Blacksburg, Virginia 24061, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2010) 122 (7-8): 1081–1093. https://doi.org/10.1130/B30067.1 Article history received: 05 Apr 2009 rev-recd: 03 Aug 2009 accepted: 24 Aug 2009 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S.P. Becker, P. Eichhubl, S.E. Laubach, R.M. Reed, R.H. Lander, R.J. Bodnar; A 48 m.y. history of fracture opening, temperature, and fluid pressure: Cretaceous Travis Peak Formation, East Texas basin. GSA Bulletin 2010;; 122 (7-8): 1081–1093. doi: https://doi.org/10.1130/B30067.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Quartz cement bridges across opening-mode fractures of the Cretaceous Travis Peak Formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low-porosity sandstone. Incremental crack-seal fracture opening is inferred based on the banded structure of quartz cement bridges, consisting of up to 700 cement bands averaging ∼5 μm in thickness as observed with scanning electron microscope–cathodoluminescence. Crack-seal layers contain assemblages of aqueous two-phase fluid inclusions. Based on fluid inclusion microthermometry and Raman microprobe analyses, we determined that these inclusions contain methane-saturated brine trapped over temperatures ranging from ∼130°C to ∼154°C. Using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth, we reconstructed the fluid temperature and pore-fluid pressure evolution during fracture opening. In combination with published burial evolution models, this reconstruction indicates that fracture opening started at ca. 48 Ma and above-hydrostatic pore-fluid pressure conditions, and continued under steadily declining pore-fluid pressure during partial exhumation until present times. Individual fractures opened over an ∼48 m.y. time span at rates of 16–23 μm/m.y. These rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

SIMULATION OF WAVES IN PORO-VISCOELASTIC ROCKS SATURATED BY IMMISCIBLE FLUIDS: NUMERICAL EVIDENCE OF A SECOND SLOW WAVE

We present an iterative algorithm formulated in the space-frequency domain to simulate the propagation of waves in a bounded poro-viscoelastic rock saturated by a two-phase fluid. The Biot-type model takes into account capillary forces and viscous and mass coupling coefficients between the fluid phases under variable saturation and pore fluid pressure conditions. The model predicts the existence of three compressional waves or Type-I, Type-II and Type-III waves and one shear or S-wave. The Type-III mode is a new mode not present in the classical Biot theory for single-phase fluids. Our differential and numerical models are stated in the space-frequency domain instead of the classical integrodifferential formulation in the space-time domain. For each temporal frequency, this formulation leads to a Helmholtz-type boundary value problem which is then solved independently of the other frequency problems, and the time-domain solution is obtained by an approximate inverse Fourier transform. The numerical procedure, which is first-order correct in the spatial discretization, is an iterative nonoverlapping domain decomposition method that employs an absorbing boundary condition in order to minimize spurious reflections from the artificial boundaries. The numerical experiments showing the propagation of waves in a sample of Nivelsteiner sandstone indicate that under certain conditions the Type-III wave can be observed at ultrasonic frequencies.

Brittle tectonics and pore-fluid conditions in the evolution of the Great Boundary Fault around Chittaurgarh, Northwestern India

This paper aims at reconstructing paleostress history and deciphering the pore-fluid pressure conditions during the reactivation of a regional-scale fault. Paleostress analyses of the mesoscopic structures suggest that three successive events of reactivation on the Great Boundary Fault occurred in thrust-type, strike-slip type and thrust-type tectonic-settings, respectively. Whereas the pore-fluid pressure was supralithostatic during the first and third events of reactivation, it was sublithostatic during the second event. Each event of reactivation induced a fluid pressure gradient, which resulted in the focused and enhanced flow of syntectonic fluids within the high permeability locales, primarily fractures and faults. Fluid inclusion study on strike-slip veins reveals that the syntectonic fluids were highly dense, Na–Ca–Cl brines of formational water origin. Stratigraphic evidence in favour of a 2-km-thick column of overburden above Kaimur sandstone beds implies that the strike-slip faulting occurred at 160–202 °C temperature and 53 MPa pressure. Variation in homogenization temperature reflects fluctuation in pore-fluid pressure during entrapment of syntectonic fluids and points to seismic pumping as a possible mechanism of fluid flow during faulting. High paleogeothermal gradient, obtained by fluid inclusion data, is ascribed to the high heat flow due to crustal stretching during the Proterozoic rifting, the basal and intermittent volcanism in the basin, and occurrence of Berach granite as the basement.

Fluid inclusions in veins from the Middle Devonian shales: A record of deformation conditions and fluid evolution in the Appalachian Plateau

Research Article| March 01, 1995 Fluid inclusions in veins from the Middle Devonian shales: A record of deformation conditions and fluid evolution in the Appalachian Plateau Mark A. Evans Mark A. Evans 1Department of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30460-8149 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1995) 107 (3): 327–339. https://doi.org/10.1130/0016-7606(1995)107<0327:FIIVFT>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mark A. Evans; Fluid inclusions in veins from the Middle Devonian shales: A record of deformation conditions and fluid evolution in the Appalachian Plateau. GSA Bulletin 1995;; 107 (3): 327–339. doi: https://doi.org/10.1130/0016-7606(1995)107<0327:FIIVFT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Fluid inclusion microthermometric analysis of multiple vein sets from the Middle Devonian shales provides insight into deformation conditions and fluid evolution in the Appalachian Plateau during, and since, the Alleghanian orogeny. The data illustrate a dynamic system in which deformation conditions and fluid compositions changed continually during basin subsidence in response to orogeny and postorogenic uplift and denudation.Two types of inclusions are present in the veins: (1) hydrocarbon inclusions that contain CH4, C2H6, CO2, higher hydrocarbons (more than two carbon atoms), and bitumen and (2) two-phase aqueous inclusions that contain very saline (up to 27 wt% NaCl equivalent) NaCl-CaCl2 brines. The veins formed in response to stresses associated with the Alleghanian orogeny during subsidence of the Middle Devonian shale section in the central Plateau province. Each vein set contains inclusions of hydrocarbons that are thermally more mature than those in the previously formed set. The earliest vein set, CFJV1 (cross-fold joints and veins 1), formed during the onset of hydrocarbon generation at a depth of ∼2.3 km and contains inclusions that are composed of degraded bitumen. The next youngest vein set, CFJV2a, formed under lithostatic pore-fluid pressure conditions at a depth of at least 2.3 km and contains primary inclusions with a condensate-like composition. CFJV2b also formed under lithostatic conditions, at a depth of 3.0 km, and contains primary CH4-C2H6-CO2 inclusions. The younger CFJV3a formed at at least 3.9 km depth under lithostatic conditions and contains primary CH4-CO2 inclusions.The youngest vein set, strike-parallel SJV (strike joints and veins), formed during maximum burial, or early during uplift, of the Middle Devonian shale section. SJV, along with reopened cross-fold joints and veins, contains barite with nearly pure CH4 inclusions and two-phase aqueous inclusions. The presence of barite in all of the vein sets is a result of the breaching of the Middle Devonian shale overpressure seal in response to relaxation of Alleghanian compressive stresses and the influx of surrounding brines into the shale section. The trapping of fluids in the barite continued throughout the uplift history because of microfracturing. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.