Fluid-filled Fractures Research Articles

Fully coupled poroelastic peridynamic formulation for fluid-filled fractures

A new fully coupled poroelastic peridynamic formulation is presented and its application to fluid-filled fractures is demonstrated. This approach is capable of predicting porous flow and deformation fields and their effects on each other. Moreover, it captures the fracture initiation and propagation in a natural way without resorting to an external failure criterion. The peridynamic predictions are verified by considering two benchmark problems including one- and two-dimensional consolidation problems. Moreover, the growth of a pre-existing hydraulically pressurized crack case is presented. Based on these results, it is concluded that the current peridynamic formulation has a potential to be used for the analysis of more sophisticated poroelastic problems including fluid-filled rock fractures as in hydraulic fracturing.

Imaging fracture permeability using magnetotellurics

We present 1D anisotropic inversions of magnetotelluric data in two regions of the Otway Basin; Koroit, Victoria, and Penola, South Australia. In the Koroit region we have delineated an electrically anisotropic layer at approximately 2.5 to 3.5 km depth; this corresponds to the upper part of the Lower Cretaceous Crayfish Group, a known reservoir unit. The anisotropy strike is consistent between stations at approximately 160° east of north. We interpret the anisotropy at Koroit as resulting from pervasive NNW oriented, fluid-filled fractures, resulting in enhanced bulk electrical and hydraulic conductivity. This interpretation is consistent with permeability data from well formation tests. It is also consistent with the orientation of mapped faults in the area, which are favourably oriented for reactivation in the current stress field. In Penola, no persistent anisotropic layer has been defined even though the areas are geologically similar. The difference in the resistivity structure may reflect differences in the density of fractures or their fill material. Alternatively, it may reflect small differences in the amount by which the fractures are open, resulting from differences in the stress field and fracture orientation in each area.

An oligotrophic deep-subsurface community dependent on syntrophy is dominated by sulfur-driven autotrophic denitrifiers

Subsurface lithoautotrophic microbial ecosystems (SLiMEs) under oligotrophic conditions are typically supported by H2 Methanogens and sulfate reducers, and the respective energy processes, are thought to be the dominant players and have been the research foci. Recent investigations showed that, in some deep, fluid-filled fractures in the Witwatersrand Basin, South Africa, methanogens contribute <5% of the total DNA and appear to produce sufficient CH4 to support the rest of the diverse community. This paradoxical situation reflects our lack of knowledge about the in situ metabolic diversity and the overall ecological trophic structure of SLiMEs. Here, we show the active metabolic processes and interactions in one of these communities by combining metatranscriptomic assemblies, metaproteomic and stable isotopic data, and thermodynamic modeling. Dominating the active community are four autotrophic β-proteobacterial genera that are capable of oxidizing sulfur by denitrification, a process that was previously unnoticed in the deep subsurface. They co-occur with sulfate reducers, anaerobic methane oxidizers, and methanogens, which each comprise <5% of the total community. Syntrophic interactions between these microbial groups remove thermodynamic bottlenecks and enable diverse metabolic reactions to occur under the oligotrophic conditions that dominate in the subsurface. The dominance of sulfur oxidizers is explained by the availability of electron donors and acceptors to these microorganisms and the ability of sulfur-oxidizing denitrifiers to gain energy through concomitant S and H2 oxidation. We demonstrate that SLiMEs support taxonomically and metabolically diverse microorganisms, which, through developing syntrophic partnerships, overcome thermodynamic barriers imposed by the environmental conditions in the deep subsurface.

Computational Fluid Dynamics Simulation of Two-dimensional Natural Convection in a Fractured Porous Medium

ABSTRACTBuoyancy induced flow is one type of flow that can occur in relation to a wide range of hydrocarbon reservoir, deep circulation of groundwater and flow around buried radioactive waste. The objective of the present study was to study the transient two-dimensional natural convection in a fractured porous medium. A computational fluid dynamics (CFD) simulation was carried out to study the velocity and temperature distribution between the two bases of a horizontal solid slab containing a fluid-filled fracture that crosses the slab. The governing equations include continuity, momentum and energy equations with Boussinesq approximation were solved simultaneously using appropriate boundary conditions. This study was confined to work out for low-Rayleigh-number flows. The simulation results were compared with experimental data to validate the accuracy of the CFD work. The CFD results were in excellent agreement with experimental data from the literature. In addition, the effective parameters such as the aspect-ratio and fracture distribution on natural convection were investigated. The results indicated that increasing the aspect-ratio of fracture caused an increase of the maximum velocity in the fracture and temperature profile in the model was affected by fractures communication. Furthermore, the effect of porous medium with a tilted fracture through it was discussed.

Laboratory evidence for Krauklis-wave resonance in fractures and implications for seismic coda wave analysis

Krauklis waves are of major interest because they can lead to resonance effects in fluid-filled fractures. This resonance is marked by seismic signals with a dominant signature frequency, which may reveal fracture-related rock properties. In our laboratory study, we used homogeneous Plexiglas samples containing a single well-defined (i.e., manufactured) fracture. We recorded the signals obtained from propagating ultrasonic P- and S-waves (source frequency: 0.6, 1, and 2.25 MHz) along a sample without a fracture and samples with a fracture with different inclination angles of 30°, 45°, and 60° with respect to the short axis. The experimental results obtained from an incident S-wave confirmed that the presence of the fracture led to resonance effects at frequencies lower than the dominant source frequency, which slowly decayed over time in the recorded seismic coda after the first arrival. The resonance frequency was independent of the fracture orientation and the source frequency. We have interpreted this narrow-banded coda signal as a resonance in the fracture, and the frequency at which this occurred was an intrinsic property of the fracture size and elastic properties. To verify our laboratory results, we used an analytical solution, which provided a relationship between the fracture width, fracture length, resonance frequency, and temporal quality factor (i.e., exponential decay over time). The temporal quality factor obtained from our laboratory data agreed very well with the analytical solution. Hence, we concluded that the observed signature frequency (approximately 0.1 MHz) in the seismic coda was indeed a resonance effect. Finally, we have developed possible applications on the reservoir scale to infer fracture-related properties based on seismic coda analysis.

Fluid-Filled Fracture Propagation With a Phase-Field Approach and Coupling to a Reservoir Simulator

Summary A quantitative assessment of hydraulic-fracturing jobs relies on accurate predictions of fracture growth during slickwater injection for single and multistage fracturing scenarios. This requires consistent modeling of underlying physical processes, from hydraulic fracturing to long-term production. In this work, we use a recently introduced phase-field approach to model fracture propagation in a porous medium. This approach is thermodynamically consistent and captures several characteristic features of crack propagation such as joining, branching, and nonplanar propagation as a result of heterogeneous material properties. We describe two different phase-field fracture-propagation models and then present a technique for coupling these to a fractured-poroelastic-reservoir simulator. The proposed coupling approach can be adapted to existing reservoir simulators. We present 2D and 3D numerical tests to benchmark, compare, and demonstrate the predictive capabilities of the fracture-propagation model as well as the proposed coupling scheme.

Initialization of phase-field fracture propagation in porous media using probability maps of fracture networks.

It is well known in the geophysical community that surface deflection information/micro-seismic data are considered to be one of the best diagnostics for revealing the volume of rock fracture. However, the in-exactness of the data representing the deformation induced to calibrate and represent complex fracture networks created and connected during hydraulic fracturing presents a challenge. In this paper, we propose a technique that implements a phase-field approach to propagate fractures and their interaction with existing fracture networks using surface deflection data. The latter one provides a probability map of fractures in a heterogeneous reservoir. These data are used to initialize both the location of the fractures and the phase-field function. In addition, this approach has the potential for optimizing well placement/spacing for fluid-filled fracture propagation for oil and gas production and or carbon sequestration and utilization. Using prototype models based on realistic field data, we demonstrate the effects of interactions between existing and propagating fractures in terms of several numerical simulations with different probability thresholds, locations, and numbers of fractures. Our results indicate that propagating fractures interact in a complex manner with the existing fracture network. The modeled propagation of hydraulic fractures is sensitive to the threshold employed within the phase-field approach for delineating fractures.

AVO inversion for a non-welded interface: estimating compliances of a fluid-filled fracture

Though well known for layer boundaries, the use of amplitude-versus-offset (AVO) variations for non-welded boundaries like fractures is not yet investigated. Depending on the seismic wavelength used, fractures can be regarded as thin, compliant zones in rocks, in different scales. We explore the potential of multiangle AVO inversion of P-P and P-S reflections from a fracture to estimate fracture properties.We conduct laboratory experiments to measure reflection responses of dry and wet fractures. The observed P-P reflections of the wet fracture and the fracture aperture are very well predicted by the non-welded interface model.We invert the angle-dependent P-P reflectivity of the fracture to estimate both normal and tangential fracture compliances. The estimated value of the normal compliance is accurate, and it is also possible to obtain the value of the non-zero tangential compliance.We find that supplementing the information of converted P-S reflections in theAVOinversion greatly improves the estimate of the tangential compliance. The calculated compliance ratio clearly shows the existence of fluid in the fracture. This finding can be crucial for new applications in a wide range of scale—from earthquake seismology, deep and shallow seismic exploration, to non-destructive material testing.

Crustal anisotropy and ductile flow beneath the eastern Tibetan Plateau and adjacent areas

Abstract Crustal anisotropy beneath 71 broadband seismic stations situated at the eastern Tibetan Plateau and the Sichuan Basin is investigated based on the sinusoidal moveout of P-to-S conversions from the Moho and an intra-crustal discontinuity. Significant crustal anisotropy is pervasively detected beneath the study area with an average splitting time of 0.39 ± 0.18 s . The resulting fast orientations are mostly parallel to the major shear zones in the Songpan–Ganzi Terrane, and can be explained by fluid-filled fractures, favoring the model of rigid block motion with deformations concentrated on the block boundaries. In the vicinity of the Xianshuihe–Xiaojiang Fault Zone in the southern Songpan–Ganzi Terrane, our results, when combined with previously revealed high crustal Poisson's ratio in the area, support the existence of mid/lower crustal flow. The Longmenshan Fault Zone and adjacent areas are dominated by strike-orthogonal fast orientations, which are consistent with alignments of cracks associated with compressional stress between the Plateau and the Sichuan Basin. The observations suggest that crustal thickening is the main cause of the high topographic relief across the Longmenshan Fault Zone.

Mapping fractures using 1D anisotropic modelling of magnetotelluric data: a case study from the Otway Basin, Victoria, Australia

We present 1D anisotropic inversion of magnetotelluric (MT) data as a potential tool for mapping structural permeability in sedimentary basins. Using 1D inversions of a 171 site, broadband MT data set from the Koroit region of the Otway Basin, Victoria, Australia, we have delineated an electrically anisotropic layer at approximately 2.5 to 3.5 km depth. The anisotropy strike is consistent between stations at approximately 160° east of north. The depth of anisotropy corresponds to the top depth of the Lower Cretaceous Crayfish Group, and the anisotropy factor increases from west to east. We interpret the anisotropy as resulting from north–northwest oriented, fluid-filled fractures resulting in enhanced electrical and hydraulic conductivity. This interpretation is consistent with permeability data from well formation tests. It is also consistent with the orientation of mapped faults in the area, which are optimally oriented for reactivation in the current stress field.

Including poroelastic effects in the linear slip theory

Numerical simulations of seismic wave propagation in fractured media are often performed in the framework of the linear slip theory (LST). Therein, fractures are represented as interfaces and their mechanical properties are characterized through a compliance matrix. This theory has been extended to account for energy dissipation due to viscous friction within fluid-filled fractures by using complex-valued frequency-dependent compliances. This is, however, not fully adequate for fractured porous rocks in which wave-induced fluid flow (WIFF) between fractures and host rock constitutes a predominant seismic attenuation mechanism. In this letter, we develop an approach to incorporate WIFF effects directly into the LST for a 1D system via a complex-valued, frequency-dependent fracture compliance. The methodology is validated for a medium permeated by regularly distributed planar fractures, for which an analytical expression for the complex-valued normal compliance is determined in the framework of quasistatic poroelasticity. There is good agreement between synthetic seismograms generated using the proposed recipe and those obtained from comprehensive, but computationally demanding, poroelastic simulations.

A Phase-Field Method for Propagating Fluid-Filled Fractures Coupled to a Surrounding Porous Medium

The recently introduced phase-field approach for pressurized fractures in a porous medium offers various attractive computational features for numerical simulations of cracks such as joining, branching, and nonplanar propagation in possibly heterogeneous media. In this paper, the pressurized phase-field framework is extended to fluid-filled fractures in which the pressure is computed from a generalized parabolic diffraction problem. Here, the phase-field variable is used as an indicator function to combine reservoir and fracture pressure. The resulting three-field framework (elasticity, phase field, pressure) is a multiscale problem that is based on the Biot equations. The proposed numerical solution algorithm iteratively decouples the equations using a fixed-stress splitting. The framework is substantiated with several numerical benchmark tests in two and three dimensions.

Local time–space mesh refinement for simulation of elastic wave propagation in multi-scale media

This paper presents an original approach to local time–space grid refinement for the numerical simulation of wave propagation in models with localized clusters of micro-heterogeneities. The main features of the algorithm are–the application of temporal and spatial refinement on two different surfaces;–the use of the embedded-stencil technique for the refinement of grid step with respect to time;–the use of the Fast Fourier Transform (FFT)-based interpolation to couple variables for spatial mesh refinement. The latter makes it possible to perform filtration of high spatial frequencies, which provides stability in the proposed finite-difference schemes.In the present work, the technique is implemented for the finite-difference simulation of seismic wave propagation and the interaction of such waves with fluid-filled fractures and cavities of carbonate reservoirs. However, this approach is easy to adapt and/or combine with other numerical techniques, such as finite elements, discontinuous Galerkin method, or finite volumes used for approximation of various types of linear and nonlinear hyperbolic equations.

Rock interface strength influences fluid-filled fracture propagation pathways in the crust

Rock interface strengths have often been assumed to be zero in numerical and analogue models of fracture propagation and magma intrusion in the crust. Rock strength tests were performed to explore the role that rock interfaces have on the geometry and propagation dynamics of fluid-filled fractures in the crust. We used a 1 kN test machine to study 5 mm thick cuboidal specimens cut from a sandstone-siltstone rock core, where the strata were known to host magma intrusions and the rock interface between the units was intact. By measuring the load required to grow a crack running along the lithological contact between the layers we calculate its fracture toughness Kc. The siltstone had an average Kc of 0.56 ± 0.03 MPa m1/2 compared to the sandstone at 0.42 ± 0.02 MPa m1/2. The rock interface had intermediate average fracture toughness to the parent units at 0.45 ± 0.03 MPa m1/2. These results have important implications on fracture propagation pathways through rocks, as well as for the geometry and propagation dynamics of magma intrusions in the crust.

The geologic record of deep episodic tremor and slip

Research Article| March 01, 2014 The geologic record of deep episodic tremor and slip Nicholas W. Hayman; Nicholas W. Hayman Institute for Geophysics, University of Texas, 10100 Burnet Road, R2200, Austin, Texas 78758, USA Search for other works by this author on: GSW Google Scholar Luc L. Lavier Luc L. Lavier Institute for Geophysics, University of Texas, 10100 Burnet Road, R2200, Austin, Texas 78758, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Nicholas W. Hayman Institute for Geophysics, University of Texas, 10100 Burnet Road, R2200, Austin, Texas 78758, USA Luc L. Lavier Institute for Geophysics, University of Texas, 10100 Burnet Road, R2200, Austin, Texas 78758, USA Publisher: Geological Society of America Received: 31 Jul 2013 Revision Received: 12 Nov 2013 Accepted: 14 Nov 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (3): 195–198. https://doi.org/10.1130/G34990.1 Article history Received: 31 Jul 2013 Revision Received: 12 Nov 2013 Accepted: 14 Nov 2013 First Online: 09 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 Nicholas W. Hayman, Luc L. Lavier; The geologic record of deep episodic tremor and slip. Geology 2014;; 42 (3): 195–198. doi: https://doi.org/10.1130/G34990.1 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 SocietyGeology Search Advanced Search Abstract Transient and episodic slow slip accommodates a great deal of tectonic strain and may be mechanically linked with locked regions of seismically hazardous faults. Best documented in subduction zones and associated with nonvolcanic tremor, most proposed mechanisms for slow slip revolve around the transition between stable and unstable frictional sliding. The dilemma is that slow slip is generated at a wide range of crustal depths, including at pressure-temperature conditions where frictional deformation mechanisms give way to predominantly viscous ones. We present a model for how fracture and viscous flow within mid-crustal shear zones can produce episodic creep transients. Our model for such transients stems from geological examples of shear zones that formed at temperatures and pressures of >500 °C and >0.6 GPa during early orogenesis, following Late Cretaceous subduction of a backarc ocean basin. Within these shear zones, relatively strong lenses of metabasalt localized fluid-filled fractures that were subsequently deformed by viscous flow in surrounding quartzofeldspathic gneiss. The spatial and temporal characteristics of the modeled creep events are similar to those of slow-slip events observed in modern subduction zones. We therefore suggest that some episodic tremor and slip can originate through combined fracture and viscous flow across shear zones comprising mixtures of strong and weak materials. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Krauklis wave initiation in fluid-filled fractures by seismic body waves

Krauklis waves are a special wave mode that is bound to and propagates along fluid-filled fractures. They can repeatedly propagate back and forth along a fracture and eventually fall into resonance emitting a seismic signal with a dominant characteristic frequency. They are of great interest because this resonant behavior can lead to strongly frequency-dependent propagation effects for seismic body waves and may explain seismic tremor generation in volcanic areas or affect microseismic signals in fractured fluid reservoirs. It has been demonstrated that Krauklis waves can be initiated by a seismic source inside the fracture, for example by hydrofracturing. Here, the aim is to study Krauklis wave initiation by an incident plane P- or S-wave in numerical simulations. Both seismic body waves are reflected and scattered at the fracture, but also, two Krauklis waves are initiated with significant amplitude, one at each fracture tip (i.e., at the diffraction-points of the fracture). Generally, the incident S-wave initiates larger-amplitude Krauklis waves compared to the incident P-wave case. For both incident wave modes, the initiation of Krauklis waves strongly depends on the fracture orientation. In the case of an incident P-wave, large-amplitude Krauklis waves are initiated at moderate (12°–40°) and high ([Formula: see text]) inclination angles of the fracture with a distinct gap at approximately 50°. The dependency of Krauklis wave initiation on fracture orientation is almost inversed in the case of an incident S-wave and the largest-amplitude Krauklis waves are initiated at an S-wave incidence angle of approximately 50°. The initiation of large-amplitude Krauklis waves by both P- and S-waves has important implications for earthquake signals propagating through fluid-bearing fractured rocks (volcanic areas, fluid-reservoirs) or for seismic exploration surveys in fractured reservoir situations.

鉱物脈組織から読み解く地殻流体流動

Fluids in the Earth's crust play crucial roles in various geological phenomena including earthquakes, volcanism and formation of ore deposits. Mineral-filling veins are fossils of fluid-filled fractures, but it is not easy to extract information on nature of fluid flow during vein formation. Mineral veins in the metamorphic rocks show systematic variation in internal textures (grain shape, mineral distribution, crystallographic orientation, relation to the host rock) as a function of aperture size and host rock. Hydrothermal experiments for silica precipitation have revealed that the occurrences and mineralogy of silica minerals are strongly controlled by solution chemistry (supersaturation, minor elements), rock substrates (quartz distribution, grain size), fracture geometry (aperture distribution, surface roughness) as well as temperature, which results in a variation in vein texture. Combining modeling and analyses of natural and synthetic veins, vein texture is a potentially useful indicator of hydrological properties (flow velocity, flow direction, degree of supersaturation, duration of sealing etc.) in the crust.

Seismic wavefield modeling in media with fluid-filled fractures and surface topography

We present a finite difference (FD) method for the simulation of seismic wave fields in fractured medium with an irregular (non-flat) free surface which is beneficial for interpreting exploration data acquired in mountainous regions. Fractures are introduced through the Coates-Schoenberg approach into the FD scheme which leads to local anisotropic properties of the media where fractures are embedded. To implement surface topography, we take advantage of the boundary-conforming grid and map a rectangular grid onto a curved one. We use a stable and explicit second-order accurate finite difference scheme to discretize the elastic wave equations (in a curvilinear coordinate system) in a 2D heterogeneous transversely isotropic medium with a horizontal axis of symmetry (HTI). Efficiency tests performed by different numerical experiments clearly illustrate the influence of an irregular free surface on seismic wave propagation in fractured media which may be significant to mountain seismic exploration. The tests also illustrate that the scattered waves induced by the tips of the fracture are re-scattered by the features of the free surface topography. The scattered waves provoked by the topography are re-scattered by the fractures, especially Rayleigh wave scattering whose amplitudes are much larger than others and making it very difficult to identify effective information from the fractures.

Spatial variation of the aftershock activity across the Kachchh Rift Basin and its seismotectonic implications

We analyzed 3365 relocated aftershocks with magnitude of completeness (Mc) ≥1.7 that occurred in the Kachchh Rift Basin (KRB) between August 2006 and December 2010. The analysis of the new aftershock catalogue has led to improved understanding of the subsurface structure and of the aftershock behaviour. We characterized aftershock behaviour in terms of a-value, b-value, spatial fractal dimension (D s ), and slip ratio (ratio of the slip that occurred on the primary fault and that of the total slip). The estimated b-value is 1.05, which indicates that the earthquake occurred due to active tectonics in the region. The three dimensional b-value mapping shows that a high b-value region is sandwiched around the 2001 Bhuj mainshock hypocenter at depths of 20–25 km between two low b-value zones above and below this depth range. The D s -value was estimated from the double-logarithmic plot of the correlation integral and distance between hypocenters, and is found to be 2.64 ± 0.01, which indicates random spatial distribution beneath the source zone in a two-dimensional plane associated with fluid-filled fractures. A slip ratio of about 0.23 reveals that more slip occurred on secondary fault systems in and around the 2001 Bhuj earhquake (Mw 7.6) source zone in KRB.

Tracking volcanic and geothermal activity in the Tongariro Volcanic Centre, New Zealand, with shear wave splitting tomography

We apply a simplified two-dimensional tomographic inversion of recorded delay times of shear wave splitting and a spatial averaging of fast direction of anisotropy to data from temporary seismic deployments in the Tongariro Volcanic Centre in order to identify regions of changing seismic anisotropy. We observe a region of strong anisotropy (>0.025s/km greater than the surrounding area) centered on Mt. Ruapehu in 1995, the time of a major magmatic eruption. This is interpreted to be due to an increase in fluid-filled fractures during the eruption. We also observe strong anisotropy (~0.018s/km greater than the surrounding area) and a change in fast direction (~80°) at Mt. Tongariro in 2008 and examine the temporal evolution of this anomaly using clusters of earthquakes and permanent seismic stations in operation since 2004. This anomaly is attributed to a change in the geothermal system. A pronounced and unchanging feature is observed at Waiouru, even when the source and receiver locations differ. We therefore conclude that the transient features of strong anisotropy associated with volcanic and geothermal activity detected with this method are also robust.