Presence Of Pore Fluids Research Articles

Impact of poroelastic effects on the inversion of fracture properties from amplitude variation with offset and azimuth data in horizontal transversely isotropic media

The identification and characterization of fractures is an important objective in many areas of earth and environmental sciences. Amplitude variation with offset and azimuth (AVOAz) analysis of seismic reflection data is a key method for achieving these tasks. Theoretical and experimental studies have shown that the presence of pore fluids together with the strong mechanical contrast between the fractures and their embedding background give rise to wave-induced fluid flow (WIFF) effects. This implies that the effective stiffness tensor of a fluid-saturated fractured rock defining its seismic response becomes viscoelastic and frequency-dependent. In spite of this, AVOAz analysis typically relies on end-member-type elastic stiffness models that either assume a relaxed (i.e., equilibrated) or unrelaxed (i.e., unequilibrated) state of the wave-induced fluid pressure in the rock. In general, however, neither the appropriateness of the chosen model nor the associated errors in the inversion process are known. To investigate this topic, we have considered a poroelastic medium containing parallel vertical fractures and generate synthetic seismic AVOAz data using the classic Rüger approximations for PP-wave reflection coefficients in horizontally transversely isotropic media. A Markov chain Monte Carlo method is used to perform a Bayesian inversion of the synthetic seismic AVOAz data. We quantify the influence of WIFF effects on the AVOAz inversion results when elastic relaxed and unrelaxed models are used as forward solvers of inversion schemes to estimate the fracture volume fraction, the elastic moduli, and the porosity of the background rock, as well as the overall weakness of the medium due to the presence of fractures. Our results indicate that, when dealing with single-frequency data, relaxed elastic models provide biased but overall better inversion results than unrelaxed ones, for which some fracture parameters cannot be resolved. Improved inversion performance is achieved when using frequency-dependent data, which illustrates the importance of accounting for poroelastic effects.

The use of super-critical carbon dioxide as the working fluid in enhanced geothermal systems (EGSs): A review study

Naturally-occurring high-temperature geothermal energy resources contribute significantly to the world’s emerging energy demand by allowing renewable and clean power generation, and the technology of enhanced geothermal systems (EGSs) continues to progress at deeper levels, providing the ability to extract heat stored in deep underground at 2–5 km depths, which would have previously been considered impractical. Water-based EGSs are fairly well established, and a number of research studies have been carried out on the performance of water as the working fluid in geothermal reservoirs. Besides, the applicability of super-critical carbon dioxide (ScCO2) as the working fluid in EGSs by replacing water was recently investigated by research studies due to a number of advantages accompanied in CO2 as the working fluid in EGSs. The aim of this review study is to comprehensively investigate the applicability of ScCO2 as a working fluid in EGSs by summarising the recent research studies conducted on the operation of CO2-based geothermal systems. The study is further intended to highlight the advantages of ScCO2 in the EGS environment compared to conventional water-based methods. Of the advantages, a reduction of the external power requirement for the pumping of ScCO2 due to enhanced injectivity and flowability is the most attractive. In terms of heat extraction, higher mass flowrates can be achieved using ScCO2 as a working fluid, depending on the density-to-viscosity ratio, and the ratio is doubled with ScCO2 compared to water in the temperature range of 200–250 °C. Moreover, an accelerated energy recovery can be achieved through the higher thermal extraction rates of ScCO2, which are 50% greater than for water. Importantly, ScCO2 has the ability of inducing an interconnected fracture network with multiple secondary branches under comparatively low breakdown pressures compared to water. ScCO2 being a linear molecule with lower viscosity has the percolation ability into microcracks of the rock matrix, and thereby, the pressurisation of these microcracks induce multiple flow paths long the rock matrix by interconnecting the microcracks. In addition, the use of ScCO2 eliminates the problems associated with silica dissolution followed by precipitation, takes place in surface piping, heat exchangers and other surface equipment, because dry ScCO2 is less reactive in hot-dry reservoirs. However, in water-based EGSs, enhanced reactivity of water with the increase of pH accelerates the dissolution and precipitation of minerals, and is a critical issue in water-based EGSs. The enhanced reactivity of ScCO2 in the presence of pore fluid may also affect the long-term integrity of EGSs, because it alters the internal pore structure, due to the dissolution and precipitation of primary and secondary rock minerals, resulting in altered porosity and permeability, which ultimately affect the energy extraction process.

Interaction Between Fluid and Porous Media with Complex Geometries: A Direct Pore‐Scale Study

AbstractEffect of boundary stress on permeability in porous media at pore‐scale is investigated by means of numerical methods for computing deformation of rock material in the presence of pore fluid. We implement a partitioned, strongly coupled solver for transient fluid‐solid simulations with independent conformed interface meshes for the fluid and solid domains. Fluid flow and solid computations were performed using the finite element method. External load is prescribed along the solid boundary in gradual steps to examine the extent of pore‐scale deformation along internal grain walls while applying a fixed constraint at the opposite end to ensure uniform stress distribution across the media. Hydrodynamic interactions between the pore walls and the matrix leading to interface displacements by fluid viscous forces are also investigated by examining fluid flow with different combinations of Reynolds number. Coupling is achieved via traction and displacements boundary conditions at the interface with respect to Lagrangian coordinates. The role of particle shape in the matrix deformation process is also considered in this paper. In order to examine the how particle shape affects local affects local permeability/stress, we examine fluid movement based on varying Reynolds numbers in Berea sandstone obtained from Micro‐CT image scans at high resolution, and artificially constructed random polydisperse 3‐D sphere packs. It was found that permeability alteration under the influence of uniaxial stress was more in the sandstone sample than in the sphere‐pack and that the sphere‐pack deformed less than the actual sample due to pore geometry, surface smoothness, and homogeneity.

Effect of temperature and pore fluid on the strength of porous limestone

AbstractThe presence of pore fluid in rocks generally reduces brittle strength. The role of water weakening in porous carbonate rocks and the effect of enhanced deformation on concomitant transport property evolution are poorly understood. The interaction between pore fluid and deformation mechanisms in carbonate rocks is complicated by crystal plasticity and solubility in calcite at low temperatures and pressures. To explore this system, we perform conventional triaxial deformation experiments on Indiana Limestone (~16% porosity) over a range of temperatures (23–75°C) and effective confining pressures (10–50 MPa). Samples are deformed with pore fluids of different saturation states with respect to calcite host rock. Samples tend to fail by distributed deformation. Sample permeability is reduced by up to an order of magnitude during deformation, even in samples exhibiting dilatancy. Compactive yield strength, associated with shear‐enhanced compaction, is greatly reduced in saturated samples compared to dry samples. Dilatancy is suppressed and compaction enhanced at elevated pressures. Greater weakening effects are observed when pore fluid is far‐from‐equilibrium with carbonate rock. Observed weakening results from the interaction between microcracking, mechanical twinning, and pressure solution.

Dilatant hardening of fluid‐saturated sandstone

AbstractThe presence of pore fluid in rock affects both the elastic and inelastic deformation processes, yet laboratory testing is typically performed on dry material even though in situ the rock is often saturated. Techniques were developed for testing fluid‐saturated porous rock under the limiting conditions of drained, undrained, and unjacketed response. Confined compression experiments, both conventional triaxial and plane strain, were performed on water‐saturated Berea sandstone to investigate poroelastic and inelastic behavior. Measured drained response was used to calibrate an elasto‐plastic constitutive model that predicts undrained inelastic deformation. The experimental data show good agreement with the model: dilatant hardening in undrained triaxial and plane strain compression tests under constant mean stress was predicted and observed.

Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions

The Alpine Fault is a major plate-boundary fault zone that poses a major seismic hazard in southern New Zealand. The initial stage of the Deep Fault Drilling Project has provided sample material from the major lithological constituents of the Alpine Fault from two pilot boreholes. We use laboratory shearing experiments to show that the friction coefficient µ of fault-related rocks and their precursors varies between 0.38 and 0.80 depending on the lithology, presence of pore fluid, effective normal stress, and temperature. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a gouge sample located very near to the principal slip zone exhibits µ = 0.67, which is high compared with other major fault zones targeted by scientific drilling, and suggests the capacity for large shear stresses at depth. A consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence. Critical nucleation patch lengths estimated using representative values of the friction velocity-dependent parameter a−b and the critical slip distance D c , combined with previously documented elastic properties of the wall rock, may be as low as ~3 m. This small value, consistent with a seismic moment M o = ~4 × 1010 for an M w = ~1 earthquake, suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3–7 km.

Liquefaction of sedimentary rocks during impact crater development

Impact crater development on every planetary body requires catastrophic movement of large volumes of crustal rocks. The process produces well-known features such as brecciation and frictional melting, but a mechanism that explains how rocks accommodate the strain during the cratering flow remains unclear. Here, we investigate target rocks from the Araguainha impact crater (central Brazil) that typify what happens to a consolidated, fluid-saturated sedimentary rock at ∼2 km below the surface prior to the impact event. Sandstone units record a pattern of chaotic large-scale folds and pervasive microscopic (grain-to-grain) brecciation that result from rock strength degradation triggered by the impact. Field mapping and extensive textural observations indicate that these sandstones experienced initial microstructural damage from the shock wave and that this process may have weakened grain-to-grain bonds and started the process of pervasive microbrecciation. Accompanying heating and decompression lead to vaporization and expansion of fluids in the sandstone pores, magnifying the process of brecciation by effectively liquefying the rock mass and allowing for chaotic folding (at a range of scales up to blocks 100 m in length) in the central uplift. This is a vaporization-assisted microbrecciation, and it may have inhibited the formation of pseudotachylites, because energy was dissipated by pervasive microcracking, vaporization of pore fluids, and large scale chaotic folding, rather than localized displacement on brittle faults and frictional heating. We suggest that impact liquefaction of sedimentary rocks depends on whether the presence of pore-fluids and related micro-brecciation are sufficient to dissipate most of the impact energy.

An exploratory study of the seismic properties of thermally cracked, fluid- saturated aggregates of sintered glass beads

A synthetic rock analogue with simple microstructure was used to advance our understanding of the influence of cracks and pore fluids on seismic properties. The glass beads with ~ 300 μm diameter were sintered near the glass transition with average 1~2% porosity and subsequently quenched from high temperature into water at room temperature to introduce cracks with uniformly low aspect ratio α ~ 0.0007. Jackson-Paterson attenuation apparatus was used for both torsional and flexural mode forced oscillations at seismic frequencies to extract shear and Young’s modulus respectively, with or without the presence of pore fluids (e.g. argon, water) of varying viscosities. In-situ permeability was extracted by using pore-fluid re-equilibration method. Shear modulus is found lower with longer oscillation periods for the cracked and argon pore fluid saturated material possibly indicating the pore-fluid relaxation mechanism at sufficiently longer periods, with minimal strain-energy dissipation 1/Q < 0.003. The averaged elastic moduli for different oscillation periods and permeability are discovered to be extremely sensitive to variation of effective pressure. The crack closure effects can be observed easily at the effective pressure level at ~ Eα, consistent with the theoretical prediction.

Compressional waves in fluid-saturated porous solid containing a penny-shaped crack

Diffraction of normal compression waves by a penny-shaped crack in a fluid-saturated porous medium is investigated. Two wave types are considered, namely, compressional wave of the first kind, and the second kind. The former, also known as fast wave, propagates primarily through the solid, whereas the latter or slow wave, propagates mainly in the fluid. Each wave propagates in the medium along with induced wave of the same type in the companion constituent of the material. Application of Biot’s theory in conjunction with integral transform technique reduces the problem to a mixed boundary-value problem whose solution is in turn governed by a Fredholm integral equation of the second kind. Near-field and far-field solutions are obtained in terms of the dynamic stress-intensity factor and the scattering cross section, respectively. They are of particular importance to the linear elastic fracture mechanics (LEFM) and in the scattering theory of elastic waves. The mode I stress-intensity factors are computed numerically for a set of selected material property values, and shown graphically for various mass density and viscosity-to-permeability ratios. The obtained results reveal significant impact of the presence of pore fluid upon the stress-intensity factors, both magnitudes and frequencies at their peak values. The influence of the fluid is also observed from the calculated scattering cross sections of the scattered far-field. Accuracy of the present solution procedure is verified by comparing the numerical results with existing results in the limiting case of dry elastic materials.

The Molecular-Scale Fundament of Geothermal Fluid Thermodynamics

Chemical interactions between fluids and minerals play a central role in numerous geological processes. In a hydrothermal environment, such reactions are usually referred to as fluid-rock interaction or water-rock interaction and lead to alteration of rocks by changing the mineral assemblages and compositions. During fluid-rock interaction the fluid’s chemical composition also changes, some components being depleted upon secondary mineral formation and others being enriched due to primary mineral dissolution. An example is the interaction between rocks at the seafloor and heated seawater during hydrothermal convection at mid-ocean ridges. There, the rocks are altered, extracting Mg and SO4 from the circulating seawater and releasing metals such as Cu into the fluid. The chemically modified seawater is released back into the ocean at black smoker sites and diffuse discharge sites. This process contributes to buffering the chemical composition of the oceans over geological time periods. In sedimentary systems, the evaporation of aqueous fluids in marine and intracontinental settings leads to the formation of evaporites from complex, multicomponent brines. The diagenesis of sedimentary rocks is closely related to mineral dissolution and precipitation reactions in the presence of pore fluids that often flow over long distances in aquifers. The majority of ore deposits have formed by the interaction of hot hydrothermal fluids with rocks, dissolving the ore constituents as trace elements from a large volume of source material, transporting and focusing them during hydrothermal fluid flow and eventually precipitating them in concentrated form in a smaller rock volume due to chemical precipitation. The precipitation may result from the chemical effects of the reactions between the fluid and the rock. For example, the pH may increase by neutralization of acid fluids, a process that is often key to Pb-Zn sulfide mineralization in limestone and marble. Changes in the physico-chemical conditions (e.g., a temperature decrease) …

Pore fluid in experimental calcite-bearing faults: Abrupt weakening and geochemical signature of co-seismic processes

While it is widely recognized that fluids influence fault strength and earthquake nucleation, propagation and arrest, their effects on co-seismic sliding friction are only conjectured. To shed light on these effects, 55 high velocity (>1ms−1) friction experiments were conducted at room temperature on Carrara marble samples in the presence of pore fluid (up to 15MPa pore pressure), room-humidity and “vacuum” (10−4mbar) conditions. In all the experiments, the friction coefficient evolved from a peak value of 0.6–0.8 to a steady-state value of 0.1 in about 1–1.5m of slip. However, experiments performed in the presence of pore fluid had a large and more abrupt decrease in friction at the initiation of sliding (65% after 20mm of slip), whereas experiments performed under vacuum and room humidity conditions showed initial velocity-strengthening behavior followed by a more gradual reduction in friction. This indicates that calcite-bearing rocks are more prone to slip in the presence of water. Under room-humidity conditions, CO2 was detected during the entire duration of the experiment. In the presence of pore fluid, HCO3− and Ca2+ were detected for slips >0.1m. The lack of decarbonation products (HCO3− and Ca2+) in pore fluid experiments for slip <0.1m implies that the abrupt weakening is not related to decarbonation (or that the abundance of the reaction products is below the resolution of the analytical methods). Given the modest thermal expansion of water, the estimated thermal pressurization during the abrupt weakening appears to be negligible. Instead, we suggest that abrupt weakening is due to subcritical crack-growth, hydrolytic weakening and brittle failure of the asperities on the sliding surfaces. Modeling shows that the occurrence in nature of co-seismic (water-present) decarbonation reactions similar to those triggered in the laboratory could yield sufficient reaction product to be detected in aquifers located in the proximity of active faults.

Laboratory triggering of stick‐slip events by oscillatory loading in the presence of pore fluid with implications for physics of tectonic tremor

The physical mechanism by which the low‐frequency earthquakes (LFEs) that make up portions of tectonic (also called non‐volcanic) tremor are created is poorly understood. In many areas of the world, tectonic tremor and LFEs appear to be strongly tidally modulated, whereas ordinary earthquakes are not. Anomalous seismic wave speeds, interpreted as high pore fluid pressure, have been observed in regions that generate tremor. Here we build upon previous laboratory studies that investigated the response of stick‐slip on artificial faults to oscillatory, tide‐like loading. These previous experiments were carried out using room‐dry samples of Westerly granite, at one effective stress. Here we augment these results with new experiments on Westerly granite, with the addition of varying effective stress using pore fluid at two pressures. We find that raising pore pressure, thereby lowering effective stress can significantly increase the degree of correlation of stick‐slip to oscillatory loading. We also find other pore fluid effects that become important at higher frequencies, when the period of oscillation is comparable to the diffusion time of pore fluid into the fault. These results help constrain the conditions at depth that give rise to tidally modulated LFEs, providing confirmation of the effective pressure law for triggering and insights into why tremor is tidally modulated while earthquakes are at best only weakly modulated.

A unified method to predict diffuse and localized instabilities in sands

A simplified method to analyse diffuse and localized bifurcations of sand under drained and undrained conditions is presented in this paper. This method utilizes results from bifurcation analysis and critical state plasticity theory to detect the onset of pure and dilatant shear band formation, static liquefaction and drained shear failures systematically. To capture the soil collapse observed in experiments, the instability state line concept originated by Chu, Lo and Lee in 1993 is adopted. Emphasis is given to examine how the presence of pore-fluid may facilitate or delay instability after yielding occurs. The predictions of instabilities are compared with experimental data from triaxial compression tests on Toyoura and Changi sands.

Impact of Fractures on CO2Storage Monitoring: Keys for an Integrated Approach

The monitoring of CO2 storage in fractured reservoirs (depleted hydrocarbon fields or brine aquifers) requires the study of the impact of fracturation and fluid substitution on seismic data. Seismic data can provide information about the additional compliance due to the fractures and the fluids through the analysis of seismic azimuthal anisotropy with an appropriate rock physics model. We introduce a rock physics model built in collaboration with geologists, providing a realistic description of fractured media. This model concerns fractured geological media in the presence of fluids characterized by some degree of matrix porosity, the presence of pore fluids, connected and/or non-connected fractures, the presence of several fracture sets, and an inherent seismic anisotropy. The direct application of this rock model shows that the P -wave anisotropy value measured through seismic data can be explained by several sets of different parameters such as the fracture density, the pore fluid compliance or the porosity. The presence of inherent layer-induced anisotropy can also modify the P -wave anisotropy and thus the interpretation of this value in terms of fluid substitution in a fractured porous medium. As far as fluid substitution monitoring is concerned, if seismic data are acquired before and after this substitution, a change in the P -wave anisotropy value can be linked to the modification of the compliance of the fluid content in the same medium exhibiting the same fracture network and the same porosity. This relative value can only be correctly interpreted in terms of fluid substitution provided we have some constraints on a few of the parameters involved in the P -wave anisotropy value such as the porosity, and a rough idea of the level of normalized fracture compliance. Then, a multidisciplinary approach is mandatory to constrain these parameters. For instance, borehole and outcrop geological information can give the upper limit of the fracture density expected at depth in the same formation. Furthermore, rock mechanics helps in understanding the fracturation state at depth to identify the predominant fractures in regard to the interpretation of seismic anisotropy in terms of fluid substitution inside the fracture network.

Geometry and subsidence history of the Dead Sea basin: A case for fluid‐induced mid‐crustal shear zone?

Pull‐apart basins are narrow zones of crustal extension bounded by strike‐slip faults that can serve as analogs to the early stages of crustal rifting. We use seismic tomography, 2‐D ray tracing, gravity modeling, and subsidence analysis to study crustal extension of the Dead Sea basin (DSB), a large and long‐lived pull‐apart basin along the Dead Sea transform (DST). The basin gradually shallows southward for 50 km from the only significant transverse normal fault. Stratigraphic relationships there indicate basin elongation with time. The basin is deepest (8–8.5 km) and widest (∼15 km) under the Lisan about 40 km north of the transverse fault. Farther north, basin depth is ambiguous, but is 3 km deep immediately north of the lake. The underlying pre‐basin sedimentary layer thickens gradually from 2 to 3 km under the southern edge of the DSB to 3–4 km under the northern end of the lake and 5–6 km farther north. Crystalline basement is ∼11 km deep under the deepest part of the basin. The upper crust under the basin has lowerPwave velocity than in the surrounding regions, which is interpreted to reflect elevated pore fluids there. Within data resolution, the lower crust below ∼18 km and the Moho are not affected by basin development. The subsidence rate was several hundreds of m/m.y. since the development of the DST ∼17 Ma, similar to other basins along the DST, but subsidence rate has accelerated by an order of magnitude during the Pleistocene, which allowed the accumulation of 4 km of sediment. We propose that the rapid subsidence and perhaps elongation of the DSB are due to the development of inter‐connected mid‐crustal ductile shear zones caused by alteration of feldspar to muscovite in the presence of pore fluids. This alteration resulted in a significant strength decrease and viscous creep. We propose a similar cause to the enigmatic rapid subsidence of the North Sea at the onset the North Atlantic mantle plume. Thus, we propose that aqueous fluid flux into a slowly extending continental crust can cause rapid basin subsidence that may be erroneously interpreted as an increased rate of tectonic activity.

Propagation of torsional surface wave in anisotropic poroelastic medium under initial stress

The present work deals with the possibility of propagation of torsional surface wave in fluid saturated poroelastic layer lying over nonhomogeneous elastic half space. Both the media are assumed to be under compressive initial stress. The half space has two types of inhomogeneity, viz; hyperbolic and quadratic. The dispersion equation for torsional wave in porous layer has been derived and observed that the presence of fluid in pores increases the velocity of the torsional surface wave but the phase velocity diminishes due to the presence of compressive initial stress in the porous layer. It is also observed that the velocity of the torsional surface wave increases due to the increase of initial stress in inhomogeneous half space. The inhomogeneity factor due to quadratic and hyperbolic variations in rigidity, density and initial stress of the medium decreases the phase velocity as it increases.

The role of rock fragmentation in the motion of large landslides

By considering the implications of the comminution generally associated with very large landslides, we arrive at a simple explanation for the remarkably low frictional resistance to motion demonstrated by large intact blockslides (e.g. Waikaremoana, New Zealand), volcanic debris avalanches (e.g. Socompa, Chile) and large rock avalanches (e.g. Falling Mountain, N.Z.), which allows such mass movements to achieve unexpectedly high velocities and long runout distances. During rapid grain flow under high direct stress, the overall grain motion generates stresses in many force chains that strain individual grains to failure; most of the elastic strain energy accumulated in these force chains before failure is returned at failure to the resulting grain fragments, resulting in apparent instantaneous pressures of ~ 3 Q on the surroundings, where Q is the ambient strength of the previously intact grains (~ GPa). These intense pressures support some of the direct force on the shear layer, so that the effective (intergranular) stress in the shear layer is reduced. Because frictional resistance is proportional to effective stress, this reduces the overall frictional resistance to shear. The steady-state effective stress is that which just allows fragmentation to continue; the resistance to motion estimated from this relationship explains to much better than order-of-magnitude accuracy the reported motions of the large, rapid mass movements. We also deduce that grain fragmentation can be sustained for sufficiently long to explain the phenomena without reducing the mean grain size by volume in the granular layer to unrealistically small values. The presence of pore fluid does not appear to influence the effect of fragmentation dynamics in a major way. The proposed mechanism requires further laboratory and simulation studies to reduce its current dependence on limited field data, but its success suggests that it is worthwhile investigating further as an explanation for large mass movements in the types of brittle rock in which fragmentation occurs.

高温高圧条件下での岩石の弾性波速度測定とその地質学的意義

We review the methods of measuring the velocities of elastic-waves in rocks and summarize the temperature-dependence of elastic-wave velocities under high-temperature and high-pressure conditions. The elastic-wave velocities in rocks are strongly affected by several phenomena such as thermal cracking, phase transition of minerals, partial melting of rocks, and dehydration of hydrous minerals. These phenomena are strongly affected by pressure-temperature conditions and chemical compositions of rocks and minerals. Thus, it is very difficult to predict the elastic wave velocities of rocks and minerals under high-pressure and high-temperature conditions theoretically. Laboratory measurements of the velocities of elastic-waves in rocks under high-pressure and high-temperature conditions have provided useful data for estimating physical and geological properties in the crust and upper mantle. We also mention the next issues to be studied in relation to the velocity of elastic waves in rocks. It is important to measure elastic-wave velocities in rocks under high-temperature and high-pressure conditions in the presence of pore-fluids.

Hydrogeochemistry, groundwater ages and sources of salts in a granitic batholith on the Canadian Shield, southeastern Manitoba

The hydrogeochemistry of the Lac du Bonnet granitic batholith has been determined for the region of the Whiteshell Research Area (WRA) in southeastern Manitoba, Canada. This work forms part of the geosciences studies performed for the Canadian Nuclear Fuel Waste Management Program over the period 1980–1995 by Atomic Energy of Canada Limited (AECL). Knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the Canadian Shield. Groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay-rich overburden, from packer-isolated borehole zones intersecting fractures or fault zones in the bedrock, and from boreholes in unfractured rock in AECL's Underground Research Laboratory (URL). Eighty-six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the Lac du Bonnet batholith. In addition, 28 groundwaters from piezometers in a large wetland area near the URL have been sampled and analysed to determine the influence of clay-rich overburden on the bedrock hydrogeochemistry. Analyses have been made for major and minor ions, pH, Eh, trace metals, and stable and radioactive isotopes, to characterise these groundwaters and relate them to their hydrogeologic regimes. Shallow groundwaters in the fractured bedrock are generally dilute (TDS <0.3 g/l), Ca–Na–HCO 3 waters and show little indication of mixing with Ca–Mg–HCO 3–SO 4 groundwater from overburden sediments. The near-modern levels of 3H and 14C, and a warm-climate 2H/ 18O signature in these groundwaters, indicates that the upper ∼200 m of fractured bedrock contains an active groundwater circulation system with a residence time of tens to hundreds of years. Deeper fracture groundwaters (200–400 m depth) in recharge areas, are more alkaline, Na–Ca–HCO 3 waters and evolve to Na–Ca–HCO 3–Cl–SO 4 waters with increasing distance along the flow path. Isotopic data indicate the presence of a glacial melt-water component suggesting that the residence times of these waters are 10 3–10 5 a. These waters form a transition zone between the upper, advective flow regime and a deeper regime in sparsely fractured rock where groundwater in fractures and fracture zones is largely stagnant. At these depths (> 500 m), Na–Ca–Cl–SO 4 waters of increasing salinity (up to 50 g/l) with depth are found and in some fractures the waters have evolved to a Ca–Na–Cl composition. Isotopic data indicate that these waters are warm-climate and pre-glacial in origin, with residence times of over 1 Ma. Pore fluids observed to drain from the unfractured rock matrix in the URL facility are almost pure Ca–Cl in composition, ∼90 g/l salinity, and have a 2H/ 18O composition displaced well to the left of the global meteoric water line, about which all other WRA groundwaters lie. This information indicates that these pore fluids have undergone prolonged water-rock interaction and have residence times of 10 1–10 3 Ma. Most of the deeper fracture groundwaters and pore fluids have low Br/Cl ratios and moderate to high δ 34S values of dissolved SO 4 which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith. These fluids may have entered the batholith during early Paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western Canada. The hydrogeochemical data and interpretations show that below ∼500 m in the WRA, fracture-hosted groundwaters are very saline, reducing and old, and are, therefore, indicative of stagnant conditions over the period of concern for nuclear waste disposal (1 Ma). The intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics. The pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 10 3 Ma.

Torsional Surface Waves in an Initially Stressed Anisotropic Porous Medium

It is well known that the homogeneous elastic half space does not allow the torsional surface wave to propagate. The present article attempts to study the possibility of propagation of such waves in a liquid-filled initially stressed poroelastic layer over nonhomogeneous half space. The study concludes that torsional surface waves may propagate in the poroelastic layer. The presence of fluid in pores and initial compressive stresses diminishes the velocity. The nonhomogeneity in rigidity and density of the half space also has a certain role to play in initiating, enhancing, or diminishing the velocity of torsional surface waves.