Mobile Fluid Research Articles

Key Factors for Depressurization-Induced Gas Production from Oceanic Methane Hydrates

Oceanic methane hydrate (MH) deposits have been found at high saturations within reservoir-quality sands in the Eastern Nankai Trough and the Gulf of Mexico. This study investigates the key factors for the success of depressurization-induced gas production from such oceanic MH deposits. A numerical simulator (MH21-HYDRES: MH21 Hydrate Reservoir Simulator) was used to study the performance of gas production from MH deposits. We calculated the hydrate dissociation behavior and gas/water production performance during depressurization for a hypothetical MH well. Simulation runs were conducted under various initial reservoir conditions of MH saturation, temperature, and absolute permeability. A productivity function (PF) was introduced as an indicator of gas productivity, which is a function of gas production rate, water production rate, and discount rate. The simulations showed that recovery factors over 36% and maximum gas production rates over 450 000 Sm3/d were expected for the most suitable conditions of a class 3 deposit (i.e., an isolated MH deposit that is not in contact with any hydrate-free zone of mobile fluids). However, gas productivity was affected by formation temperature and initial effective permeability. The values of PF increased with increasing formation temperature when the initial permeability of the deposit was higher than a threshold value (the threshold permeability); however, it decreased for the deposit below the threshold permeability. The threshold permeability was estimated to be between 1 and 10 mD in the class 3 deposit. These results suggest that key factors for the success of depressurization-induced gas production from oceanic MH are as follows: (1) The initial effective permeability of the MH deposit is higher than the threshold value, and (2) the temperature of the MH deposit is as high as possible.

Topography of Accumulation of Stagnant Lymph and Tissue Fluid in Soft Tissues of Human Lymphedematous Lower Limbs

The knowledge of where does excess tissue fluid accumulate in obstructive lymphedema is indispensable for rational physical therapy. However, it has so far been limited to that obtained from lymphoscintigraphic, ultrasonographic, and MR images. None of these modalities provide composite pictures of dilated lymphatics and expanded tissue space in dermis, subcutis, and muscles. So far, only anatomical dissection and histological processing of biopsy material can visualize the tissue lymphatic network and the sites of accumulation of the excess of mobile tissue fluid. We visualized the "tissue fluid and lymph" space in skin and subcutaneous tissue of foot, calf, and thigh in various stages of lymphedema in specimens obtained during lymphatic microsurgical procedures or tissue debulking, using special staining techniques. The volume of accumulated fluid was calculated from the densitometric data of stained tissue sections. We found that lymph was present only in the subepidermal lymphatics, whereas the collecting trunks were obliterated in most cases. Mobile tissue fluid accumulated in the spontaneously formed spaces in the subcutaneous tissue, around small veins and above and underneath muscular fascia. Deformation of subcutaneous tissue by free fluid led to formation of interconnecting channels. The volume of subcutaneous free fluid ranged around 50% of total tissue volume and there were no significant differences in various stages of lymphedema. This could be explained by the presence of thick layers of subcutaneous fat tissue even in the most advanced stage of lymphedema. In lymphedema caused by obliteration of collecting trunks, lymph is present only in the subepidermal lymphatics, whereas the bulk of stagnant tissue fluid accumulates in the subcutaneous tissue and above and beneath muscular fascia. These findings should be useful for designing pneumatic devices for limb massage as well as for rational manual lymphatic drainage in terms of sites of massage and level of applied external pressures.

An assessment of the mantle and slab components in the magmas of an oceanic arc volcano: Raoul Volcano, Kermadec arc

Raoul Volcano occupies a simple oceanic subduction setting in the northern part of the Kermadec arc on the Pacific–Australian convergent plate boundary. The primary inputs to the magmatic system that feeds the volcano are a subduction component derived from the subducting old Pacific oceanic lithosphere and its veneer of pelagic sediment, and the overlying peridotitic mantle wedge. Conservative trace elements that are very incompatible during mantle melting are relatively depleted in Raoul lavas indicating a source that has been depleted during an earlier melting event. Major element co-variations indicate magma genesis by 25% near fractional melting of a mantle source that is weakly depleted (2% melt extraction) relative to a fertile MORB source. An important influence on the composition of the mantle component is progressive melt extraction coupled with minimal advection of fresh material into the sub-arc zone followed by melt extraction from a melting column beneath the spreading centre of an adjacent back arc basin. High field strength element and rare earth element systematics indicate involvement of a subduction-related component of constant composition. Two fluid components can be distinguished, one enriched in large ion lithophile elements inferred to be an aqueous fluid that is continuously added to the ascending melt column and the other a less mobile fluid that transfers Th. A homogeneous subduction-related component of constant composition and magnitude arises if the slab-derived flux migrates from the slab–mantle interface to the sub-arc melting column by repeated episodes of amphibole formation and decomposition its composition is then governed by the distribution coefficients of pyroxene and its magnitude by the degree of amphibole saturation of mantle peridotite. The results from Raoul Volcano are comparable to those from other oceanic subduction-related arcs such as South Sandwich and Marianas suggesting that this is a general model for oceanic arcs.

Nitrogen isotopic exchange during maturation of organic matter

The definition of kerogen as the insoluble fraction of sedimentary organic matter leads to the assumption that all of its chemical constituents are autochthonous. Evidence is presented that some of the organic N in kerogen derives from allochthonous mobile and chemically reactive N species (e.g., ammonium, HCN and low molecular weight organic N containing compounds) that are delivered by migrating fluids to stationary kerogen. Reactions of humic compounds (e.g., protokerogen) with ammonium in soil and sediment are facilitated biologically. At higher temperatures, reactions between kerogen in rocks and N containing compounds in mobile fluids are driven geochemically. N isotopic exchange between kerogen and NH4+ or NH3 is evident from experiments where 15N enriched ammonium chloride solutions and source rocks containing kerogen types I, II, IIS and III were heated over 5 years at temperatures of 100, 144 and 196°C. The resulting data corroborate earlier results from hydrous pyrolysis experiments (330°C for up to 144h) and prove that ammonium is readily converted abiogenically to organic N in kerogen at temperatures that approach those in naturally maturing sediments. The propensity of different types of kerogen to react with NH4+ or NH3 increases in the order II<I<IIS<III. Ammonium ion mobility in formation fluids and the ability to isotopically interact with kerogen makes them an effective isotopic buffer for sedimentary organic N. Isotopic exchange tends to homogenize the isotopic compositions of participating pools of N and thus limits the isotopic ranges of organic and inorganic N. Recognition of N exchange in thermally matured sediments offers fresh perspectives and opportunities to constrain the extent and pathways of N containing geofluid migration, especially when synoptically characterizing organic and mineral bound N moieties across gradients of thermal maturity.

Economic Analysis for Enhanced CO2 Injection and Sequestration Using Horizontal Wells

Abstract Carbon dioxide flooding has been recognized widely as one of the most effective enhanced oil recovery processes applicable for light to medium oil reservoirs. Moreover, the injection of CO2 into an oil reservoir is a promising technology for reducing greenhouse emissions while increasing the ultimate recovery of oil. Numerical reservoir simulation is an important and inexpensive tool for designing EOR CO2 projects and predicting optimal operational parameters. In this work, reservoir simulations performed with a compositional simulator were applied to investigate the macroscopic mechanisms of a CO2 injection process. Horizontal injectors were used to increase injectivity. Compared to traditional vertical wells, horizontal wells are more attractive to improve CO2 flooding economics by increasing injection rate, improving areal sweep and increasing CO2 storage. The effects of several important parameters on the performance of the CO2 process were studied to optimize the process. Operational parameters such as different production schemes, the injector pressure and injection rate were investigated to determine the optimal - operating conditions for simultaneous objectives of higher recovery and higher CO2 storage. The application of CO2 flooding using horizontal wells can shorten- project life, which is critical to its economics. The simulation results served as the basic input parameters for the economic analysis performed. Furthermore, net present value (NPV) and profitability index results were used to optimize the profitability of the project and to compare the CO2 application using vertical and horizontal wells. The analysis used actual design parameters, including equipment and operating costs. The evaluation emphasized the importance of reservoir characteristics, optimum design of operation parameters and economic factors in the economic feasibility of CO2 injection projects for enhanced oil recovery and sequestration. Introduction The carbon dioxide flooding process can increase oil recovery by means of swelling, evaporating and lowering oil viscosity(1, 2). Many injection schemes using CO2 have been applied(3), including CO2 gas injection (continuously), CO2 gas slug followed by water, and others. There are some important factors to be considered during the design of CO2 flooding, including the availability and amount of CO2 to inject, the reservoir conditions, whether mobility control techniques are required and other general operating conditions(4, 5). Among these factors, the knowledge of reservoir conditions is essential to the injection/production process and, thereafter, the economic success of the project. These include the reservoir temperature and pressure, reservoir permeability and porosity, fractures and faults, etc. Field tests of CO2 floods have shown that reservoir heterogeneities, such as fractures, strata discontinuities and pinch-outs, can reduce the effectiveness of the process. CO2 is a highly mobile fluid because of its low viscosity, so fingering and channeling of CO2 or bypassing of oil can affect the volumetric sweep efficiency in CO2 flooding. In this case, mobility control becomes an important issue for the improvement of CO2 applications(6). With the increase of productivity performance and the decrease of drilling and completion costs, horizontal wells became more cost effective. This paper compares the conventional CO2 flooding process using vertica

Influence of pressure and temperature on the physico-chemical properties of mobile phase mixtures commonly used in high-performance liquid chromatography

To fulfil the increasing demand for faster and more complex separations, modern HPLC separations are performed at ever higher pressures and temperatures. Under these operating conditions, it is no longer possible to safely assume the mobile phase fluid properties to be invariable of the governing pressures and temperatures, without this resulting in significantly deficient results. A detailed insight in the influence of pressure and temperature on the physico-chemical properties of the most commonly used liquid mobile phases: water–methanol and water–acetonitrile mixtures, therefore becomes very timely. Viscosity, isothermal compressibility and density were measured for pressures up to 1000 bar and temperatures up to 100 °C for the entire range of water–methanol and water–acetonitrile mixtures. The paper reports on two different viscosity values: apparent and real viscosities. The apparent viscosities represent the apparent flow resistance under high pressure referred to by the flow rates measured at atmospheric pressure. They are of great practical use, because the flow rates at atmospheric pressure are commonly stable and more easily measurable in a chromatographic setup. The real viscosities are those complying with the physical definition of viscosity and they are important from a fundamental point of view. By measuring the isothermal compressibility, the actual volumetric flow rates at elevated pressures and temperatures can be calculated. The viscosities corresponding to these flow rates are the real viscosities of the solvent under the given elevated pressure and temperature. The measurements agree very well with existing literature data, which mainly focus on pure water, methanol and acetonitrile and are only available for a limited range of temperatures and pressures. As a consequence, the physico-chemical properties reported on in this paper provide a significant extension to the range of data available, hereby providing useful data to practical as well as theoretical chromatographers investigating the limits of modern day HPLC.

Implications of Rate‐Limited Mass Transfer for Aquifer Storage and Recovery

Pressure to decrease reliance on surface water storage has led to increased interest in aquifer storage and recovery (ASR) systems. Recovery efficiency, which is the ratio of the volume of recovered water that meets a predefined standard to total volume of injected fluid, is a common criterion of ASR viability. Recovery efficiency can be degraded by a number of physical and geochemical processes, including rate-limited mass transfer (RLMT), which describes the exchange of solutes between mobile and immobile pore fluids. RLMT may control transport behavior that cannot be explained by advection and dispersion. We present data from a pilot-scale ASR study in Charleston, South Carolina, and develop a three-dimensional finite-difference model to evaluate the impact of RLMT processes on ASR efficiency. The modeling shows that RLMT can explain a rebound in salinity during fresh water storage in a brackish aquifer. Multicycle model results show low efficiencies over one to three ASR cycles due to RLMT degrading water quality during storage; efficiencies can evolve and improve markedly, however, over multiple cycles, even exceeding efficiencies generated by advection-dispersion only models. For an idealized ASR model where RLMT is active, our simulations show a discrete range of diffusive length scales over which the viability of ASR schemes in brackish aquifers would be hindered.

Una nueva interpretación cuantitativa de curvas de avance con mesetasy largos limbos de recesión a partir de ensayos de trazadoren el acuífero cárstico artesiano de Stuttgart, Alemania

In 1998 and 1999, two multi-tracer experiments were conducted in the artesian karst aquifer of the mineral springs of Stuttgart, Germany. The breakthrough curves (BTCs) monitored at the springs showed very long tails or developed plateau-like concentration levels for more than 200 days. Initially, this observation was qualitatively explained by exchange between cavities with stagnant water and the active conduits. Since then, a new analytical solution for tracer transport in karst aquifers has become available, the “two-region non-equilibrium model” (2RNE), which assumes the presence of mobile and immobile fluid regions, and mass transfer between these two regions. The experiments were thus revisited, and it was possible to provide a more quantitative explanation of the observed behaviour. The new model simulated all BTCs very well, thus confirming the earlier qualitative explanation. The prolonged BTCs can be attributed to intermediate storage in cavities containing quasi-immobile groundwater, and slow release into active fractures and conduits. The results also demonstrate that karst aquifers are not always fast-flushing systems, but contaminants can sometimes remain in immobile fluid regions for long periods.

Do airway secretions play an underappreciated role in acute respiratory distress syndrome?

We review the evidence that airway secretions may have an underappreciated role in acute respiratory distress syndrome, contributing to physiologic disarrangements, ventilator dependence and perhaps to injury generation. As common manipulations of ventilator settings, position and fluid status have the potential to influence these problems, explorations into the secretion dynamics of acute lung injury may be fertile ground for developing therapeutic advances. Principles that govern the interaction of airflow and airway fluids suggest that mobile fluids and secretions are pumped by well-selected ventilatory patterns toward the airway opening. Conversely, other selections may inhibit these fluids from clearance or encourage their translocation between lung regions. Recent laboratory work demonstrates that choices for tidal volume and positive end-expiratory pressure may localize or disperse proteinaceous lung edema or bacteria. Gravitational factors may interact with ventilatory pattern for benefit or harm. Capability of ventilation and positioning to mobilize secretions implies the potential for clearance or containment of inflammatory mediators and infection. Ventilatory and positional prescriptions could be designed to meet one of either conflicting targets. Additional experimental and clinical investigations are required before adopting these proposed therapeutic principles into practice.

Modeling of solute transport in snow using conservative tracers and artificial rain‐on‐snow experiments

We report a study of solute transport in snow, using artificial rain‐on‐snow experiments with conservative anions (F−, Br−, and SO42−). The tracers were mixed into tap water and sprayed onto the snow surface from two water supply tanks. The water flux out of the base of the snowpack was recorded, and discharge samples were collected and analyzed for the three tracers. The chemical concentration of tracers in the discharge was negatively associated with the water flux. The objectives of the experiment were to test whether the mobile‐immobile model (MIM) with variable mobile‐immobile water exchange coefficient can simulate both positive and negative concentration‐discharge relationships in this and previous tracer experiments. By simulating our experimental data, we confirm that it is necessary for the exchange coefficient to increase with water velocity. In addition, we use the model to show that with a diurnal variation of clean water flux, a negative concentration‐discharge relationship occurs when solutes are evenly distributed in the mobile and immobile fluids, while a positive relationship occurs when the solutes were present only in the immobile fluid near the surface. This result may help in explaining the complicated concentration‐discharge relationships observed in catchments.

Helium isotopes as geochemical indicators of a serpentinized fore‐arc mantle wedge

New helium isotope data from hot springs on eastern Kyushu Island were obtained in order to elucidate the geographic distribution of 3He/4He ratios in the fore‐arc regions of southwest Japan. Systematic, regional variations in 3He/4He ratios are clearly distinguished across the fore arc from Kyushu Island (0.093–0.79 RA, where RA denotes the atmospheric 3He/4He ratio of 1.4 × 10−6), to Shikoku Island (0.17–3.6 RA) and to the Kii Peninsula (0.24–7.4 RA). The distribution of mantle helium relative to total helium reveals a close association with the geophysical evidence of a serpentinized fore‐arc mantle wedge including the occurrence of deep, nonvolcanic, low‐frequency tremors (LFTs), believed to be caused by upwardly mobile aqueous fluids derived from slab dehydration. Flux of mantle helium carried by aqueous fluids via a non serpentinized fore‐arc mantle wedge is very low on Kyushu Island where LFTs have not been detected and 3He/4He ratios are lower than the atmospheric ratio. On the other hand, hot springs above the regions where LFTs occur in the Kii Peninsula and northern Shikoku Island have high 3He/4He ratios, indicating a significant contribution of mantle helium carried by aqueous fluids via a serpentinized fore‐arc mantle wedge. The helium isotopes in hot and natural spring gases may be a useful geochemical indicator of a serpentinized fore‐arc mantle wedge, especially in areas with sparse seismic station coverage.

Porosity Distributions in Carbonate Reservoirs Using Low-Field NMR

Abstract Alberta contains significant deposits of oil and gas in carbonate formations. Carbonates tend to have fairly tight matrix structures, resulting in low primary porosity and permeability. Laboratory characterization of carbonate properties is a slow and tedious process, however, core data is often collected in order to augment and tune logging tool predictions. In this application, having a good understanding of carbonate pore systems at the core analysis level is key to proper reservoir characterization. Low-field NMR is an emerging technology that shows great promise for rock characterization measurements. In this paper, low-field NMR technology is investigated for determining primary and secondary porosity through the interpretation of NMR spectra. This data was also used to establish the bound and mobile fluid distributions existing in the porous medium. The data set for this experimental work consists of a large collection of core samples from various fields in Alberta and Saskatchewan. CT data were analyzed to obtain the primary and secondary porosity fractions, which were used to find corresponding NMR cutoff values that separate the NMR spectra into primary and secondary porosity. A distinct relationship was observed between the primary porosity fraction and the irreducible water saturation, Swi. The fraction of NMR amplitude in the last peak of the NMR spectra can also be correlated to CT secondary porosity. Another important relationship observed is that the geometric mean relaxation time of the last NMR peak correlates well with the cutoff between primary and secondary porosity. The bound and mobile fluid distributions are generally distinguished through the identification of T2cutoff values. A correlation was found to predict T2cutoff for this wide range of samples. This study shows that information from the fully saturated NMR spectrum can be used to estimate primary and secondary porosity fractions in carbonates, as well as bound and mobile fluid fractions. Introduction Porosity of carbonates is a complex problem that has had only limited attention in the literature(1). In general, carbonate porosity is divided into primary and secondary porosity. These different types of porosity are not easily distinguishable unless the primary pores and the diagenesis processes that occurred are studied(1). Despite these difficulties, it is very important to recognize and attempt to quantify the different porosity types and mobile/immobile fluid fractions in carbonates in order to help in developing carbonate reservoirs and to estimate the pore connections and recovery efficiency in these reservoirs. As various researchers have found, Nuclear Magnetic Resonance (NMR) can capture pore size information of the porous media(2–4). Thus, in theory, it describes both the primary and secondary porosity. However, separating the signal into different porosity components remains a daunting task. Part of this difficulty arises from the fact that there is no clear distinction between primary and secondary pore size distributions, as they overlap with each other. Chang et al.(3) have previously tried to separate the signal of vugs in NMR response. In carbonates, however, even the definition of vugs can be quite different. Chang et al.(3) used the term vugs to describe cavities that are formed in the matrix by diagenesis, with sizes ranging from about 100 μm to cavern size.

A new simulation method for the separation of binary mixtures in a four-column simulated moving bed

In this paper, a new resolution method for the dynamic simulation of a simulated moving bed is presented. A general rate model for each solute and each column is presented for the case of a linear isotherm. A hybrid resolution is developed, in which an analytical solution is used for the solid stationary phase, while the mobile fluid phase concentrations are obtained by numerical resolution. It is illustrated, through a numerical study, that the resolution method is robust. The method is validated by comparison with previously published experimental results. It is also shown that these numerical simulations are obtained much faster than those based on the physical separation process, allowing the use of this resolution method for on-line applications and process control.

Colmatation-suffosion filtration in a porous medium with mobile and immobile fluids

The problem of colmatation-suffosion filtration of disperse systems in a two-dimensional porous medium consisting of two zones with mobile and immobile fluids has been studied with account for diffusion phenomena. The effects of lateral and longitudinal diffusion and colmatation-suffosion phenomena on collector properties of a porous medium have been determined.

Towards an acoustic model-based poroelastic imaging method: II. experimental investigation

Soft biological tissue contains mobile fluid. The volume fraction of this fluid and the ease with which it may be displaced through the tissue could be of diagnostic significance and may also have consequences for the validity with which strain images can be interpreted according to the traditional idealizations of elastography. In a previous paper, under the assumption of frictionless boundary conditions, the spatio-temporal behavior of the strain field inside a compressed cylindrical poroelastic sample was predicted ( Berry et al. 2006). In this current paper, experimental evidence is provided to confirm these predictions. Finite element modeling was first used to extend the previous predictions to allow for the existence of contact friction between the sample and the compressor plates. Elastographic techniques were then applied to image the time-evolution of the strain inside cylindrical samples of tofu (a suitable poroelastic material) during sustained unconfined compression. The observed experimental strain behavior was found to be consistent with the theoretical predictions. In particular, every sample studied confirmed that reduced values of radial strain advance with time from the curved cylindrical surface inwards towards the axis of symmetry. Furthermore, by fitting the predictions of an analytical model to a time sequence of strain images, parametric images of two quantities, each related to one or more of three poroelastic material constants were produced. The two parametric images depicted the Poisson’s ratio ( ν s ) of the solid matrix and the product of the aggregate modulus ( H A ) of the solid matrix with the permeability ( k) of the solid matrix to the pore fluid. The means of the pixel values in these images, ν s = 0.088 (standard deviation 0.023) and H Ak = 1.449 (standard deviation 0.269) × 10 –7 m 2 s –1, were in agreement with values derived from previously published data for tofu ( Righetti et al. 2005). The results provide the first experimental detection of the fluid-flow-induced characteristic diffusion-like behavior of the strain in a compressed poroelastic material and allow parameters related to the above material constants to be determined. We conclude that it may eventually be possible to use strain data to detect and measure characteristics of diffusely distributed mobile fluid in tissue spaces that are too small to be imaged directly. (E-mail: gearoid.berry@icr.ac.uk)

Multitracer Test Approach to Characterize Reactive Transport in Karst Aquifers

A method to estimate reactive transport parameters as well as geometric conduit parameters from a multitracer test in a karst aquifer is provided. For this purpose, a calibration strategy was developed applying the two-region nonequilibrium model CXTFIT. The ambiguity of the model calibration was reduced by first calibrating the model with respect to conservative tracer breakthrough and later transferring conservative transport parameters to the reactive model calibration. The reactive transport parameters were only allowed to be within a defined sensible range to get reasonable calibration values. This calibration strategy was applied to breakthrough curves obtained from a large-scale multitracer test, which was performed in a karst aquifer of the Swabian Alb, Germany. The multitracer test was conducted by the simultaneous injection of uranine, sulforhodamine G, and tinopal CBS-X. The model succeeds to represent the tracer breakthrough curves (TBCs) of uranine and sulforhodamine G and verifies that tracer-rock interactions preferably occur in the immobile fluid region, although the fraction of this region amounts to only 3.5% of the total water. However, the model failed to account for the long tailing observed in the TBC of tinopal CBS-X. Sensitivity analyses reveal that model results for the conservative tracer transport are most sensitive to average velocity and volume fraction of the mobile fluid region, while dispersion and mass transfer coefficients are least influential. Consequently, reactive tracer calibration allows the determination of sorption sites in the mobile and immobile fluid region at small retardation coefficients.

Efficient algorithm for modeling transport in porous media with mass exchange between mobile fluid and reactive stationary media

We compare two approaches to numerically solve the mathematical model of reactive mass transport in porous media with exchange between the mobile fluid and the stationary medium. The first approach, named the “monolithic algorithm,” is the approach in which a standard finite-difference discretization of the governing transport equations yields a single system of equations to be solved at each time step. The second approach, named the “system-splitting algorithm,” is here applied for the first time to the problem of transport with mass exchange. The system-splitting algorithm (SSA) solves two separate systems of equations at each time step: one for transport in the mobile fluid, and one for uptake and reaction in the stationary medium. The two systems are coupled by a boundary condition at the mobile– immobile interface, and are solved iteratively. Because the SSA involves the solution of two smaller systems compared to that of the monolithic algorithm, the computation time may be greatly reduced if the iterative method converges rapidly. Thus, the main objective of this paper is to determine the conditions under which the SSA is superior to the monolithic algorithm (MA) in terms of computation time. We found that the SSA is superior under all the conditions that we tested, typically requiring only 0.3–50% of the computation time required by the MA. The two methods are indistinguishable in terms of accuracy. Further advantages to the SSA are that it employs a modular code that can easily be modified to accommodate different mathematical representations of the physical phenomena (e.g., different models for reaction kinetics within the stationary medium), and that each module of the code can employ a different numerical algorithm to optimize the solution.

Towards an acoustic model-based poroelastic imaging method: I. Theoretical Foundation

The ultrasonic measurement and imaging of tissue elasticity is currently under wide investigation and development as a clinical tool for the assessment of a broad range of diseases, but little account in this field has yet been taken of the fact that soft tissue is porous and contains mobile fluid. The ability to squeeze fluid out of tissue may have implications for conventional elasticity imaging, and may present opportunities for new investigative tools. When a homogeneous, isotropic, fluid-saturated poroelastic material with a linearly elastic solid phase and incompressible solid and fluid constituents is subjected to stress, the behaviour of the induced internal strain field is influenced by three material constants: the Young’s modulus ( E s ) and Poisson’s ratio (ν s ) of the solid matrix and the permeability ( k) of the solid matrix to the pore fluid. New analytical expressions were derived and used to model the time-dependent behaviour of the strain field inside simulated homogeneous cylindrical samples of such a poroelastic material undergoing sustained unconfined compression. A model-based reconstruction technique was developed to produce images of parameters related to the poroelastic material constants ( E s , ν s , k) from a comparison of the measured and predicted time-dependent spatially varying radial strain. Tests of the method using simulated noisy strain data showed that it is capable of producing three unique parametric images: an image of the Poisson’s ratio of the solid matrix, an image of the axial strain (which was not time-dependent subsequent to the application of the compression) and an image representing the product of the aggregate modulus E s ( 1 − ν s ) / ( 1 + ν s ) ( 1 − 2 ν s ) of the solid matrix and the permeability of the solid matrix to the pore fluid. The analytical expressions were further used to numerically validate a finite element model and to clarify previous work on poroelastography. (E-mail: gearoid.berry@icr.ac.uk)

Two-scale modeling in porous media: Relative permeability predictions

We present a numerical analysis of fluid flow through a porous medium with two distinct characteristic scales. The system considered is a monodisperse matrix with porosity ϕ and permeability Kpm with an embedded second phase, characterized by a phase content or saturation s and phase length scales Lϕ and Ls. Both two- and three-dimensional simulations are performed to compute the mobile fluid phase relative permeability kr,m and its dependence on s and Kpm. The relative permeability is found to vary as a power law of saturation, with a quasilinear behavior for low permeability, and increasing values of the exponent as Kpm increases. For media with low permeability, the linearity of kr,m is attributed to the drag force, whereas for high Kpm, the decrease of kr,m with s is due primarily to viscous forces. An analytical model for kr,m is also presented to aid the interpretation and to corroborate the simulation results. In the second part, in order to elucidate the role of the length scales on kr,m, simulations explicitly resolving both porous media and second-phase scales are performed. The relative permeability is found to drop rapidly when both scales are of the same order (Ls≈Lϕ) or when Ls&amp;lt;Lϕ. Three regimes (Darcy, Brinkman, Stokes) are consequently identified based on the length scales.

Rate controls on the chemical weathering of natural polymineralic material. I. Dissolution behaviour of polymineralic assemblages determined using batch and unsaturated column experiments

Chemical weathering rates for a mudstone obtained from a mining environment were investigated using a combination of batch reactors and hydrologically unsaturated column experiments. Results of tracer tests were combined with relationships between solute concentrations, mass fluxes, flow rates and residence times, and used to calculate element release rates and infer rate-controlling mechanisms for the two different experimental environments. Elements that eluted from column experiments exhibited either: (1) concentrations independent of flow rate and column length coupled with mass flux increasing with flow rate, or (2) an inverse relationship between concentration and flow rate coupled with mass flux increasing with both column length and flow rate. The former is attributed to equilibrium-controlled release of particular elements (Si, Al), while the latter is ascribed to transport-controlled release of others (Mg, Mn, Ca, Na, K, S). Tracer tests using NaBr solutions revealed that some elements were also affected by ion exchange (Mg, Mn, Ca, Na), but these effects were temporary and did not mask underlying dissolution rate-controlling mechanisms. Analysis of characteristic diffusive lengths was used to distinguish between transport rates limited by the transfer of solutes between immobile and mobile water within the columns, and rates limited by slow diffusion across partially reacted mineral layers. The analysis suggests that transfer between immobile and mobile water limited the element release rates, with diffusion hindered by low diffusion coefficients within the unsaturated medium, and by low interfacial areas between mobile and immobile fluids. Results of the batch experiments showed different characteristics. Element concentrations either rose to a plateau or increased linearly with time. Rate-controlling mechanisms associated with these characteristics were equilibrium (Fe) and surface kinetic reactions (Si, Mg, Mn, Ca, K, S), respectively. Surface area-normalized element release rates for Mg, Mn, Ca and S are consistently a factor of 4 higher than those from the column experiments. This is a significant difference and cannot be attributed to differences in mineral preparation or to factors influencing individual minerals. It must, therefore, reflect the difference in the rate-controlling mechanism, that is, transport vs. surface-kinetic control. These results suggest that some proportion of the commonly recorded discrepancy between laboratory and field weathering rates is due to hydrological differences between the two environments, and that hydrological characterization of weathering environments is, therefore, as important as physical–geochemical characterization of reacting solids. An important practical implication of the work is that substantial reservoirs of solutes are held in immobile water in the unsaturated environment, and that these could be released by soil disturbance or flooding.