Showing papers on "Effective porosity published in 2009"

Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast

Abstract: [1] Weathering of rocks as a result of exposure to water and the atmosphere can cause significant changes in their chemistry and porosity. In low-porosity rocks, such as basalts, changes in porosity, resulting from chemical weathering, are likely to modify the rock's effective diffusivity and permeability, affecting the rate of solute transport and thus potentially the rate of overall weathering to the extent that transport is the rate limiting step. Changes in total porosity as a result of mineral dissolution and precipitation have typically been used to calculate effective diffusion coefficients through Archie's law for reactive transport simulations of chemical weathering, but this approach fails to account for unconnected porosity that does not contribute to transport. In this study, we combine synchrotron X-ray microcomputed tomography (μCT) and laboratory and numerical diffusion experiments to examine changes in both total and effective porosity and effective diffusion coefficients across a weathering interface in a weathered basalt clast from Costa Rica. The μCT data indicate that below a critical value of ∼9%, the porosity is largely unconnected in the basalt clast. The μCT data were further used to construct a numerical pore network model to determine upscaled, effective diffusivities as a function of total porosity (ranging from 3 to 30%) for comparison with diffusivities determined in laboratory tracer experiments. By using effective porosity as the scaling parameter and accounting for critical porosity, a model is developed that accurately predicts continuum-scale effective diffusivities across the weathering interface of the basalt clast.

Unstructured finite volume discretization of two-dimensional depth-averaged shallow water equations with porosity

Abstract: This paper deals with the numerical discretization of two-dimensional depth-averaged models with porosity. The equations solved by these models are similar to the classic shallow water equations, but include additional terms to account for the effect of small-scale impervious obstructions which are not resolved by the numerical mesh because their size is smaller or similar to the average mesh size. These small-scale obstructions diminish the available storage volume on a given region, reduce the effective cross section for the water to flow, and increase the head losses due to additional drag forces and turbulence. In shallow water models with porosity these effects are modelled introducing an effective porosity parameter in the mass and momentum conservation equations, and including an additional drag source term in the momentum equations. This paper presents and compares two different numerical discretizations for the two-dimensional shallow water equations with porosity, both of them are high-order schemes. The numerical schemes proposed are well-balanced, in the sense that they preserve naturally the exact hydrostatic solution without the need of high-order corrections in the source terms. At the same time they are able to deal accurately with regions of zero porosity, where the water cannot flow. Several numerical test cases are used in order to verify the properties of the discretization schemes proposed.

Hydraulic Properties of Variously Weathered Granitic Bedrock in Headwater Catchments

Abstract: Recent studies have emphasized the importance of bedrock in hydrologic processes occurring in headwater catchments. To understand water flow processes through variously weathered bedrock, we measured the saturated hydraulic conductivity, K s , and water retention characteristics of weakly to highly weathered Tanakami granite and Rokko granite core samples. On the basis of these core-scale properties, along with the core shape and in situ K s measurements, we defined two groups of bedrock: C M class (weakly weathered) and C L to D L class (moderately to highly weathered). The C M class bedrock cores had almost no effective porosity (i.e., the amount of porosity that effectively contributes to water flow) and therefore extremely small core-scale K s , indicating that the matrix could be regarded as essentially impermeable. The in situ K s was much larger than the core-scale values, however, and the core shape showed apparent fractures, suggesting that water did flow preferentially through the fractures. The volumetric water content of the C L – to D L –class bedrock water retention curves changed little in the dry range but changed gradually in the wet range, resulting in a moderate core-scale K s of 10 −5 to 10 −3 cm s −1 . The core-scale K s values were well explained by the parameters characterizing the water retention curve. The similarity of the in situ K s to the core-scale values, and the lack of fractures in the core shape, suggested that water flow could be characterized as matrix flow. The hydraulic properties of weathered granite at other sites followed the trends observed at our sites, implying wide applicability of the findings in this study to various types of weathered granite.

Behavior of a shallow water table under periodic flow conditions

Abstract: A new laboratory data set on the behavior of a shallow water table in a sand column aquifer subject to simple harmonic periodic forcing at its base is presented and discussed. The data are analyzed using the dynamic effective porosity, which is defined as the ratio of the rate of change in total moisture to the rate of change in water table elevation; thus, a reduction in this parameter means that the extent of moisture exchange has been reduced relative to a given water table fluctuation. The data show a clear decrease in the dynamic effective porosity with increasing proximity of the water table to the sand surface, which is consistent with previous research under a steadily rising or falling shallow water table. The observed reduction in moisture exchange due to shallowness of the water table has implications for periodic flow scenarios such as the propagation of water table waves in coastal and beach groundwater systems. That is, as moisture exchange is reduced, less work is being done by the flow, and thus, energy dissipation rates for shallow water tables will be reduced relative to the case of a deeper water table. At present no account of the influence of water table shallowness has been included in theories describing water table wave dispersion. The present experiments, in conjunction with the dynamic effective porosity concept, provide a framework in which this gap in knowledge can be further investigated. Additional experiments were designed such that the free surface transgressed the sand surface for part of the oscillation period to investigate the influence of meniscus formation and deformation at the sand surface on periodic flow dynamics. The observed behavior is consistent with previous observations of steady infiltration above shallow water tables, namely, a rapid drop (rise) in pore pressure with the onset of meniscus formation (deformation). A simple "wetting and drying'' model is derived, accounting for the variation in effective porosity caused by the free surface transgressing the sand surface, which is shown to accurately capture the observed behavior. A finite element solution of the Richards equation in which the transient upper boundary condition is easily mimicked by means of a surface element with special storage features also shows excellent agreement with the observed data.

Porosity and permeability development in mechanically compacted silt‐kaolinite mixtures

Abstract: Summary Prediction of porosity and permeability in mudstones are challenging because of the uncertainties associated with textural and mineralogical composition. This study investigates the development of porosity and permeability in thirteen mechanically compacted brine-saturated synthetic mudstones consisting of silt and kaolinite mixtures. The drained uniaxial mechanical compaction tests were performed under room temperature and atmospheric pressure. Total porosity and vertical permeability (parallel to the direction of applied stress) were measured continuously for each sample under increasing effective stress up to 50 MPa. Results show that mineralogy is a dominant factor controlling the development of porosity and permeability in silt-clay mixtures. Kaolinite-dominated samples have much lower porosity and permeability compared to silt-dominated samples. At 20 MPa effective stress that corresponds to about 2 km depth of burial of normally compacted basin, the pure kaolinite sample was compacted to about 20% porosity whereas the pure silt sample had retained 35% porosity. The permeability of pure kaolinite and silt samples were about 0.003 and 0.2 mD respectively at the same effective stress. The porosity and permeability reduction were not systematic with increasing kaolinite content in silt aggregates or vice versa. The porosity and permeability difference between pure silt and pure kaolinite aggregates were relatively less at low effective stresses but increased significantly with increasing effective stress. At high effective stresses the permeability was 2-3 orders of magnitude lower in pure kaolinite than in pure silt. Calculating permeability in mudstones from porosity without considering mineralogy will therefore introduce significant error in permeability estimation. Our experimental results provide valuable constraints on porosity-stress/depth, permeability-stress/depth and porosity-permeability relationships for shallow mudstones that will have practical use for basin modelling, pore pressure prediction and fluid flow modelling in shallower part of the basins (<80-100 0 C) where mechanical compaction is the dominant process.

Evaluation of Anomalous Solute Transport in a Large Heterogeneous Soil Column with Mobile-Immobile Model

Abstract: This study uses the mobile-immobile model (MIM) and the traditional convection-dispersion equation (CDE) to analyze the observed breakthrough curves (BTCs) at different distances in a 1,250-cm-long saturated and highly heterogeneous soil column. It provides a simple method to determine the mobile water fraction independently as the ratio of effective porosity over total porosity of the packed soil materials. The effective porosity is calculated a priori as the ratio of measured flow rate and estimated pore-water velocity. It is found that there is a significant amount of immobile water in the soil column, resulting in the anomalous early breakthrough and tailing behaviors of the measured BTCs. Comparing to the CDE, the measured asymmetric BTCs at various scales can be better described by the MIM, especially their early arrival and long tailing parts. The degree of anomalous transport behavior in this large heterogeneous soil column is reduced with transport scale due to the increased mobile water fraction...

Soil Water Retention Curves Characterization of a Natural Forested Hillslope using a Scaling Technique Based on a Lognormal Pore‐Size Distribution

Abstract: Heterogeneous water flow is known to be an important factor of hydrological processes in a natural forested hillslope. To model heterogeneous water flow, the characterization of spatial variability in water retention curve (WRC) is required. The scaling technique introduced by Miller and Miller is effective to characterize it for conditions of constant standard deviation (STD) in pore-size distribution and porosity, but this is not necessarily appropriate for forested hillslopes. We tested the conventional scaling method and the two proposed methods that presume that field soils do not exhibit constant STD and porosity. The observed WRCs were fitted using a model, which assumes a lognormal pore-radius distribution and contains three parameters: the matric pressure head related to the median pore radius, ψ m ; the STD of the log-transformed pore radius distribution, σ; and the effective porosity, θ e . In Method 1, which corresponds to the conventional scaling method, ψ m was optimized for each soil, whereas the values of σ and θ e were common for the whole data set. In Method 2, σ was optimized for each soil, and in Method 3, θ e was optimized for each soil, whereas the values of the remaining parameters were common for the whole data set. Method 3 produced the best description of spatial variability in the WRCs. This result indicates that in the natural forested hillslope, variability in the pore-size distribution is characterized by variability in effective porosity. In practical aspect, we suggested an alternative simpler method to Method 3. In this method, the θ e for each location was estimated from soil penetration resistances measurable in situ. This method explained 59.3% of the spatial variability in WRCs on the studied natural forested hillslope.

Comparison of modeling methods for the determination of effective porosities and diffusion coefficients in through-diffusion tests.

Abstract: [1] Diffusion coefficients and effective, or transport, porosities are important parameters in the assessment of the fate and transport of contaminants in fractured rock systems and when contaminant transport needs to be considered in zones of low to negligible advective flows This study presents and discusses various solutions, analytical and semianalytical, to Fick's law for the typical through-diffusion experiment and applies these solutions to the results of three through-diffusion experimental data sets These through-diffusion experiments were conducted on two natural breccia samples, with bulk measured porosities of approximately 137% and 168%, and one manufactured porous plate sample with a bulk measured porosity of 409%, such that the samples displayed order of magnitude differences in their diffusivities For each experiment, the derived effective porosities and intrinsic diffusion coefficients for each method are compared It was shown that the semianalytical solution provides an improved method for reliably estimating the effective porosity from through-diffusion experimental data; however, there is little difference between methods in the derived intrinsic diffusion coefficients

Influences of Sedimentary History on the Mechanical Properties and Microscopic Structure Change of Horonobe Siliceous Rocks

Abstract: As part of the research and development program on geological disposal of high-level radioactive waste Japan Atomic Energy Agency is implementing the Horonobe URL Project. This paper shows the results obtained from the laboratory tests carried out to understand mechanical properties of Horonobe siliceous rocks for site characterization. Accordingly, the relationship among microscopic observation, sedimentary history and mechanical properties of Horonobe siliceous rocks was discussed in this paper. Such laboratory tests as uniaxial compression test, triaxial compression test, isotropic consolidation test were carried out. Uniaxial compression tests were carried out for specimen sampled at about 50 m interval of eleven deep boreholes more than 500 m deep from the ground surface. Triaxial compression tests including isotropic consolidation test were also carried out using cores of the boreholes. The triaxial tests were carried out under drained and undrained conditions since change in pore structure was expected for the sedimentary soft rocks with large effective porosity. Physical and mechanical properties showed significant variations along the burying depth due to dissolution of the minute porosity in diatom occured from increasing of overburden, temperature etc. The consolidated undrained and drained triaxial compression tests showed different behaviors of strain-softening, pore pressure and dilatancy between diatomaceous and siliceous mudstones. Stress-strain behavior changed from strain-softening to ductile behavior under high confining pressure and pore pressure increased gradually even after peak strength for diatomaceous mudstone. On the other hand, for siliceous mudstone strain-softening behavior was observed and pore pressure decreased rapidly after peak stress regardless of confining pressure value. Diatomaceous mudstone yielded under hydrostatic pressure of 10 MPa in isotropic consolidation test. This yielding was regarded as pore collapse based on the variation of hydraulic conductivity which was estimated from variation of volumetric strain in isotropic consolidation test, effective porosity and microscopic observation before and after the yielding.

Methods for determining the petrophysical property cutoffs of extra-low porosity and permeability sandstone reservoirs——an example from the Xishanyao Formation reservoirs in Yongjin oilfield

Abstract: With the porosity range of 4% to 6% and the permeability range of 0.01×10-3 μm2-0.30×10-3 μm2,the Xishanyao Formation reservoirs in the Yongjin oilfield of the Junggar Basin are typical extra-low porosity and extra-low permeability reservoirs.According to the static and dynamic data and petrophysical experiments,we have determined the effective porosity and permeability cutoffs of these reservoirs through methods of empirical statistics,oil-bearing occurrence test,physical property test,nuclear magnetic resonance,minimal pore throat radius and displacement pressure.The results show that the lower limit of porosity is 6% and the lower limit of permeability is 0.08×10-3 μm2,providing basis for determining effective thickness and estimate reserves.

Correlation of unsaturated soil and dielectric property for monitoring of subsurface characteristics: development of unsaturated dielectric mixing models and its application

Abstract: The behavior due to rainfall infiltrating the ground plays a role in landslides, groundwater recharge and various other ground responses. Most of these geotechnical behaviors have a correlation between soil pore space and soil volumetric water content in the unsaturated and saturated soil porous media. Therefore, the soil porosity associated with soil pores and the distribution of volumetric water content are significantly important hydrological characteristics. In the case of shallow slope failure such as landslide, the infiltration activity due to the connectivity of soil pore spaces in a porous media is induced. Slope failure may be attributed to the effect of a wetting front with the slope due to liquid infiltration, which changes the volumetric water content, soil matric suction and shear strength of the slope. This study was performed with an unsaturated injection test using a frequency domain reflectometry (FDR) dielectric device which measures the dielectric constant of unsaturated soil and the study then proposed the unsaturated dielectric mixing models to calculate soil porosity and effective porosity of unsaturated soils. From the experimental results the ratio of effective porosity to porosity of soils are measured in a range of 70–85%. These experimental results show a decrease of about 5–10% for unsaturated soil compared to the ratio of effective porosity to porosity of saturated soil. The infiltration passages of tracer material are restricted within the pore connectivity in the unsaturated soil which is caused by dead-pores in the soil. Using the FDR device and the unsaturated dielectric mixing models, we can consider the acquisition of physical properties to detect the infiltration activity, the response of the dielectric constant along with the injected tracer and hydrological parameters for the unsaturated soil porous media.

Matrix porosity calculation in volcanic and dolomite reservoirs and its application

Abstract: Matrix porosity calculations of fractured and vuggy reservoirs, such as volcanics and weathered dolomite, are one of the problems urgently needed to solve in well-log evaluation. In this paper, we first compare the an empirical formula for porosity calculation from full diameter rhyolite core experiments with the matrix porosity formulas commonly used. We discuss the applicability of the empirical formula in fractured and vuggy reservoirs, such as intermediate-basic volcanics and weathered dolomite. Based on core analysis data, the error distribution of the calculated porosity of our empirical formula and the other porosity formulas in these reservoirs are given. The statistical error analysis indicates that the our empirical formula provides a higher precision than the other porosity formulas. When the porosity is between 1.5% and 15%, the acoustic experiment formula can be used not only for acidic volcanics but also in other fractured and vuggy reservoirs, such as intermediate-basic volcanics and weathered dolomite. Moreover, the formula can reduce the effects of borehole enlargement and rock alteration on porosity computation.

Prediction of the Secondary Porosity Zone of Fuyu Oil Layer in the West of Daqing Placanticline

Abstract: In order to find the reservoir which develop the secondary porosity in the context of generally low porosity and low permeability,on the basis of the analysis of contributing factor of secondary porosity and diagenesis research of the reservoir,Combined impact of the formation of secondary porosity factors,we have predicted the porosity developed area of Fuyu oil layer in the West of Daqing placanticline.Research shows that secondary porosity in the region is mainly affected by the formation of carbonate and organic acids which come from Qingshankou group,and by the impact of fault controlled and overpressure,secondary porosity mainly distributed in the surrounding region of Qijia sag,Gulong sag and Longhupao terrace,some secondary porosity distributed in the western slope which developed by meteorological water eluviations.

Experimental assessment of the infiltration properties of a coarse soil medium in a dielectric infiltration test.

Abstract: An accurate determination of the hydrological characteristics of porous media, such as the values of the porosity and effective porosity, are essential. This is important to understand the transport processes of infiltration and the movement of water and contaminants in the porous media. In this study, a laboratory soil column experiment to estimate the porosity and effective porosity of Toyoura standard sand samples, using a dielectric method termed the frequency domain reflectometry with vector network analyzer (FDR-V), was performed. The FDR-V device uses high-frequency microwaves, ranging from 0.1 to 3 GHz, to measure the complex dielectric constants of the sample. From the measured complex dielectric constant, the two parameters of the soil samples then were derived, using a proposed dielectric mixture model and tracer concentration model. The effective porosity of the soil sample is measured at 0.311 with the FDR-V dielectric method and 0.345 by the soil tracer column test. Comparing this with the calculated porosities of the soil sample, the ratio of effective porosity is approximately 78% for the dielectric method, and approximately 86% for the tracer concentration method. These different values can be explained to be within the measurement range with regard to the measurement volume for the soil column. These results indicate that measurement by the dielectric method using an FDR-V device is an efficient and useful tool for estimating the hydrological parameters of porous media because of the dielectric response of earth materials.

Relationship of Sandstone Diagenesis and Relatively High Porosity Areas in Nantun and Tongbomiao Formation:A Case from the Southern of Wuerxun Depression in Hailaer Basin

Abstract: The types of sandstone from Nantun and Tongbomiao Formation of Wuerxun depression in Hailaer basin are mainly debris-feldspar sandstone,sightly feldspar sandstone and feldspar-debris sandstone.The diageneses include compaction,cementation,metasomatism and dissolution,etc.With depth increasing,the porosity and permeability decrease.There are four relatively high porosity arears in about 1 500 m,1 850-2 050 m,2 300 m and 2 600-2 800 m.The reasons of relatively high porosity areas appearance are the preservation of primary porosity and the development of secondary porosity.The weakness of compaction and cementation,existence of clay coating and microcrystalline quartz are favor to the preservation of primary porosity;in the processing of thermal evolution of organic content and the conversion of clays,secondary porosity forming occurs.

A simple two layer model for simulation of adsorbing and nonadsorbing solute transport through field soils

Abstract: . While rapid movement of solutes through structured soils constitutes the risk of groundwater contamination, simulation of solute transport in field soils is challenging. A modification in an existing preferential flow model was tested using replicated Chloride and Lithium leachings carried out at constant flow rates through four soils differing in grades and type of structure. Flow rates generated by +10 mm, −10 mm, −40 mm, and −100 mm water heads at the surface of 35 cm diameter 50 cm height field columns. Three well-structured silty clay soils under ponding had concurrent breakthrough of Chloride and Lithium within a few cm of drainage, and a delayed and reduced peak concentration of Lithium with decrease in flow rate controlled by the negative heads. Massive sandy loam soil columns had delayed but uniform breakthrough of the solutes over the flow rates. Macropore flow in well-structured silty clay/clay loam soils reduced retardation, R (1.5 to 4.5) and effective porosity, θe (0.05 to 0.15), and increased macropore velocity, vm (20 to 30 cm cm−1 drainage) compared to the massive sandy soils. The existing simple preferential flow equation (single layer) fitted the data well only when macropore flow was dominant. The modified preferential flow equations (two layers) fitted equally well both for the adsorbing and nonadsorbing solutes. The later had high goodness of fit for a large number of solute breakthroughs, and gave almost identical retardation coefficient R as that calculated by two-domain CDE. With fewer parameters, the modified preferential flow equation after testing on some rigorous model selection criteria may provide a base for future modeling of chemical transport.

Reservoir Quality Assessment of Tight Gas Reservoirs - Links to Producibility

Abstract: Quantification of porosity types and sizes coupled with in-situ reservoir capillary pressure data allows one to estimate the potential hydrocarbon pore volume in a hydrocarbon transition zone. This data coupled with economic gas rate, production and reservoir quality data (such as sandstone compositional and textural trends on a field or basin scale) can provide a tool to evaluate Conventional vs Tight-Gas zones in a prospect. Conventional gas reservoirs consist predominantly of primary intergranular porosity with large pore-throat sizes and varying amounts of secondary porosity. In contrast, tight-gas reservoirs (i.e. a reservoir which requires artificial stimulation to produce at economic rates) consist predominantly of secondary porosity with pore-throat sizes well below 1 micron in diameter.

Numerical simulation with finite element method on natural gas hydrate decomposition by depressurization in porous media

Abstract: Decomposition kinetics of hydrate,heat transfer and gas-water two-phase flow are the three main and primary mechanisms during the hydrates decomposition in porous media.Based on these three mechanisms,a three-phase model was developed to simulate the process of natural gas hydrate decomposition by depressurization in porous media,and the variation of permeability and effective porosity due to hydrates decomposition was considered in this model.A finite element program was developed and verified.The spatial and temporal distributions of pressure,temperature,hydrate saturation,effective porosity and the flow velocity for gas and water in the process of hydrates decomposition were simulated by this model.And the variation laws of location of decomposition front and cumulative gas production with time were also researched.The results show that the effective porosity and permeability are improved due to hydrates decomposition.The process of hydrates decomposition is thermonegative,and the temperature in decomposition front decreases obviously.Hydrates decomposition rate and gas production rate increase with invironment temperature increasing,and the decomposition rate decreases with pressure increasing.The environment temperature and outlet pressure are two important influencing factors for hydrates decomposition.