Showing papers on "Effective porosity published in 2007"

Validated Leverett Approach for Multiphase Flow in PEFC Diffusion Media I. Hydrophobicity Effect

Abstract: This work is the second part of a series of papers to describe the multiphase transport mechanism in thin-film polymer electrolyte fuel cell ! PEFC" diffusion media ! DM" . The present work is devoted to delineating the effects of compression. Direct measurements of drainage capillary pressure-saturation curves for SGL series carbon paper DM tailored with a range of mixed wettability were performed at room temperature under various compressions ! 0, 0.6, and 1.4 MPa" typically encountered in a fuel cell assembly. Based on these benchmark data, an appropriate form of the Leverett approach, including a Leverett-type empirical function that incorporates the effect of compression and the mixed wettability characteristics of the tested DM samples, was developed. The presented approach can determine the capillary pressure as a function of hydrophobic additive content, liquid saturation, compression, and uncompressed porosity of the DM. Compression leads to an increase in capillary pressure, effectively caused by the corresponding reduction in effective porosity. Any increase in hydrophobicity amplifies the compression effect, yielding a higher capillary pressure for the same saturation level. Furthermore, the fraction of connected hydrophilic pores is observed to be reduced with an increase in compression, leading to a favorable reduction in water storage capacity of the fuel cell DM.

Porosity and Permeability in Sediment Mixtures

Abstract: Porosity in sediments that contain a mix of coarser- and finer-grained components varies as a function of the porosity and volume fraction of each component. We considered sediment mixtures representing poorly sorted sands and gravely sands. We expanded an existing fractional-packing model for porosity to represent mixtures in which finer grains approach the size of the pores that would exist among the coarser grains alone. The model well represents the porosity measured in laboratory experiments in which grain sizes and volume fractions were systematically changed within sediment mixtures. Permeability values were determined for these sediment mixtures using a model based on grain-size statistics and the expanded fractional-packing porosity model. The permeability model well represents permeability measured in laboratory experiments using air- and water-based permeametry on the model sediment mixtures.

Fluid substitution in shaley sediment using effective porosity

Abstract: The traditional method of fluid substitution in porous rock requires the total porosity and the elastic modulus of the mineral phase as input and assumes that the fluid reaches instantaneous hydraulic equilibrium throughout the pore space. This assumption may not be appropriate for shaley sediment because of the low permeability of shale and the resulting immobility of the water in it. To address this problem, we propose an alternative method that uses effective porosity instead of total porosity. Effective porosity is lower than total porosity if porous shale is present in the system. A new, composite mineral phase is introduced, which includes the porous water-saturated shale together with the nonporous minerals and whose elastic modulus is an average of those of its components, including the porous shale. This alternative method increases the sensitivity of the elastic properties of sediment-to-pore-fluid changes and therefore may be used as a physics-based theoretical tool to better explain and interpret seismic data during exploration as well as variations in seismic response as hydrocarbon production progresses.

Constraints on velocity-depth trends from rock physics models

Abstract: Estimates of depth, overpressure and amount of exhumation based on sonic data for a sedimentary formation rely on identification of a normal velocity–depth trend for the formation. Such trends describe how sonic velocity increases with depth in relatively homogeneous, brine-saturated sedimentary formations as porosity is reduced during normal compaction (mechanical and chemical). Compaction is ‘normal’ when the fluid pressure is hydrostatic and the thickness of the overburden has not been reduced by exhumation. We suggest that normal porosity at the surface for a given lithology should be constrained by its critical porosity, i.e. the porosity limit above which a particular sediment exists only as a suspension. Consequently, normal velocity at the surface of unconsolidated sediments saturated with brine approaches the velocity of the sediment in suspension. Furthermore, porosity must approach zero at infinite depth, so the velocity approaches the matrix velocity of the rock and the velocity–depth gradient approaches zero. For sediments with initially good grain contact (when porosity is just below the critical porosity), the velocity gradient decreases with depth. By contrast, initially compliant sediments may have a maximum velocity gradient at some depth if we assume that porosity decreases exponentially with depth. We have used published velocity–porosity–depth relationships to formulate normal velocity–depth trends for consolidated sandstone with varying clay content and for marine shale dominated by smectite/illite. The first relationship is based on a modified Voigt trend (porosity scaled by critical porosity) and the second is based on a modified time-average equation. Baselines for sandstone and shale in the North Sea agree with the established constraints and the shale trend can be applied to predict overpressure. A normal velocity–depth trend for a formation cannot be expressed from an arbitrary choice of mathematical functions and regression parameters, but should be considered as a physical model linked to the velocity–porosity transforms developed in rock physics.

Permeability and Volumetrics of Porous Asphalt Concrete : A Theoretical and Experimental Investigation

Abstract: This paper deals with the theoretical and experimental analysis of percolation phenomena of water into Porous Asphalt concretes (PA). The main objects are the following: a) comparison of different indicators dealing with percolation processes; b) finding the best volumetric descriptor of percolation phenomena. The developed physical-based model is the basis for the design and analysis of experiments. A chart of equivalence for cross-referencing typical results obtained from permeability tests is proposed. Theory showed that, when managing permeability performance, effective porosity, neff, can be very important. On the other hand, goodness of fit did not provide sufficient data to clearly identify neff as a superior method/indicator.

Porosity Prediction using 3D Seismic Inversion Kadanwari Gas Field, Pakistan

Abstract: In the western part of the Kadanwari field a gasbearing stratigraphic trap was identified in the G-sand of Lower Goru formation by drilling the X well. However, due to poor reservoir properties the well was not commercial. Thus, the challenge was to locate and quantitatively evaluate a drilling target in the sand body, which would have porosity values beyond the known producible threshold in the area. With this aim, 250 km of 3D seismic data was acquired in the area. The data was processed to pre-stack time migration, and acoustic impedance inversion was performed. This inverted data was used to generate a pseudo porosity volume to locate a “sweet spot” for porosity in the area. A linear relationship between Porosity and Acoustic Impedance was built based on the wire-line and CPI logs of the well X. The equation was obtained as a regression line for the clean G-sand porosity and acoustic impedance. The equation was applied to the available AIMP volume to obtain a pseudo-porosity volume. The inversion-derived pseudoporosity was compared with the CPI porosity at well X. As expected, the pseudo-porosity obtained from inversion provided a result with frequency content similar to the seismic data. For this reason the calculated porosity was not able to describe the details of internal variations of the G-Sand but capable to catch the bulk character of the target sand only. Using this pseudoporosity volume, a pseudo-porosity map was generated. On the basis of this work, the successful well Y was drilled. Due to the presence of complex mineralogy the effective porosity of well Y is still a question, however the sonic porosity was very close to the predicted porosity of 17-18%. On the basis of this success the Kadanwari Joint Venture has decided to acquire 3D seismic in the all field area, to apply the same technique in order to fully evaluate the remaining potential. According to author s knowledge this technique has not been used previously in the area for porosity prediction.

Simultaneous Prediction of Saturated Hydraulic Conductivity and Drainable Porosity Using the Inverse Problem Technique

Abstract: The saturated hydraulic conductivity K and the effective porosity f are two important input parameters needed for lateral drain spacing design, as well as some other applications. The technical and economic justification, of most drainage projects, is mainly connected to these two parameters. The current design procedure is based upon calculation of the lateral spacing, using some average values of K and f within the drainage area. The objectives of this study were to introduce a new method for simultaneous estimation of K and f parameters using the inverse problem technique, and to evaluate five different unsteady drainage analytical models of the Boussinesq equation, suggested by different researchers for simultaneous prediction of the parameters. Consequently, five different analytical models for predicting water table profiles were solved, using the inverse problem technique. Each model was then evaluated. A physical drainage model of 2.2 m length, 0.3 m width, and 0.5 m height was established in the ...

Time Scales in the Evolution of Solution Porosity in Porous Coastal Carbonate Aquifers by Mixing Corrosion in the Saltwater-Freshwater Transition Zone

Abstract: Dissolution of calcium carbonate in the saltwater-freshwater mixing zone of coastal carbonate aquifers up to now has been treated by coupling geochemical equilibrium codes to a reactive-transport model. The result is a complex nonlinear coupled set of differential transport-advection equations, which need high computational efforts. However, if dissolution rates of calcite are sufficiently fast, such that one can assume the solution to be in equilibrium with respect to calcite a highly simplified modelling approach can be used. To calculate initial changes of porosity in the rock matrix one only needs to solve the advection-transport equation for salinity s in the freshwater lens and its transition zone below the island. Current codes on density driven flow such as SEAWAT can be used. To obtain the dissolution capacity of the mixed saltwater-freshwater solutions the calcium equilibrium concentration ceq(s) is obtained as a function of salinity by PHREEQC-2. Initial porosity changes can then be calculated by a simple analytical expression of the gradient of the spatial distribution s(x, y) of salinity, the distribution of flow fluxes q(x,y) and the second derivative of the calcium equilibrium concentration ceq(s) with respect to salinity s. This modelling approach is employed to porosity evolution in homogeneous and heterogeneous carbonate islands and coastal aquifers. The geometrical patterns of porosity changes and the reasons of their origin will be discussed in detail. The results reveal initial changes of porosity in the order of several 10-6 per year. This places the time scale of cavern evolution to orders from several tens of thousands to a hundred thousand years.

Physical and mechanical performance of porous concrete for drainage base

Abstract: As base course material,porous concrete should have good physical and mechanical performance.According to tests and analysis,the relationships between effective porosity and total porosity,permeability coefficient and porosity of porous concrete are adduced.The strength development with time,relationships between flexural tensile strength,split strength and compression strength,the relationship between compression strength and porosity,and two types of double logarithm fatigue equation of porous concrete are got.The relationships between flexural tensile elastic module and flexural tensile strength,compression elastic module and compression strength of porous concrete are drawn.At last,the temperature contraction coefficient and dry shrinkage coefficient of porous concrete are advanced.

Classification method for extra-low permeability reservoirs

Abstract: Aimed at the shortage of classification evaluation for extra-low permeability reservoirs,several new techniques developed in reservoir research are introduced in recent years.Four classification evaluation parameters for extra-low permeability reservoir are proposed,including effective porosity,mainstream throat radius,movable fluid saturation and start-up pressure gradient.The reservoir classification evaluations are carried out to nine blocks.The results show that the seepage capacity and effective reservoir space could be effectively reflected.This classification method will be important to guide the oil/gas field development.

Compressional-wave velocity variation in the upper Macaé formation: A well-log regression analysis study

Abstract: In this contribution, a regression analysis methodology allows prediction of P-wave velocities in locations where sonic logs are for some reason unavailable. The proposed methodology uses fundamental geophysical well logs (i.e., nuclear and resistivity logs) for least-squares estimation of regression coefficients. Fractional effective porosity and clay volume as well as bulk density form the set of assumed independent variables in the corresponding regression models. Considering P-wave velocities varying according to linear, parabolic and multivariate linear models, the calibration process confirms previous results showing porosity as the most important dependence parameter in the regressions. When calibration is extended to a well far away from the original location, it reveals that combinations of certain obtained regressions can predict P-wave velocities in the formation under study with reasonable accuracy.

The lower limits of physical properties for deep clastic reservoirs in north Songliao basin and its relationship with microscopic features

Abstract: Based on the research on lithology,physical properties, pore structure,and pore microscopic features of sandstone and conglomerate reservoirs with different productivity,the porosity permeability versus productivity charts for sandstone and conglomerate have been established with the consideration of gas testing results.According to the research,the lower limits of effective porosity and permeability for reservoirs where oil can be accumulated and produced are determined.The lower limit of porosity for sandstone is 4.1%5.5%,and that of permeability is 0.06×10-3μm2.The lower limit of porosity for conglomerate is only 2.7%,and that of permeability is 0.05×10-3μm2,because the single conglomerate layer is thick,the microfractures are vegetal and the expulsion pressure is low.Further study indicates that the difference of the main pore type,capillary pressure curve characteristics,diagenetic features and the dominant factors for reservoirs between the gas pay and dry formation results in the gas-bearing difference of reservoirs.Due to the great difference of microscopic features,such as pore type and expulsion pressure,between sandstone and conglomerate,the lower limits of physical properties vary greatly.

Microscopic effects and porous medium

Abstract: The objective of this paper is to show an experimental work elucidating the effect of rock sample sizes on porosity. Otherwise, this testing reveals two domains : - The microscopic effects where a great variations in porosity should be observed. The homogeneous porous medium where the effect of adding or subtracting one or several pores, has no significant influence on the porosity

Characterization and modelling of argillaceous porous medium by compressional and shear acoustic waves

Abstract: This chapter focuses on the characterization and modeling of argillaceous porous medium by compressional and the shear acoustic waves. The prediction of effective porosity and elastic moduli in mixed phases depends on volume fractions and the elastic modulus of each phase as well as their geometrical combination. With volume fractions specification, it is only possible to predict the mixed upper and lower elastic moduli bounds. The results showed that the clay compaction evolution and the water drainage cause density increase. It is observed that clay is initially put in suspension in water and then arranges itself progressively to raise a laminated structure. The ultimate stage of compaction tends to form a mineral block of a much reduced porosity, in which anisotropy tends to disappear. Taking into account of geometrical structure, and Reuss and Voigt formulations, it is possible to express clays bulk and shear moduli. To use the good values for mineral elastic moduli, it is advisable to have a good reliability about the clay mineralogy for a given depth. Clay mineralogy determination could be done with two methods—X-Ray diffraction on plugs, and gamma ray spectroscopy.