Effective porosity

About: Effective porosity is a research topic. Over the lifetime, 1199 publications have been published within this topic receiving 26511 citations.

Simulations of Single-Fluid Flow in Porous Media

Abstract: Several results of lattice-gas and lattice-Boltzmann simulations of single-fluid flow in 2D and 3D porous media are discussed. Simulation results for the tortuosity, effective porosity and permeability of a 2D random porous medium are reported. A modified Kozeny–Carman law is suggested, which includes the concept of effective porosity. This law is found to fit well the simulated 2D permeabilities. The results for fluid flow through large 3D random fibre webs are also presented. The simulated permeabilities of these webs are found to be in good agreement with experimental data. The simulations also confirm that, for this kind of materials, permeability depends exponentially on porosity over a large porosity range.

The relationships among stress, effective porosity and permeability of coal considering the distribution of natural fractures: theoretical and experimental analyses

Abstract: Although the relationships among stress, effective porosity and permeability of coal are a fundamental research topic that has been studied for decades and are widely used in analyzing the mechanical behavior of coal seams and predicting coalbed methane production, most relevant studies are based on idealized models and do not consider the influence of natural fracture distributions. To obtain a comprehensive understanding of the interrelationships among stress, effective porosity and permeability of coal, a series of effective porosity and permeability determinations have been conducted under different overburden stresses using an automated permeameter–porosimeter considering the directional distribution of natural fractures in coal. The experimental results show that the directional distribution of natural fractures provides a substantial contribution to the anisotropy of the effective porosity and absolute permeability of coal, which exponentially decrease with increasing overburden stress. An existing permeability model was modified to reflect the influence of the natural fracture distribution on the power law relationship between effective porosity and permeability, i.e., the exponent is not constant, but a variable related to the natural fracture distribution. The anisotropic effective porosity sensitivity and stress sensitivity of coal are also discussed, and the coal mass is shown to have the highest effective porosity sensitivity and lowest stress sensitivity in the direction perpendicular to the bedding planes compared to those in other directions.

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.

Exact Solutions for a Thermal Nonequilibrium Model of Fluid Saturated Porous Media Based on an Effective Porosity

Abstract: An effective porosity concept has been introduced to account for the effects of tortuosity and thermal dispersion on the individual effective thermal conductivities of the solid and fluid phases in a fluid-saturated porous medium. Using this effective porosity concept, a thermal nonequilibrium model has been proposed to attack locally thermal nonequilibrium problems associated with convection within a fluid-saturated porous medium. Exact solutions are obtained, assuming a plug flow, for the two cases of thermally fully developed convective flows through a channel, namely, the case of isothermal hot and cold walls and the case of constant heat flux walls. These exact solutions for the cases of metal foam and air combination reveal that the local thermal equilibrium assumption may hold for the case of isothermal hot and cold walls, but may fail for the case of constant heat flux walls. [DOI: 10.1115/1.4004354]

Effect of alteration, formation absorption, and standoff on the response of the thermal neutron porosity log in gabbros and basalts: Examples from Deep Sea Drilling Project-Ocean Drilling Program Sites

Abstract: This study focuses on the effects of hydrous alteration minerals, formation absorption, and standoff on the response of the thermal neutron porosity log in the basaltic and gabbroic rocks logged at sites 395, 418, 504, 642, and 735 during the Ocean Drilling Program. The concentration of hydrogen present in the rocks in the form of free water (pore space) and bound water (hydrous minerals) is the primary factor controlling the neutron elastic scattering process, while the presence of other elements, such as chlorine, gadolinium, boron, lithium, and samarium in the fluids and in the rock matrix can largely affect the thermal diffusion phase. These neutron absorbers cause an increase of the capture cross section, and in turn of the apparent thermal porosity. Further perturbations occur when the recording conditions depart from those under which the tool has been calibrated; a large and irregular hole diameter and a lack of eccentralization both produce erroneous porosity readings. The effect of hydrous alteration minerals on the thermal neutron porosity log has been estimated from 922 core oxide measurements using an analysis program that calculates the slowing-down length and converts it into apparent porosity. The results show that the computed apparent porosity ranges from less than 1% in fresh basalts and gabbros to about 30% in highly altered units. Depending on the alteration mineral assemblage, natural gamma ray, capture cross section, or hydrogen logs have been used to continuously predict the effect of bound hydrogen at each site. Corrected porosities generally show excellent agreement with core data for massive units, whereas they are higher for pillow basalts and fractured zones. The discrepancy is interpreted as the result of (1) difference in the volume of rock investigated (core specimens do not sample large vugs and fractures) and (2) frequent variations in the hole size and lack of tool contact with the borehole wall (standoff), not completely accounted for in the corrections. High capture cross section values measured on selected basalt samples seem to be associated with more altered basalts, suggesting a larger concentration of strong neutron absorbers in secondary minerals. The concentrations of elements with large capture cross section (Σ) measured at three of the five sites studied yield a maximum increase of 1% in the apparent neutron porosity. This is too low to explain the porosity values, some greater than 7%, observed in some of the fresh and unfractured massive units. A 1- to 2-inch standoff due to the lack of eccentralization of the tool string used in the Ocean Drilling Program is considered the most probable cause of such discrepancy. This interpretation is confirmed by the neutron porosity values recorded at site 735, where the nuclear string was for the first time fully eccentralized, and only a correction for borehole size and the presence of hydrous minerals has been applied.