Showing papers on "Permeability (earth sciences) published in 1991"

Numerical calculation of equivalent grid block permeability tensors for heterogeneous porous media

Abstract: A numerical procedure for the determination of equivalent grid block permeability tensors for heterogeneous porous media is presented. The method entails solution of the fine scale pressure equation subject to periodic boundary conditions to yield, upon appropriate averaging of the fine scale velocity field, the coarse scale or equivalent grid block permeability. When the region over which this coarse scale permeability is computed constitutes a representative elementary volume (REV), the resulting equivalent permeability may be interpreted as the effective permeability of the region. Solution of the pressure equation on the fine scale is accomplished through the application of an accurate triangle-based finite element numerical procedure, which allows for the modeling of geometrically complex features. The specification of periodic boundary conditions is shown to yield symmetric, positive definite equivalent permeability tensors in all cases. The method is verified through application to a periodic model problem and is then applied to the scale up of areal and cross sections with fractally generated permeability fields. The applicability and limitations of the method for these more general heterogeneity fields are discussed.

Two‐phase flow in heterogeneous porous media: 2. Model application

Abstract: The migration of a dense, nonaqueous phase liquid through heterogeneous porous media is examined using numerical simulation Laboratory measurements of capillary pressure-saturation curves were performed on samples obtained from a sand aquifer and scaled to permeability to provide a data base of model input parameters Numerical simulations incorporating 25,200 finite difference cells are carried out in a spatially correlated, random permeability field to illustrate the influence of fluid properties on the migration of a nonwetting liquid below the water table The simulation results are characterized by spatial moments to reflect the relative degrees of lateral spreading exhibited by the migrating nonwetting body in the presence of lenses of differing permeability In addition, numerical simulations were performed in a solution domain containing a single lens of lower permeability material in order to examine the local-scale sensitivity to porous media and fluid properties The results of the study show the migration of a nonwetting liquid to be extremely sensitive to subtle variations in the capillary properties of the porous medium and to be influenced strongly by the fluid physical properties

Evaluation of Unsaturated Zone Air Permeability Through Pneumatic Tests

Abstract: Predicting the steady state distribution of air pressure in the unsaturated zone resulting from a pneumatic test provides a method for determining air-phase permeability. This technique is analogous to the inverse problem of well hydraulics; however, air flow is more complicated than ground water flow because of air compressibility, the Klinkenberg effect, variations in air density and viscosity that result from temperature fluctuations in the unsaturated zone and the possibility of inducing water movement during the pneumatic test. An analysis of these complicating factors reveals that, when induced water movement can be neglected, a linear version of the airflow equation can provide an appropriate approximation for the purpose of determining air-phase permeability. Two analytical solutions for steady state, two-dimensional, axisymmetric airflow to a single well partially screened in the unsaturated zone are developed. One solution applies where there is a stratum of relatively low air permeability, separating the stratum in which the well is completed, from the atmosphere. The other solution applies where there is no separating stratum between the domain and atmosphere. In both situations the water table forms the lower horizontal boundary. Applications of both solutions to determine air permeability from data collected during pneumatic tests are presented.

A model for coupled fluid-flow and mixed-volatile mineral reactions with applications to regional metamorphism

Abstract: The effect of fluid flow on mixed-volatile reactions in metamorphic rocks is described by an expression derived from the standard equation for coupled chemical-reaction and fluid-flow in porous media. If local mineral-fluid equilibrium is assumed, the expression quantitatively relates the time-integrated flux at any point in a flow-system to the progress of devolatilization reactions and the temperature- and pressure-gradients along the direction of flow. Model calculations indicate that rocks are generally devolatilized by fluids flowing uptemperature and/or down-pressure. Flow down-temperature typically results in hydration and carbonation of rocks. Time-integrated fluid fluxes implied by visible amounts of mineral products of devolatilization reactions are on the order of 5·102–5·104 mol/cm2. The model was applied to regionally metamorphosed impure carbonate rocks from south-central Maine, USA, to obtain estimates of fluid flux, flow-direction, and in-situ metamorphic-rock permeability from petrologic data. Calculated time-integrated fluxes are 104–106 cm3/cm2 at 400°–450° C, 3,500 bars. Fluid flowed from regions of low temperature to regions of high temperature at the peak of the metamorphic event. Using Darcy's Law and estimates for the duration of metamorphism and hydrologic head, calculated fluxes are 0.1–20·10-4 m/year and minimum permeabilities are 10-10–10-6 Darcy. The range of inferred permeability is in good agreement with published laboratory measurements of the permeability of metamorphic rocks.

Empirical prediction of porosity and permeability in sandstones

Abstract: Current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process-oriented models. Empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models. Of these parameters, the most important are detrital composition, grain size, sorting, and temperature history or pressure history or both. Despite its limitations, the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability. Predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within +/- 2% porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest. The effectiveness of the empirical predictive approach is illustrated by case studies from the Taranaki basin (New Zealand) and Yacheng field (People's Republic of China). These studies indicate that empirical predictions are basin specific or even play specific, and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target. Process-oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability. Until adequate quantification of the sandstone diagenesis processes is achieved, empirical models have a distinct advantage over process-oriented models in providing reliable predictions of reservoir quality in many sandstone intervals.

Hydrogeology of thrust faults and crystalline thrust sheets: Results of combined field and modeling studies

Abstract: Field, laboratory, and modeling studies of faulted rock yield insight into the hydraulic character of thrust faults. Late-stage faults comprise foliated and subparallel faults, with clay-rich gouge and fracture zones, that yield interpenetrating layers of low-permeability gouge and higher-permeability damage zones. Laboratory testing suggests a permeability contrast of two orders of magnitude between gouge and damage zones. Layers of differing permeability lead to overall permeability anisotropy with maximum permeability within the plane of the fault and minimum permeability perpendicular to the fault plane. Numerical modeling of regional-scale fluid flow and heat transport illustrates the impact of fault zone hydrogeology on fluid flux, fluid pore pressure, and temperature in the vicinity of a crystalline thrust sheet.

A generalized Blake‐Kozeny equation for multisized spherical particles

Abstract: The objectives of this study were twofold First, in view of the pausicity of data in the literature, additional experiments were conducted for laminar flow through a porous medium made up of multisized particles Second, a theoretical analysis was performed to provide a physical basis for correlating the permeability data

Correlations for predicting air permeabilities and 222Rn diffusion coefficients of soils.

Abstract: The rate of 222Rn gas transport through earthen materials controls 222Rn releases to the atmosphere and to indoor environments. The key soil-related parameters characterizing 222Rn transport in earthen materials are the 222Rn diffusion coefficient and the soil air permeability. Simple correlations have been developed for predicting the 222Rn diffusion coefficient and the air permeability of soils based on fraction of water saturation, total porosity, and arithmetic mean particle diameter. Correlations are based on 1,073 diffusion coefficient measurements and 137 soil air permeability measurements. The geometric standard deviations between the correlation predictions and the measurements are 1.98 for the diffusion coefficients and 2.31 for the soil air permeabilities.

Permeability of High-Strength Lightweight Concrete

Abstract: Information on the resistance of high strength lightweight concrete 50 to 100 MPa to water penetration and accelerated chloride penetration is presented. Testing techniques are also discussed. The permeability of high strength-lightweight concrete appears to be very low, but it may be higher than that of normal weight concrete at a similar strength level. The peremeability of high strength lightweight concrete appears to be more dependent on the porosity of the mortar matrix than the porosity of the lightweight aggregate. There appears to be an optimum cement content for permeability. A too high cement content may increase the permeability. No direct relationship between water permeability and electrical conductivity was observed, but a direct relationship between water permeability and accelerated rate of chloride penetration was observed. Hence, accelerated testing of chloride penetration appears to be a more valuable way to test the permeability than testing the electrical conductivity.

Cyclical closed-system freeze-thaw permeability testing of soil liner and cover materials

Abstract: The impact of closed-system freeze–thaw cycles on the permeability of clay, till, and sand–bentonite mixtures was examined in a laboratory test program. The bentonite content of the sand–bentonite mixtures varied from 4.5 to 25%. Test samples were prepared by dry mixing, moisture conditioning, and compacting materials according to ASTM D698, method A specifications. The molding water content of these samples was at or slightly wet of optimum. Trimmed test samples were placed in a triaxial permeameter and their unfrozen or initial permeability was established. These permeability tests were conducted for approximately 10 000 min using low hydraulic gradients and confining stress. Changes in specimen volume were also measured during permeability testing. Once the initial permeability of a specimen was established, the test was halted and the specimen was frozen inside the permeameter with no access to water. After freezing, the specimens were allowed to thaw before a second permeability test was conducted. T...

Influence of deformation on the fluid transport properties of salt rocks

Abstract: While the fluid transport properties of rocks are well understood under hydrostatic conditions, little is known regarding these properties in rocks undergoing crystal plastic deformation. However, such data are needed as input in the field of radioactive waste disposal in salt formations. They are also needed to understand deformation, shear zone evolution and associated fluid movement, metamorphism and mineralization in the deep crust and even upper mantle. The present study is concerned with the development of a fundamental understanding of the influence of crystal plastic deformation on dilatancy and permeability evolution in salt rocks and salt/anhydrite rocks. It is experimentally based and seeks to explain the influence of deformation on permeability in the framework of "percolation theory", currently finding wide application in solid state physics. The results relate directly to the behaviour of salt rock in disposal systems and, viewing salt as an analogue material, provide insight into the effects of plastic deformation on the fluid transport properties of crystalline rocks in general.

Petrography and reservoir physics; III, Physical models for permeability and formation factor

Abstract: Permeability and formation factor are physical properties of porous rocks useful for assessing reservoirs. Neither property varies consistently as porosity varies. The relationship of both properties to porosity is complex, being sensitive to the structure of the porous microstructure, i.e., the sizes of pore throats, the numbers and sizes of pores, and the relationships between pores and throats. Physical models to account for these factors require parameters that describe physically relevant properties of the microstructure. A partial characterization of the relationship between pores and throats is embodied in the relationship between pore type and throat size. This relationship is derived by combining data obtained from thin sections, from which pore types are derived via image analysis, and mercury injection porosimetry, which quantifies throat size information. Parameters derived from such a combination are sufficient to construct simple physical models for permeability and electrical conductivity (inverse formation factor). These models assume a porous medium that has large numbers of flow paths parallel to the potential gradient, such that flow has little tortuosity (i.e., flow parallel to bedding). The contributions of each pore type to permeability and electrical conductivity are computed. Calculated values are close to measurement values. A constant of proportionality is the same for all samples from a reservoir, but can vary between reservoirs, is required, and must have values ranging (for sandstones) from about 2.5 to 3.5 for permeability an 5.0 to 7.0 for conductivity. These values are consistent for an efficiently packed fabric. One result of such modeling is a physical model of Archie's cementation exponent m as the ratio of the logarithms of the cross sectional throat area to pore area (per unit area).

Experimental verification of Darcy-Brinkman-Forchheimer flow model for natural convection in porous media

Abstract: Experimental results for natural convection in a horizontal porous cavity of aspect ratio A = 5 and heated from below are reported. A wide range of governing parameters are covered by careful selection of bead size, solid material, and fluid. These results fully support the effects of fluid-flow parameters (Rayleigh and Prandtl numbers), porous matrix-structure parameters (Darcy and Forchheimer numbers), and the conductivity ratio as predicted by the formulation based on the Darcy-Brinkman-Forchheimer (DBF) equations of motion. The DBF flow model, with variable porosity and variable thermal conductivity in the wall regions, predicts reasonably well in comparison with the experimental data. However, the difference between the predictions and the measurements increases as the ratio of solid-to-fluid thermal conductivity becomes very large. 32 refs.

Change in Apparent Viscosity of CO2 Foam With Rock Permeability

Abstract: Review of new and previous high-pressure experiments measuring the mobility of CO 2 foam in porous rocks (sandstones and carbonates).

Analysis of subsidence due to a point sink in an anisotropic porous elastic half space

Abstract: Analytical solutions of the long-term consolidation deformation and excess pore water pressure due to fluid withdrawal from a saturated anisotropic porous elastic half space are presented. In the analysis, both the permeability and the elastic properties are considered cross-anisotropic. Cases of pervious and impervious half-space boundary are studied. The consolidation settlements as effected by the anisotropy and boundary conditions for pore water are illustrated and discussed.

Travis peak core permeability and porosity relationships at reservoir stress

Abstract: In this paper relationships of permeability to porosity are shown from analyses of more than 2,100 core plugs from nine wells in the Travis Peak, a low-permeability, tight-gas sandstone formation in northeast Texas. Effects of reservoir vs. ambient stress are shown for permeability, porosity, and the Klinkenberg factor. The relationship of brine permeability to gas permeability is also shown.

The water permeability of concrete and its relationship with strength

Abstract: As part of a much broader programme to evaluate the performance of concretes for use in reinforced and pre-stressed containments for liquid gases, seventeen concrete mixes, ranging in strength from 16 to 100N/mm2, were subjected to screening tests by measurement of the water permeability coefficient. Specimens were stored in sealed conditions at 20° C and tests were also carried out to determine the compressive and tensile splitting strengths. The relationships between permeability and strength are discussed, as well as the influence of concreting materials, mix proportions and curing.

A Method for Correcting Dimensionless Fracture Conductivity for Non-Darcy Flow Effects

Abstract: This paper provides a method of extending the concept of dimensionless fracture conductivity to the design of fracture conductivity in wells to be produced at rates that cause non-Darcy flow down the fracture. Such flow rates are common in gas wells and occasionally occur in oil wells. the correction requires knowledge of the expected producing rate and calculation of a Reynolds number characterizing flow in the fracture near the wellbore.

Network model investigation of gas transport in bidisperse porous adsorbents

Abstract: A capillary network model consisting of a micropore network permeated by one of macropores of randomly varying size has been constructed. Although simplified (to keep computer space and time requirements low) in relation to a real bidisperse porous adsorbent or catalyst, it embodies the salient pore structural features likely to determine the gas-transport behavior of such porous solids. Suitable model calculations of Knudsen gas-phase and surface diffusion enabled us to validate useful approximate methods for the more economical evaluation of network permeability and demonstrate certain important characteristic effects of nonrandom bidisperse pore structure on transport behavior and their practical consequences, especially in connection with the experimental determination of surface diffusion coefficients

Factors Influencing Unsteady Relative Permeability of a Mixed-Wet Reservoir Rock

Abstract: Capillarity, viscous fingering, and heterogeneity influence the flow in a core plug and hence affect the relative permeability determined from an unsteady test. Several unsteady water/oil relative permeability tests were carried out in a mixed-wet core while in-situ 3D saturation distribution was monitored by a computerized-tomography (CT) scanner. Results illustrate included in this paper that, in the early part of the Johnson-Bossler-Naumann (JBN) method, relative permeability is dominated by fingering and heterogeneity effects. The later part of this method ({gt}1 PV), however, represents the relative permeability of the end-face saturation and is influenced by the capillary number and throughput. Thus, laboratory results must be scaled to the field on the basis of the flow parameters: end-effect, capillary, instability, and heterogeneity numbers.

A “Conductance” Mesh Approach to the Permeability of Natural and Simulated Fracture Patterns

Abstract: The “conductance” mesh method is used to determine the permeability properties of natural and simulated, two-dimensional, fracture patterns In the method, the fractures and their matrix are discretized on a grid of 200 × 200 elements The permeability of the whole pattern in the direction of the pressure gradient is determined from the total flow through the network By applying the model in varying orientations, a picture of the directional permeability is constructed Using the model, the effects of varying matrix permeability and fracture aperture on the global permeability of a natural fracture pattern and 10 simulations are investigated The simulated fracture patterns were found to increasingly underestimate the permeability of the natural pattern as the permeability contrast between rock matrix and fractures increased The lower permeability of the simulations was found to relate to their poorer connectivity, as reflected in the sizes of their “backbones” (comprising all paths across the region along the fracture network) and in the number and type of fracture intersections The results show that the effect of the natural pattern spatial distribution is to promote fracture intersection, thereby increasing connectivity and permeability

Kinetics of capillary extraction of organic vehicle from ceramic bodies. Part I: Flow in porous media

Abstract: Gas and water flow permeabilities of four powder beds with diverse characteristics were measured and compared with values predicted by a range of model equations. No single equation was found to fit all powders. This means that a general theory of capillary extraction of ceramic binders cannot rest on currently available permeability models. The kinetics of wax sorption from an unlimited supply was measured gravimetrically and was found to have parabolic dependence on time, as predicted by Darcy's Law. The sorption constant was deduced and, together with experimental values of permeability, used to calculate the average capillary pressures for the powder beds. These in turn were compared with theory. In Part II the results will be used to attempt to quantify the extraction of wax from ceramic bodies.

Simulation of block-to-block processes in naturally fractured reservoirs

Abstract: Mechanisms involved in gas/oil gravity drainage in terms of the block-to-block are examined. A new approach is proposed in which these mechanisms can be represented properly in the field-scale simulation of these reservoirs. The method involves the calculation of pseudocapillary potentials giving the correct flow behavior

Analytical models of the effective permeability of sand‐shale reservoirs

Abstract: SUMMARY A study is made of the dependence of the effective permeability of sand-shale reservoirs on the geometry of the shale inclusions. Numerical simulations are described in which factors such as the orientation and the degree of anisotropy of the shales, their volume fraction and the permeability contrast ratio between the shale and the sandstone were varied. Two different reservoir models were used in the simulations: one in which the shales were in the form of random, overlapping spheroidal inclusions, and one in which the shales were generated by a geostatistical technique. The computed effective permeability data are compared with a variety of analytical models in an attempt to discover ‘rules of thumb’for the estimation of the effective permeability of sand-shale reservoirs for use in a range of practical situations. The main technical way in which this study advances on previous work is in its use of a computationally efficient, random walk algorithm for calculating the effective permeability, which has enabled larger volumes to be simulated and, hence, a thorough investigation of finite-size effects to be made. Another advantage of the work reported here is the general nature of the sand-shale geometries used in the numerical simulations and the extensive comparisons with analytical models.

Convective instability in superposed fluid and anisotropic porous layers

Abstract: The onset of thermal convection due to heating from below in a system consisting of a fluid layer overlying a porous layer with anisotropic permeability and thermal diffusivity is studied. Flow in the porous medium is assumed to be governed by Darcy’s law; the Beavers–Joseph condition is applied at the interface between the two layers. The linear perturbation equations are solved numerically. It is found that the effects of anisotropy on the onset of thermal convection are most profound for small values of the depth ratio ζ (ratio of fluid layer thickness to porous layer thickness), since in that case, the onset of convection corresponds to significant motion in both layers. For fixed values of the vertical permeability in the porous medium, decreasing the value of ξ (ratio of horizontal to vertical permeability) leads to stabilization of the superposed layer configuration because of increased resistance to motion in the porous medium. For larger values of ζ, the onset of motion is increasingly confined to the fluid layer, with the transport of heat through the porous layer occurring primarily by conduction. Accordingly, the influence of ξ on the stability characteristics for larger ζ is less significant than the effects of an anisotropic thermal conductivity.