Showing papers on "Permeability (earth sciences) published in 1995"

Flow and Transport in Porous Media and Fractured Rock: From Classical Methods to Modern Approaches

Abstract: Part 1 Continuum versus discrete models: a hierarchy of heterogeneities and length scales long-range correlations, fractals and percolation continuum versus discrete models. Part 2 The equations of change: the equation of continuity the momentum equation the diffusion and convective-diffusion equations. Part 3 Fractal concepts and percolation theory: box-counting method and self-similar fractals self-affine fractals multifractal systems fractional Brownian motion and long-range correlations percolation processes a glance at history. Part 3 Diagenetic processes and formation of rock: diagenetic and metasomatic processes continuum models of diagenetic processes geometrical models of diagenetic processes in granular rock a geometrical model of carbonate rock diagenetic processes of fractured rock. Part 5 Morphology of porous media and fractured rock: porosity, specific surface area and tortuosity fluid saturation, capillary pressure and contact angle pore size distribution topological properties of porous media fractal properties of porous media porosity and pore size distribution of fractal porous media morphology of fractured rocks. Part 6 Models of porous media: models of macroscopic porous media models of pore surface roughness models of megascopic porous media interpolation schemes and conditional simulation. Part 7 Models of fractured rock: continuum approach - the multi-porosity models network models simulated annealing model synthetic fractal models mechanical fracture models. Part 8 Flow and transport in porous media: the volume-averaging method and derivation of Darcy's law the Brinkman and Forchheimer equations predicting the permeability, conductivity and diffusivity fractal transport and non-local formulation of diffusion derivation of Archie's law relation between permeability and electrical conductivity relation between permeability and nuclear magnetic resonance dynamic permeability. Part 9 Dispersion in porous media: the phenomenon of dispersion mechanisms of dispersion processes the convective-diffusion equation measurement of dispersion coefficients dispersion in simple systems dependence of dispersion coefficients on the Peclet number models of dispersion in macroscopic porous media long-time tails - dead-end pores versus disorder dispersion in short porous media dispersion in porous media with percolation disorder dispersion in megascopic porous media dispersion in stratified porous media. Part 10 Flow and dispersion in fractured rock: flow in a single fracture - continuum and discrete models flow in fractured rock dispersion in a single fracture dispersion in fractured rock. Part 11 Miscible displacements: factors affecting miscible displacement processes viscous fingering continuum models of miscible displacements in Hele-Shaw cells continuum models of miscible displacements in porous media. (Part contents).

Experimental Measurement of Coal Matrix Shrinkage Due to Gas Desorption and Implications for Cleat Permeability Increases

Abstract: Coal is not an inert reservoir rock and reacts to gas desorbed from its surface. Coal matrix shrinks as gas is desorbed, increasing cleat width and, therefore, permeability. Very few coal matrix shrinkage data have been reported in the literature so a series of experiments was undertaken to measure such data at reservoir pressures, temperatures, and 100% relative humidity. Strain gages were affixed to the coal sample in the face and butt cleat directions as well as the vertical direction. This work reports measured deformations of a sample of high volatile C bituminous coal from the San Jan Basin during sorption and desorption of first methane then CO 2 . A pressure cycle was also run with helium, a nonsorbing gas, to determine mechanical compliance of the sample. Observed strain gage behaviors are discussed and shrinkage coefficients for both gases reported. Matrix shrinkage was found to correlate with gas content rather than pressure, confirming the work of a previous investigator. Shrinkage coefficients varied more among replicate gages aligned in the same direction than between gages in different directions. Anisotropic shrinkage effects are discussed. Using a matchstick geometry model, equations are derived for permeability change due to matrix shrinkage. Coefficients reported here are used in example calculations of absolute permeability and porosity increases during coalbed depletion.

Permeability characterization. Part 1: A proposed standard reference fabric for permeability

Abstract: A three-dimensionally woven fabric is proposed as a standard reference material for permeability characterization. The 3-D woven fabric requires care in cutting and handling, although it is more robust than 2-D woven or braided fabrics. If prepared carefully, the permeability of the 3-D woven fabric can be measured reproducibly within 15% in either radial flow or saturated 1-D flow geometries. The material was characterized for permeability in radial, unsaturated and saturated 1-D, and through-thickness flow geometries. The transient results demonstrated the importance of structural heterogeneity on the unsaturated flow behavior, and agree qualitatively with a simplistic model of flow in heterogeneous unsaturated porous media. The effects of heterogeneity were manifested in the proposed SRM by an increasing trend in the “unsaturated permeability.” Experiments were also conducted with a random mat that displayed transient flows dominated by wicking. The effects of wicking on the macroscopic flow behavior were manifested by transients in the “unsaturated permeability” in which a decreasing trend was observed.

Scale Dependency of Hydraulic Conductivity Measurements

Abstract: The hydraulic conductivity of five stratigraphic units in a carbonate aquifer has been measured with slug, pressure, and pumping tests, and with two calibrated digital models. The effective test radii range from less than one to greater than 10,000 meters. On log-log plots hydraulic conductivity increases approximately linearly with test radius to a range between 20 and 220 meters, but thereafter, it is constant with scale. The increase in magnitude of hydraulic conductivity is similar to scaling effects reported at seven additional sites in a variety of geologic media. Moreover, the increase in magnitude correlates with an increase in variance of log-hydraulic conductivity measured at successively greater separation distances. The rate of increase in both parameters, and particularly the range, have characteristic values for different pore systems. The larger ranges are consistently present in units with greater secondary porosity. Therefore, scaling effects provide a qualitative measure of the relative importance of secondary and primary permeability, and they can potentially be used to distinguish the dominant type of pore system.

Geotechnical Properties of Oil-Contaminated Kuwaiti Sand

Abstract: Large quantities of oil-contaminated sands resulted from exploded oil wells, burning oil fires, the destruction of oil storage tanks, and the formation of oil lakes in Kuwait at the end of the Gulf War. An extensive laboratory testing program was carried out to determine the geotechnical characteristics of this material. Testing included basic properties, compaction and permeability tests, and triaxial and consolidation tests on clean and contaminated sand at the same relative density. Contaminated specimens were prepared by mixing the sand with oil in the amount of 6% by weight or less to match field conditions. The influence of the type of oil, and relative density was also investigated by direct shear tests. The results indicated a small reduction in strength and permeability and an increase in compressibility due to contamination. The preferred method of disposal of this material is to use it as a stabilizing material for other projects such as road construction.

Permeability for cross flow through columnar-dendritic alloys

Abstract: Experiments for measuring permeability in columnar-dendritic microstructures have provided data only up to a volume fraction of liquid of 0.66. Hence, the permeability for flow perpendicular to the primary dendrite arms in columnar-dendritic microstructures was calculated, extending our data base for permeability to volume fractions of liquid as high as 0.98. Analyses of the dendritic microstructures were undertaken first by detecting the solid-liquid interfaces with a special computer program and then by generating a mesh for a finite-element fluid flow simulation. Using a Navier-Stokes solver, the velocity and pressure at the nodes were calculated at the microstructural level. In turn, the average pressure gradient was used to calculate the Darcy permeability. Permeabilities calculated by this versatile technique provided data at high volume fractions of liquid that merged with the empirical data at the lower volume fractions.

Network modeling of permeability evolution during cementation and hot isostatic pressing

Abstract: Permeability of rocks may evolve as a complicated function of porosity during hydrothermal compaction. Recent laboratory studies on hot-pressed calcite and naturally lithified Fontainebleau sandstone reveal that there exist three regimes with distinctly different permeability-porosity relationships. At relatively high porosities, permeability changes with porosity reduction following a power law (k∝ϕα) with an exponent α≈3 (regime 1). At low porosities, the power law no longer applies and an accelerated reduction in permeability is found (regime 2). Finally, the permeability becomes too small to be measured, which implies that the pore space is disconnected and there is no percolation (regime 3). Similar behavior and three separate regimes have also been observed in the evolution of electrical conductivity with porosity in hot-pressed quartz. In this study, we developed a unified model based on percolation theory and the simulation of random networks to analyze the evolution of permeability and electrical conductivity in regimes 1 and 2. We incorporated a random shrinkage model and a connectivity loss model in a three-dimensional cubic network to account for the two distinct regimes. In regime 1, we kept the network connectivity at 100% and reduced the diameter of an arbitrary bond by a shrinkage factor randomly distributed between 0 and 1. In regime 2, we reduced the network connectivity from 100% to the percolation threshold while maintaining the same pore size distribution. For Fontainebleau sandstone, the pore size distribution is constrained by microstructural observations from automated image analysis and stereological measurements. For hot-pressed calcite and quartz, since microstructural data were not available, we made preliminary measurements on one available micrograph of a calcite sample. Our simulations predict changes in permeability and pore statistics as a function of porosity which show good agreement with the laboratory data. In accordance with percolation theory, the ratio between interconnected and total porosities is given by the ratio between the order parameter and the bond occupation probability of the network. Detailed observations of the interconnected porosity and total porosity of calcite during hot isostatic pressing are in good agreement with the theoretical prediction. Implications of our modeling results on the kinetics of healing and diagenetic processes are also discussed.

Why do gels reduce water permeability more than oil permeability

Abstract: A capacity to reduce water permeability much more than oil permeability is critical to the success of gel treatments in production wells if zones cannot be isolated during gel placement. Although several researchers have reported polymers and gels that provide this disproportionate permeability reduction, the explanation for the phenomenon was unclear. In this paper, the authors examine several possible explanations for why some gels reduce water permeability more than oil permeability. Their experimental results indicate the disproportionate permeability reduction is not caused by gravity or lubrication effects. Results also indicate that gel shrinking and swelling are unlikely to be responsible for the phenomenon. Although wettability may play a role in the disproportionate permeability reduction, it does not appear to be the root cause for water permeability being reduced more than oil permeability. Results from an experiment with an oil-based gel suggest that segregation of oil and water pathways through a porous medium (on a microscopic scale) may play the dominant role in the disproportionate permeability reduction. However, additional work will be required to verify this concept.

Numerical evaluation of the permeability and the Kozeny constant for two types of porous media

Abstract: Using the lattice-Boltzmann method we have calcuated the permeability of a random array of spheres and a clay soil. We have determined the structure of the clay soil by computed tomography imaging. As observed experimentally, the semiempirical Carman-Kozeny equation gives a good estimate for the permeability of the random array of spheres. For the soil sample, our calculated value of the permeability is consistent with experimental values. The Carman-Kozeny equation provides a much less successful estimate for the permeability of the soil than the random spheres.

The crack tip solution for hydraulic fracturing in a permeable solid

Abstract: This paper extends previous work on self-similar analytical solutions for a hydraulically driven fracture propagating in a solid which is in a state of plane strain In particular, we examine the effect of fluid loss to the formation modelled by the Carter leak-off mechanism Our main new results are asymptotic solutions for arbitrary rock permeability; it is shown how these may be represented by an expansion whose leading term is the near-tip solution in the high permeability limit This term gives an integrable singularity in the near-tip fluid pressure which is slightly stronger than the singularity which arises in the impermeable case; it follows that there is always a region immediately adjacent to the fluid front where the solution is dominated by fluid loss Our most important conclusion for applications is that the solution in the practical case of intermediate permeability may be constructed as a series which starts from the loss dominated limit We also provide a detailed comparison of our predictions with results from a numerical model which includes a fluid lag zone The results are found to be in good agreement in all of the various permeability regimes

Permeability dependence of streaming potential in rocks for various fluid conductivities

Abstract: Streaming potentials have been measured on sandstone and limestone samples in a large range of permeabilities. The electrokinetic coupling coefficient increases with permeability and we explain this effect by the related variation of surface conductivity. A model is proposed to study this effect for various fluid conductivities and it is shown that the dependence of the electrokinetic coupling coefficient on permeability is stronger for high fluid resistivity and is weaker for lower fluid resistivity. When fluid resistivity is below 1 Ω.m permeability and streaming potential are no more related.

Large Strain Static and Dynamic Semisaturated Soil Behaviour

Abstract: The theoretical and computational aspects of large strain static and dynamic partially saturated soil analysis are presented. The soil is assumed to be a three-phase material with a solid phase which may yield according to a generalized plasticity theory, whose liquid phase flows according to Darcy's law and where gaseous phase remains equal to the external ambient pressure. Permeability varies not only as a function of the degree of water saturation, but also as a function of the void ratio. These aspects are shown on several examples.

Relative permeability hysteresis: Laboratory measurements and a conceptual model

Abstract: Relative permeability hysteresis has been measured for a water-wet outcrop rock sample and a mixed-wet reservoir core. For the oil phase, imbibition and drainage relative permeability curves differed significantly. The difference was much less pronounced for the water phase. Scanning curves, which characterize transitions between imbibition and drainage curves, were also measured. A notable characteristic of the oil relative permeability scanning curves is their reversibility; along most of the length of a scanning curve, oil relative permeability exhibits no hysteresis. A proposed mechanism for the reversible behavior is pinning of water/oil interfaces on surfaces of rock grains. Pinned interfaces remain anchored at fixed positions on grains despite changes in interface curvature and contact angle. In water-wet samples, pinning can occur as a result of contact-angle hysteresis. In mixed-wet rock, pinning can occur at the boundaries between water- and oil-wet grain surfaces. As long as interfaces remain pinned, pore-level fluid geometry is a function of saturation only and does not depend on the directional of saturation change.

Analytical solutions for determining vertical air permeability in unsaturated soils.

Abstract: The study of flow and transport in unsaturated soils requires a good knowledge of the soil properties. When advective gas phase transport is of interest, air permeability is the key parameter of soil characterization. Although an air permeability test can easily be done in a laboratory, the reported values are frequently found to be not very representative of the air permeability in situ. Air permeability determined by a field test is useful both in the study of gas flow through unsaturated soils and in the prediction of the saturated hydraulic conductivity. This paper presents analytical solutions for determining the vertical air permeability of unsaturated soils using observed air pressure data at the land surface and at depth in the vadose zone. One solution is developed for a single-layer soil, and the other is developed for layered soils. Example calculations using field data have demonstrated the significant potential of the solution applications in field tests.

Experimental characterization of permeability and fibre wetting for liquid moulding

Abstract: Liquid moulding processes are unique in that resin is infused into a dry fibre preform. Appropriate wet-out of the reinforcing fibres is thus a necessity for the achievement of good composite properties. For this class of manufacturing methods, both macroscopic flow, as related to Darcy's Law and characterized by permeability, and microscopic flow, as related to fibre wet-out, are important. The current research investigates factors affecting permeability and fibre wet-out as related to liquid moulding. Specifically, it is shown that fabric permeability is dependent on the type of test fluid used. Surface tension and contact angle measurements indicate that interactions at the microscopic level between fibre and test fluid account for these differences in permeability. The investigation illustrates the competing nature of macroscopic and microscopic flow in liquid moulding.

Predicting Permeability from Porosity Using Artificial Neural Networks

Abstract: Permeability values in a borehole are predicted by an artificial neural network from the porosity values at the same depths. The network used in this study employs an architecture called backpropagation that is good at making predictions. The traditional approach for permeability prediction is regression analysis. In regression analysis, the relationship between porosity and permeability is assumed to be known. In reality, the functional form of this relationship, i.e., the model equation, is unknown. In contrast, the neural-network approach assumes no functional relationship. Six wells from Big Escambia Creek (Jurassic Smackover carbonate) field in southern Alabama were used to test predicting permeability from porosity using a neural network. Porosity and spatial data alone were used to predict permeability because these data are readily available from any hydrocarbon field. Three scenarios were performed; in each one, a subset of the six wells was used for a training set, one well for calibration, and one or two wells were used for prediction. For each scenario, simple linear regression was also used to predict permeability from porosity. The neural net predicted permeability much better than did regression in one scenario; in the other two scenarios the two methods performed equally well. The neural net predicted permeability accurately using minimal da a, but other kinds of information (e.g., log- or core-derived lithologic information) are easily incorporated if available. In addition, compartmentalization of carbonate reservoirs may be recognizable by this approach.

Numerical and Analytical Study to Estimate the Effect of Two Length Scales upon the Permeability of a Fibrous Porous Medium

Abstract: A fibrous porous medium with two length scales is modeled as a bed of porous cylinders aligned perpendicular to the flow of viscous fluid. The flow behavior is described using Stokes and Darcy flow equations in the regions around (higher length scale) and within the cylinders (lower length scale) respectively. The typical ratio of higher and lower length-scale regions enable us to invoke lubrication approximation and simplify the equations to develop a closed form solution for the overall permeability of this dual-scale porous medium. A parametric analysis is performed to explore the dependence of permeability on factors such as the volumetric ratio of higher and lower length-scale regions, permeability and size of inclusions in the smaller length-scale region. The analytical model is compared with the numerical results and the trend is compared with the experiments.

A Physical Model of Cementation and Its Effects on Single-Phase Permeability

Abstract: In this paper, we present a mathematical model to quantify the effects of cements on the single-phase permeability estimate of clastic rocks. The model represents pore-filling, pore-lining, and pore-bridging cements by treating the cement particles as rectangular crystals attached radially to detrital grains. These clusters influence permeability through an increase in effective specific surface area and tortuosity. Because the model is based on physical principles, it is largely devoid of empiricism and should be applicable to a wide range of rock types. Permeability estimates by our model match experimental data from two sets of sandstone samples of five hydrocarbon-bearing formations in the North Sea. The model also confirms the well-known result that the effect of pore-bridging cement is a factor of ten greater in reducing the single-phase permeability compared to pore-lining and pore-filling cements.

Permeability and Compressibility Behavior of Bentonite-Sand/Soil Mixes

Abstract: As a seepage barrier slurry trench material should have a relatively low coefficient of permeability, in the range of 10(-7) cm/s, and at the same time should be compatible with surrounding material with regard to compressibility. Although bentonite-sand/soil mixes are used widely, there is no specific engineering approach to proportion these mixes that satisfies the above practical requirements. In this paper, a generalized approach is presented for predicting the permeability and compressibility characteristics of mixes with minimum input parameters. This approach will be helpful in proportioning mixes and predicting corresponding changes in engineering behavior. It is possible to proportion a mix to arrive at the required compressibility without affecting the permeability. This is explained using an illustrative example.