Showing papers on "Permeability (earth sciences) published in 1997"

The transition from brittle faulting to cataclastic flow: Permeability evolution

Abstract: Triaxial compression experiments were conducted to investigate influences of stress and failure mode on axial permeability of five sandstones with porosities ranging from 15% to 35%. In the cataclastic flow regime, permeability and porosity changes closely track one another. A drastic decrease in permeability was triggered by the onset of shear-enhanced compaction caused by grain crushing and pore collapse. The compactive yield stress C* maps out a boundary in stress space separating two different types of permeability evolution. Before C* is attained, permeability and porosity both decrease with increasing effective mean stress, but they are independent of deviatoric stresses. However, with loading beyond C*, both permeability and porosity changes are strongly dependent on the deviatoric and effective mean stresses. In the brittle faulting regime, permeability and porosity changes are more complex. Before the onset of shear-induced dilation C', both permeability and porosity decrease with increasing effective mean stress. Beyond C', permeability may actually decrease in a dilating rock prior to brittle failure. After the peak stress has been attained, the development of a relatively impermeable shear band causes an accelerated decrease of permeability. Permeability evolution in porous sandstones is compared with that in low-porosity crystalline rocks. A conceptual model for the coupling of deformation and fluid transport is proposed in the form of a deformation-permeability map.

Permeability and effective porosity of porous media

Abstract: The concept of permeability of porous media is discussed, and a modification of Kozeny's permeability equation to include the effect of effective porosity is introduced. An analytical expression for the specific surface area of a system constructed of randomly placed identical obstacles with unrestricted overlap is derived, and a lattice-gas cellular automaton method is then used to simulate the dependence on porosity of permeability, tortuosity, and effective porosity for a flow of Newtonian uncompressible fluid in this two-dimensional porous substance. The simulated permeabilities can well be explained by the concept of effective porosity, and the exact form of the specific surface area. The critical exponent of the permeability near the percolation threshold is also determined from the simulations.

Influence of gas production induced volumetric strain on permeability of coal

Abstract: The gas permeability of a coalbed, unlike that of conventional gas reservoirs, is influenced during gas production not only by the simultaneous changes in effective stress and gas slippage, but also by the volumetric strain of the coal matrix that is associated with gas desorption. A technique for conducting laboratory experiments to separate these effects and estimate their individual contribution is presented in this paper. The results show that for a pressure decrease from 6.2 to 0.7 MPa, the total permeability of the coal sample increased by more than 17 times. A factor of 12 is due to the volumetric strain effect, and a factor of 5 due to the gas slippage effect. Changes in permeability and porosity with gas depletion were also estimated using the measured volumetric strain and the matchstick reservoir model geometry for flow of gas in coalbeds. The resulting variations were compared with results obtained experimentally. Furthermore, the results show that when gas pressure is above 1.7 MPa, the effect of volumetric strain due to matrix shrinkage dominates. As gas pressure falls below 1.7 MPa, both the gas slippage and matrix shrinkage effects play important roles in influencing the permeability. Finally, the change in permeability associated with matrix shrinkage was found to be linearly proportional to the volumetric strain. Since volumetric strain is linearly proportional to the amount of gas desorbed, the change in permeability is a linear function of the amount of desorbing gas.

General Three-Phase Relative Permeability Model for Prudhoe Bay

Abstract: This paper describes two- and three-phase relative permeability concepts important for Prudhoe Bay. It includes a three-phase relative permeability correlation that incorporates hysteresis in gas, oil, and water relative permeability as well as the dependence of relative permeability on composition and gas/oil interfacial tension (IFT). The functional forms chosen to correlate the relative permeability data were based on interpretation of the pore-level mechanisms that determine fluid flow. The three-phase correlation reduces to traditional models in various limits and is more consistent with available data and trends in the literature than previous correlations. Although this correlation was developed for Prudhoe Bay, it can be and has been applied to other mixed-wet reservoirs with changes in the input parameters. The correlation is particularly useful in situations where both compositional effects and hysteresis are important.

Heat and mass transfer in unsaturated soils during freezing

Abstract: This paper presents an approach for predicting heat and mass transfer in freezing unsaturated soil when frost heave does not occur. The theoretical formulation uses soil-freezing and soil-water characteristic curve data to combine the heat and mass transfer relationships into a single equation for freezing or frozen regions of the soil. Numerical simulations were compared with the results of freezing tests for a closed system of fine silica flour where large moisture redistribution occurred without frost heave. Accurate predictions of ice content and water content were obtained using a single permeability versus suction function with no impedance factor. The permeability function was obtained using a recently developed method for predicting permeability functions for unsaturated soils using the soil-water characteristic curve. (A)

A Simple Model for the Variation of Permeability due to Partial Saturation in Dual Scale Porous Media

Abstract: The main focus of this work is to model macroscopically the effects of partial saturation upon the permeability of dual scale fibrous media made of fiber bundles when a Newtonian viscous fluid impregnates it. A new phenomenological model is proposed to explain the discrepancies between experimental pressure results and analytical predictions based on Darcy's law. This model incorporates the essential features of relative permeability but without the necessity of measuring saturation of the liquid for its prediction. The model is very relevant for the small scale industrial systems where a liquid is forced to flow through a fibrous porous medium. It requires four parameters. Two of them are the two permeability values based on the two length scales. One length scale is of the order of magnitude of the individual fiber radius and corresponds to the permeability of the completely staurated medium, the other is of the order of magnitude of the distance between the fiber bundles and corresponds to the permeability of the partially saturated medium. The other two parameters are the lengths of the two partially saturated regions of the flow domain. The two lengths of the partially saturated region and the permeability of the fully saturated flow domain can be directly measured from the experiments. The excellent agreement between the model and the experimental results of inlet pressure profile with respect to time suggests that this model may be used to describe the variation of the permeability behind a moving front in such porous media for correct pressure prediction. It may also be used to characterize the fibrous medium by determining the two different permeabilities and the relative importance of the unsaturated portion of the flow domain for a given architecture.

Three-dimensional flow and reaction in porous media: Implications for the Earth's mantle and sedimentary basins

Abstract: We present a theoretical and numerical study of the reorganization of a porous matrix due to fluid flow coupled with dissolution or precipitation processes. We find that under certain conditions, flow of corrosive fluids results in unstable growth of the permeability and increasing disequilibrium in fluid chemistry with time. High-permeability channels may form parallel to the direction of flow. In time, these channels cause the distribution of porosity to become increasingly correlated and anisotropic and cause flow rates to be increasingly variable. Flow coupled with crystallization has the opposite effect: With time, permeability reduction occurs at a decreasing rate. Mineral composition in the fluid approaches chemical equilibrium. Precipitation destroys existing preferred paths for flow and acts to homogenize and disperse the flow. Connectivity of the porous media is reduced. Implications of these results for two geological systems are discussed: (1) Modes of melt extraction from the Earth's mantle, where the expected different modes of flow and reaction may help explain different geochemical and geological observations at hot spots and mid-ocean ridges, and (2) Precipitation and formation of abnormal pressure zones in sedimentary basins.

Ground‐penetrating‐radar‐assisted saturation and permeability estimation in bimodal systems

Abstract: Near-surface investigations often require characterization of vadose zone hydraulic parameters. Conventional sampling or borehole techniques for estimating these parameters are costly, time consuming, and invasive, all of which limit collection of hydrogeological data at a spacing needed for detailed site characterization. Incorporation of two- or three-dimensional densely sampled geophysical data with conventional hydrological data increases the amount of data available for the characterization and thus has the potential to significantly improve the hydraulic parameter estimates over those obtained from borehole data alone. The hydraulic estimation procedure can be greatly improved by incorporating dielectric information potentially available from ground penetrating radar (GPR), a noninvasive, high-resolution geophysical method. The procedures for collecting and processing GPR data in the format needed for the proposed estimation technique are relatively new and still a topic of research; our method requires as a starting point the ability to estimate dielectric constants from GPR data. Numerical experiments were performed to investigate the general utility of the GPR-assisted estimation technique under a range of conditions. Three bimodal systems were investigated, each system being composed of a sand facies together with another facies with a larger clay volume fraction; each facies was defined using characteristic values of clay content, porosity, and permeability. Using dielectric information and petrophysical relations, degree of saturation and intrinsic permeability values at each location within the three systems were identified. For bimodal systems, a dielectric constant measurement corresponds to two possible values of saturation and intrinsic permeability at each location; single values of saturation and intrinsic permeability were estimated from these values using the principle of maximum likelihood. Results from case studies demonstrate that a combination of GPR data with conventional borehole data significantly improves the estimates of saturation and has the potential to improve the estimates of permeability over those obtained from well bore data alone. The proposed method should be especially advantageous for vadose zone characterization in areas favorable for GPR data acquisition, where detailed hydraulic parameter information is required but the drilling of numerous boreholes is prohibited.

Prudhoe Bay Gas/Oil Relative Permeability

Abstract: Prudhoe Bay is a mixed-wet reservoir where about half the oil recovery is attributable to gravity drainage. Gas/oil relative permeability data show that gravity-drainage recovery efficiency is poorer for more fine-grained sandstone and increases as the grain size increases. Gravity-drainage efficiency also increases with connate-water saturation. Dependence of recovery efficiency on grain size is related to changes in sorting. An effective grain size, defined by inverting the Carman-Kozeny relation, provides a useful parameter for correlating recovery efficiency. This estimate correlates well with visual estimates and direct measurements on disaggregated core. Grain size is also found to be a more effective parameter for correlating trapped gas than porosity, a common alternative. Lithology impacts trapped-gas level with finer-grained, more poorly sorted rock having higher trapped gas. Trapped gas decreases with increasing microporosity. Because little gas is trapped in microporosity, a zero-slope generalization of the Land curve better represents trapped-gas data.

Analytical model for particle migration within base soil-filter system

Abstract: This study highlights a mathematical (analytical) model simulating the filtration phenomenon applicable to a base soil-filter system, incorporating the hydraulic conditions and the relevant material properties such as porosity, density, friction angle, and the shape and distribution of particles. The model is founded on the concept of critical hydraulic gradient derived from limit equilibrium considerations, where the migration of particles is assumed to occur under applied hydraulic gradients exceeding this critical value. The rate of particle erosion, and hence, the filter effectiveness is quantified on the basis of mass and momentum conservation theories. By dividing the base soil and filter domains into discrete elements, the model is capable of predicting the time-dependent particle gradation and permeability of each element, thereby the amount of material eroded from or retained within the system. Laboratory tests conducted on a fine base material verify the validity of the model. The model predictions are also compared with the available empirical recommendations, including the conventional grading ratios.

Laboratory method for investigating permeability upscaling

Abstract: The purpose of this work is to describe, evaluate, and demonstrate a laboratory-based method for physically investigating permeability upscaling The method makes use of a simple instrument, the gas permeameter, to acquire rapid, precise, and nondestructive permeability measurements from heterogeneous blocks of dry rock Critical to investigating permeability upscaling is the ability to acquire data at multiple sample supports subject to consistent boundary conditions and flow geometry Such measurements, spanning almost 4 orders of magnitude on a per volume basis, are made with the gas permeameter by simply varying the size of the permeameter tip seal The precision and consistency of measurements made in this way were evaluated using a suite of data collected from blocks of three relatively homogeneous materials: Berea Sandstone and two synthetic rocks Results suggest that measurement error is small (approximately ±1% of the measured permeability) and consistent, and measurements made at different sample supports are free from systematic bias To demonstrate the ability of this method to measure and quantify upscaling processes, limited data sets were collected with four different-sized tip seals from the Berea Sandstone block Analysis reveals distinct and consistent trends diagnostic of permeability upscaling relating the sample mean (increased), variance (decreased), and semivariogram to increasing sample support

A formulation of fully coupled thermal–hydraulic–mechanical behaviour of saturated porous media—numerical approach

Abstract: The theoretical aspects of fully coupled thermohydromechanical behaviour of saturated porous media are presented. The non-linear behaviour of soil skeleton is assumed. A new concept called ‘thermal void ratio state surface’ is introduced to include thermal effects, and the stress state level influence on volume changes. The fluid phase flows according to Darcy's law and energy transport is assumed to follow Fourier's law classically. Variation of water permeability, water and solid unit weight due to thermal effects and pore pressure changes are included. A finite element package is developed based on final matrix form obtained from discretization of integral form of field equations by finite element method and integration in time. A very good agreement between the theoretical predictions and the experimental results was obtained for the several simple problems proposed by other authors. © 1997 by John Wiley & Sons, Ltd.

Determining permeability of tight rock samples using inverse modeling

Abstract: Data from gas-pressure-pulse-decay experiments have been analyzed by means of numerical simulation in combination with automatic model calibration techniques to determine hydrologic properties of low-permeability, low-porosity rock samples. Porosity, permeability, and Klinkenberg slip factor have been estimated for a core plug from The Geysers geothermal field, California. The experiments were conducted using a specially designed permeameter with small gas reservoirs. Pressure changes were measured as gas flowed from the pressurized upstream reservoir through the sample to the downstream reservoir. A simultaneous inversion of data from three experiments performed on different pressure levels allows for independent estimation of absolute permeability and gas permeability which is pressure-dependent due to enhanced slip flow. With this measurement and analysis technique we can determine matrix properties with permeabilities as low as 10 221 m 2 . In this paper we discuss the procedure of parameter estimation by inverse modeling. We will focus on the error analysis, which reveals estimation uncertainty and parameter correlations. This information can also be used to evaluate and optimize the design of an experiment. The impact of systematic errors due to potential leakage and uncertainty in the initial conditions will also be addressed. The case studies clearly illustrate the need for a thorough error analysis of inverse modeling results.

Methods for the determination of water permeability of concrete

Abstract: Two test methods have been successfully used to determine the water permeability of different concretes. The methods are based on the determination of coefficient of permeability using either a constant flow or a depth of penetration technique. The flow method has generally been found to suit concretes with higher permeability, while the penetration method is used for concretes with very low permeability. Presently no clear guidelines exist for the selection of the appropriate method for a particular type of concrete. This study was carried out to examine the correlation between the two methods. A broad guideline has also been established for the selection of the appropriate method for a particular concrete with respect to its binder composition, 28-day compressive strength, and age. The concretes examined were prepared from five types of binders and with a grade range of 35-50 MPa.

Method for directing groundwater flow and treating groundwater in situ

Abstract: A method for treating groundwater in situ in rock or soil. An elongate permeable upgradient zone (2) and an elongate permeable downgradient zone, each in hydraulic communication with a permeable subsurface treatment zone (14) and having a major axis parallel to a non-zero component of the general flow direction (8), are provided in the subsurface by any of a number of construction methods. The upgradient zone, downgradient zone, and treatment zone are situated within the subsurface medium (4) and have permeabilities substantially greater than the adjacent subsurface medium's permeability.

Borehole Flowmeter Application in Fluvial Sediments: Methodology, Results, and Assessment

Abstract: In many situations, inadequate design or performance of ground-water remediation systems is the result of underestimation of aquifer hydraulic heterogeneity, and in particular, the vertical variation of hydraulic conductivity which plays an important role in contaminant migration. Described herein are applications of the electromagnetic (EM) borehole flowmeter to fluvial sediments in Louisiana and South Carolina. The direction of natural vertical flow in the test aquifers was defined easily, and short pumping tests enabled the calculation of hydraulic conductivity profiles for each test well. The results correlated well with other information obtained independently, including natural gamma logs, driller`s logs and a hydraulic conductivity profile based on grain size analysis. Large variations in hydraulic conductivity over short vertical and horizontal distances were documented. Tests in gravel-packed wells suggested that flowmeters produce misleading data for a variety of reasons in such situations. Among other things, an annulus of high permeability around a well screen allows flow to bypass the meter, and the phenomenon is amplified by high pumping rates. The resulting error is displayed as an erroneous high permeability zone at the top of the well screen. This observation deserves further study. In its present form the EM flowmeter is awkward to handle more » on a routine basis. However, none of the present design flaws preclude its effective use. « less

The Effects of Fluid Type and Viscosity on the Steady-State and Advancing Front Permeability Behavior of Textile Preforms

Abstract: The effects of fluid type and viscosity on the permeability of both saturated and dry preforms were investigated. Fluids used were water, corn oil, and Epon 815, an epoxy resin. Preforms tested included style 162 E-glass, a plain weave E-glass fabric, and IM7/8HS, an eight harness satin carbon fabric. Two methods were used to measure the permeability of the textile preforms. The first, known as the steady-state method, measures the permeability of a saturated preform under constant flow rate conditions. The second, denoted the advancing front method, measures the permeability of a dry preform to an advancing fluid. Results from the two methods showed that fluid viscosity had no significant influence on the permeabilities of the two fabrics. Steady-state and advancing front permeabilities for the warp direction of the two fabrics were similar. In addition, advancing front permeability values were found to be similar for different fluids over a wide range of values for the capillary number. Contact angle measurements indicated that Epon 815 wets both fibers better than the corn oil. In addition, E-glass has lower contact angles with both fluids.

Experimental assessment of gas transport mechanisms in natural porous media: Parameter evaluation

Abstract: An understanding of vapor transport in natural porous media is critical to the assessment of a wide range of environmental problems. In this work a comprehensive experimental program was undertaken to evaluate the relative importance of different gaseous transport mechanisms in natural porous media. The dusty gas model was used as a framework for this evaluation. The experimental program was divided into two parts: the first emphasizes the measurement of porous media transport parameters, and the second explores flux mechanisms in organic vapor transport. Results of the first part are presented in this paper. Single and binary gas experiments were conducted to obtain governing transport parameters (coefficients of permeability, Knudsen diffusion, and diffusibility) in dry porous media. To conduct these experiments, a special experimental apparatus was built that incorporates a diffusion cell that represents an open system where the pressure gradient and absolute pressures can be regulated by controlling the flow rates of the component gases at both ends of the soil sample. Soils tested included three uniform materials, a sea sand, an Ottawa sand, and kaolinite clay, and five graded mixtures of these uniform soils. Results of the single-gas experiments illustrate the importance of Knudsen diffusion in permeability measurements. Two new transport parameter correlations are developed from the presented data and compared with available literature correlations. The correlation for the Knudsen diffusion radius has a functional dependence upon the square root of intrinsic permeability, selected as a characteristic length of the porous medium. Binary diffusion experiments, using an equimolar pair of gases, are employed to develop a correlation for a composition independent diffusibility.

Response in Seabed of Finite Depth with Variable Permeability

Abstract: Conventional investigations of the wave-seabed interaction have assumed that the seabed has an uniform permeability. However, the soil permeability in most marine sediments varies with burial depth due to consolidation under overburden pressure. Here an analytical solution describing the short-crested wave-induced soil response in a seabed with variable permeability is derived. The soil matrix considered here is hydraulically anisotropic and of finite thickness. The wave-induced soil response is obtained from a set of governing equations incorporating a variable permeability. The results indicate that the soil response is affected significantly by variable permeability. The influences of soil characteristics on the relative differences in pore pressure between the solutions for variable and uniform permeability are detailed.

A transient pressure pulse method for the mesurement of permeability of a cement grout

Abstract: The paper discusses the application of a transient pressure pulse technique for the laboratory evaluation of the permeability characteristics of a cementitious grout material with a relatively low permeability. The experimentation involves the internal pressurization of a cylindrical grout specimen with a concentric circular cavity. The time-dependent decay of the fluid pressure within the cavity is used as the basis for the computation of the permeability of the grout. The paper presents details of the experimental methodologies and the mathematical procedures which are used to evaluate the permeability. The results of a series of radial flow pressure pulse tests are used to ascertain the permeability characteristics of a cementitious grout.

Reservoir Characteristics of Low-Permeability Sandstones in the Rocky Mountains

Abstract: Understanding gas production from low-permeability sandstones requires an understanding of the in situ porosity, brine saturation, and effective gas permeability at reservoir brine saturation. Analysis of data from hundreds of cores from numerous western U.S. basins indicates that petrophysical properties of well-defined lithofacies (or log-facies) are often unique within a narrow range. Diagenesis in these sandstones commonly resulted in the destruction of much of the original intergranular porosity and left dissolved grains, clay-filled pores, and sheet-like connecting intergranular pore throats. Pore throats or channels that connect larger pores typically range in size from 1 to 0.1 micron and represent only a small portion of the total porosity. In most low-permeability sandstones, porosity is not significantly changed by confining stress changes, but in situ effective gas permeabilities range from 10 to 1,000 times less than routine air permeability. The influence of confining stress on permeability can be attributed primarily to the decrease in size of the thin, tabular pore throats that connect the larger pores. Under stress, pore throats decrease in diameter by up to 50% to 70% resulting in permeability decreases of 10 to 40 times. Gas effective permeabilities also decrease rapidly to less than 1% of absolute values at water saturations above approximately 40% to 50%. "lrreducible" water saturations increase with decreasing porosity and permeability, and, in sandstones with less than 0.01 md permeability, "irreducible" water saturations increase dramatically. Cumulative flow and storage capacity plots indicate that very thin higher permeability intervals typically yield a large percentage of the cumulative flow capacity. Increased water saturations due to drilling or stimulation result in lower effective gas permeabilities and can unknowingly be stabilized by capillary pressure forces if pore pressures are decreased. This type of formation damage can be remedied by increasing the gas pore pressure to displace mobile water.