Showing papers on "Permeability (earth sciences) published in 1998"

How Permeability Depends on Stress and Pore Pressure in Coalbeds: A New Model

Abstract: In naturally fractured formations, such as coal, permeability is sensitive to changes in stress or pore pressure (i.e., effective stress). This paper presents a new theoretical model for calculating pore volume compressibility and permeability in coals as a function of effective stress and matrix shrinkage, using a single equation. The equation is appropriate for uniaxial strain conditions, as expected in a reservoir. The model predicts how permeability changes as pressure is decreased (i.e., drawdown). Pore volume compressibility is derived in this theory from fundamental reservoir parameters. It is not constant, as often assumed. Pore volume compressibility is high in coals because porosity is so small. A rebound in permeability can occur at lower drawdown pressures for the highest modulus and matrix shrinkage values. We have also history matched rates from a {open_quotes}boomer{close_quotes} well in the fairway of the San Juan basin using various stress-dependent permeability functions. The best fit stress-permeability function is then compared with the new theory.

Compaction‐driven evolution of porosity and permeability in natural mudstones: An experimental study

Abstract: This paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones. Two intact samples of London Clay with clay fractions of 40% and 67% were compacted to 33 MPa effective stress. Clay fraction, permeability, porosity, pore size distribution, and specific surface area were measured and their evolution was monitored throughout the compaction process. Electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress. In both cases, porosity loss occurred primarily by the collapse of large pores. This process is more obvious in the coarser-grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius. Consistent with the pore size distributions, the permeability of the coarser sample ranges from ∼ 10−10 m s−1 to 10−12 m s−1 while that of the finer-grained sample ranges from ∼4 × 10−12 m s−1 to 5 × 10−14 m s−1 during progressive compaction from 2 to 33 MPa. The compressibility of the finer-grained sample is greater than that of the coarser-grained sample (0.15 as opposed to 0.07). However, in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales. The demonstration that both porosity and lithology (clay fraction) influence the permeability of mudstones should allow the development of more realistic porosity-permeability relationships which take into account lithological variations exhibited by mudstones.

Permeability and volume changes in till due to cyclic freeze/thaw

Abstract: A fine-grained nonplastic till was compacted in the laboratory in three types of rigid wall permeameters, having a volume of 0.4, 1.5, and 25 dm3, respectively, and, was thereafter exposed to a max...

Permeability of three-dimensional fracture networks

Abstract: The permeability of a three-dimensional network of polygonal fractures is determined by triangulating the network and solving the two-dimensional Darcy equation in each fracture. The general triangulation methodology and the numerical solution are presented. Networks of regular hexagonal fractures are detailed; finite-size scaling is used to analyze the data relative to the percolation threshold, but the conduction exponent $t$ is found close to its classical value in three dimensions; for large fracture densities, permeability is shown to tend towards the mean-field model of Snow [Water Resour. Res. 5, 1273 (1969)]. Finally, the influence of the shape of the fracture is studied and can be rationalized by means of the excluded volume.

Network modeling of non-Darcy flow through porous media

Abstract: Darcy's law is inadequate for describing high-velocity gas flow in porous media, which occurs in the near well-bore region of high capacity gas and condensate reservoirs. This study is directed at understanding the non-Darcy flow behavior. A pore-level network model has been developed to describe high velocity flow. The inputs to the model are pore size distributions and network coordination numbers. The outputs are permeability, non-Darcy coefficient, tortuousity and porosity. The additional pressure gradient term is found to be proportional to the square of the velocity in accordance with the Forchheimer's equation. The correlation between the non-Darcy coefficient and other flow properties (the permeability, the porosity and the tortuousity) is found to depend on the morphological parameters being changed. General correlations are derived between these flow properties.

Development and verification of a coefficient of permeability function for a deformable unsaturated soil

Abstract: The modelling of flow through saturated/unsaturated soils has become routine in geotechnical and geo-environmental engineering. The analysis requires that the coefficient of permeability for an uns...

Temperature controlled porosity/permeability reduction, fluid migration, and petroleum exploration in sedimentary basins

Abstract: Findings mainly from the Norwegian Continental Shelf indicate that at temperatures greater than approximately 60°C, internally sourced quartz cementation and diagenetic clay become important porosity and permeability reduction factors, respectively. Porosity loss due to quartz cementation in sandstones and siltstones proceeds mainly independent of effective stress or fluid pressure. Porosity loss rates approach those required to generate high fluid overpressures at approximately 120°C, which can result in hydrofracturing of the overlying low permeability shales. In thick low permeable sediment sequences, the probability of km scale vertical fluid migration increases. These thermally driven processes are capable of generating and sustaining fluid overpressure and facilitate fluid migration for up to tens of millions of years. Unlike mechanical compaction models, this model predicts that porosity loss and therefore fluid migration will continue despite the buildup of overpressure, even during periods of no sedimentation. In sealed compartments, fluid flow from more deeply buried high permeable lithologies will occur by hydrofracturing of overlying low permeable lithologies, preferentially along near vertical faults if present. When hydrofractures are induced from high permeable sediments, they will propagate vertically through overlying low permeable sediments, unless they enter lithologies with sufficient permeability and volume to bleed off the fracture fluid propagation pressure. Quantitative analysis of the mineral reactions causing porosity loss cannot only identify sediments in thermal zones which are expelling fluids, but also lithologies in thermal zones which are likely to receive those fluids, in addition to the timing and rates of fluid migration. The model has important applications for evaluating petroleum exploration risks and the potential for remigrated hydrocarbon plays both at basin scale and prospect level.

The relation among porosity, permeability, and specific surface of chalk from the Gorm field, Danish North Sea

Abstract: The origin of the difference in the relationship between permeability and porosity for Danian and Maastrichtian chalk from the Gorm field offshore Denmark has been investigated. The investigation was based on 300 sets of core data (He-expansion porosity and air permeability) from Well Gorm N-22X. On 24 of the core plugs, the specific surface was determined by BET and, on 14 of these samples, image analysis was made. The data were rationalized by the use of the Kozeny equation and it was found that each geologic unit had a characteristic relationship among porosity, permeability, and specific surface. Furthermore, it was found that the nature of porosity (intrafossil, intergranular, etc.) had no significant influence on the air permeability, so that the permeability of the chalk can be calculated from total porosity and specific surface. Kozeny’s empirical constant, c, was determined analytically from a simple porosity model and Poiseuille’s law.

Investigation of water permeability of coarse graded superpave pavements

Abstract: It is well recognized that the air void content of an asphalt mixture is an important factor that affects the performance of the pavement throughout its service life. High air voids in a finished pavement, particularly if the voids are interconnected, will adversely affect its durability and performance in various ways. Air filtration into a permeable pavement accelerates the aging or hardening process of the asphalt binder from the aggregate surface, leading to potential pavement distresses. This paper deals primarily with air voids and their influence on water permeability of asphalt pavements. Water permeability is an important yet often overlooked pavement property. The permeability of a pavement is generally assumed to be proportional to its air void content. However, the lack of void interconnection and size dimensions of the individual voids may result in a watertight pavement of relatively high void content. To date there has not been an accepted standardized testing procedure to determine the potential for water permeability of compacted asphalt mixtures. Thus, there is a need for a simple and effective approach to evaluate this important property of the pavement. The present study presents the results of an investigation of the water permeability of coarse graded Superpave mixes. The data indicates the importance of proper compaction during placement operations. This paper also describes a simple but effective laboratory method, developed during the course of this study, for measuring water permeability.

Gas Permeability in Undisturbed Soils: Measurements and Predictive Models

Abstract: Accurate prediction of changes in the gas permeability during variable soil-moisture conditions is a prerequisite for improved simulation and design of soil-venting systems for removal of volatile organic chemicals in polluted soils. Air permeability, k, as a function of soil air-filled porosity,

Network Model of Flow, Transport and Biofilm Effects in Porous Media

Abstract: In this paper, we develop a network model to determine porosity and permeability changes in a porous medium as a result of changes in the amount of biomass. The biomass is in the form of biofilms. Biofilms form when certain types of bacteria reproduce, bond to surfaces, and produce extracellular polymer (EPS) filaments that link together the bacteria. The pore spaces are modeled as a system of interconnected pipes in two and three dimensions. The radii of the pipes are given by a lognormal probability distribution. Volumetric flow rates through each of the pipes, and through the medium, are determined by solving a linear system of equations, with a symmetric and positive definite matrix. Transport through the medium is modeled by upwind, explicit finite difference approximations in the individual pipes. Methods for handling the boundary conditions between pipes and for visualizing the results of numerical simulations are developed. Increases in biomass, as a result of transport and reaction, decrease the pipe radii, which decreases the permeability of the medium. Relationships between biomass accumulation and permeability and porosity reduction are presented.

Precipitation sealing and diagenesis: 1. Experimental results

Abstract: During burial and diagenesis of granular aggregates, significant permeability reduction may be induced by the formation of low-temperature, authigenic minerals. To quantitatively assess the importance of this process, we have conducted a series of hydrothermal flow-through experiments using deionized water and labradorite/quartz sand. All experiments were conducted in a modified triaxial apparatus, configured to allow continuous permeability measurements. Under most of the conditions tested, significant permeability reduction is observed with no concurrent decrease in porosity. The overall permeability reduction sometimes exceeds 1 order of magnitude over 4 days and is positively correlated to temperature and deviatoric stress. Scanning electron microscope observations together with data from additional experiments show that the observed permeability reduction is entirely a result of secondary mineral growth. Si and Al concentrations in the postexperiment fluids are also correlated to temperature and stress, confirming the link between the chemical state of the system and permeability behavior. In all experiments, permeability reduction is fastest early and levels off in the late stages. To explain the permeability behavior as a function of time, a conceptual model is developed in which precipitation of authigenic minerals is rapid at early times while dissolution of quartz and labradorite is most active. As the system approaches equilibrium, the components necessary for secondary mineral formation are liberated at a lower rate, thereby causing precipitation to slow. Although authigenic mineral formation does not reduce total pore space in these experiments, there is a reduction in effective porosity, which results in permeability reduction.

Modeling metamorphic fluid flow with reaction-compaction permeability feedbacks

Abstract: Existing models of metasomatic flow do not allow for the effect that reaction has on the flow patterns. Instead, it is assumed that the volatiles produced are negligible in volume compared to those infiltrated and that reaction does not modify permeability. This is clearly unlikely to be true for infiltration-driven decarbonation reactions. The rates of porosity creation by reaction and porosity loss by creep have been calculated for a representative volume of calcite –quartz-wollastonite marble, and it is found that, even for a weak calcite matrix, the rate of porosity generation by reaction is likely to outstrip the collapse of porosity, as long as the system is out of equilibrium. We have applied a self-consistent 1D finite-difference model to the reaction of calcite + quartz to wollastonite in a 10m thick marble, in response to influx of H2O rich fluid, with fixed boundary conditions. The model allows us to evaluate the effect of reaction on the porosity structure and fluid pressure variation across the layer, from which local Darcy fluxes can be evaluated. The progress of reaction that we model is constrained by hydrological considerations, with the requisite parameters recalculated as reaction progresses, assuming creep compaction of rock under the stress difference between lithostatic and fluid pressures. We fnd that the volume of fluid realised by decarbonation, driven by influx of H20, is sufficient to create a back-flow, so that further advancement of the reaction front is only possible as a result of diffusion of water against the Darcy flux. The effect of creep driven by differences between fluid pressure and lithostatic pressure is to reduce the permeability of the layer and especially reduce the secondary porosity developed in the zone at and behind the advancing reaction front. We predict that in a 3D situation, the porous zone of reacted marble becomes a conduit for layer-parallel flow, and the secondary porosity is infilled by calc-silicate minerals due to silica metasomatism.

Method for determining distribution of reservoir permeability, porosity and pseudo relative permeability

Abstract: A method and apparatus for predicting distribution of reservoir permeability or porosity and pseudo relative permeability using well data (80) and 3-D seismic data (80) are disclosed, where a preferred first step (82) in the method models the reservoir property of interest to provide model logs (86) which are expressible as continuous curves of a reservoir property as a function of depth. The next step (88) selects individual data points on the continuous curves traversing a reservoir layer and determines permeable trends (96) for a multitude of column-like subvolumes which each enclose a portion of the selected data points. The subvolumes are then divided (100) into a desired number of horizontal slices, with each slice containing a number of the model data points.

Fluid permeability of deformable fracture networks

Abstract: We consider the problem of defining the fracture permeability tensor for each grid block in a rock mass from maps of natural fractures. For this purpose we implement a statistical model of cracked rock developed by M. Oda, where the permeability tensor is related to the crack geometry via a volume average of the contribution from each crack in the population. In this model, tectonic stress is implicitly coupled to fluid flow through an assumed relationship between crack aperture and normal stress across the crack. We have included three enhancements to the basic model. (1) A realistic model of crack closure under stress has been added along with the provision to apply tectonic stresses to the fracture system in any orientation. The application of compressive stress results in fracture closure, and consequently, a reduction in permeability. (2) The fracture permeability can be linearly superimposed onto an arbitrary anisotropic matrix permeability. (3) The fracture surfaces are allowed to slide under the application of shear stress, causing fractures to dilate and result in a permeability increase. Through two examples we demonstrate that significant changes in permeability magnitudes and orientations are possible when tectonic stress is applied to fracture systems.

A physical model for porosity reduction in sandstones

Abstract: The experimental elastic moduli‐porosity trends for clean sandstones can be described by the modified upper Hashin‐Shtrikman (MUHS) bound. One geometrical (but not necessarily geological) realization is: as porosity decreases, the number of the pores stays the same and each pore shrinks while maintaining its shape. This concept of uniform porosity reduction implies that permeability is proportional to the effective porosity squared, and that formation factor is proportional to the inverse of the effective porosity. The effective porosity here refers to the part of the pore‐space that dominates fluid flow. The proposed relations for permeability and formation factor agree well with the experimentally observed values. These laws are different from the often used forms of the Kozeny‐Carman equation and Archie’s law, where permeability is proportional to the total porosity cubed and formation factor is proportional to the inverse of the total porosity squared, respectively. We suggest that the uniform porosit...

Structural stability for the Darcy equations of flow in porous media

Abstract: A priori bounds are derived for the Darcy equations of flow in porous media when the porous body is subject to boundary conditions of Newton cooling type. With the aid of these a priori bounds we are able to demonstrate continuous dependence on the cooling coefficient when the boundary condition of Newton cooling type is employed. We further show that the solution depends continuously on a change in the equation of state employed in the body force in the Darcy equation. The model is allowed to change from one of Boussinesq convection type to one more general, and structural stability is established.

Permeability Damage Due to Asphaltene Deposition : Experimental and Modeling Aspects

Abstract: The flow properties of several asphaltenic crudes were studied at reservoir temperature in rocks of different morphology and mineralogy. The experiments performed showed a progressive reduction in permeability to oil during injection, varying in rate according to the system considered. The existence of organic deposits was verified by Rock-Evalpyrolysis measurements made on sections of samples taken at the end of flow at different distances from the entry face. This technique enables the profile of the deposits to be quantified. The interpretation of the permeability damage experiments and their simulation are treated by comparing the asphaltenes in oil to colloidal particles in suspension, capable of being deposited at the surface of the pores and thus reducing the permeability of the porous medium. The first simulations were carried out using the PARISIFP particle damage model, which has recently been extended to the case of multi-layer deposition. A satisfactory qualitative agreement is observed with the experimental results.

Permeability porosity relationships from numerical simulations of fluid flow

Abstract: Numerical calculations of permeability are obtained from a lattice Boltzmann simulation of flow in simplified 2D porous media over a range of solid fractions. The evolution equation governing the flow behaviour incorporates the effect of porous medium geometry through the definition of solid density ns, a real number, at each node of the simulation grid. The results obtained for homogeneous media are compared to commonly used theoretical and empirical relationships relating rock properties to permeability. Behaviour consistent with a Kozeny-Carman type relationship between porosity ϕ and permeability k is obtained for low to intermediate solid fractions. At high solid fractions the rapid decrease in k is consistent with a percolation process giving a power-law relationship for ϕ and k. Both the critical porosity and power-law exponent are in agreement with quoted values for the lattice geometry used. A comparison of the results for homogeneous media with k values, obtained by embedding a spanning planar fracture into the matrix, illustrates the importance of matrix-fracture flow interactions. The results for this case are consistent with experimental observations and illustrate the difficulties involved in using simplified assumptions to predict permeability from porosity in fractured porous rock.