Showing papers on "Permeability (earth sciences) published in 1999"

Permeability of shaly sands

Abstract: The permeability of a sand shale mixture is analyzed as a function of shale fraction and the permeability of the two end-members, i.e., the permeability of a clay-free sand and the permeability of a pure shale. First, we develop a model for the permeability of a clay-free sand as a function of the grain diameter, the porosity, and the electrical cementation exponent. We show that the Kozeny-Carman-type relation can be improved by using electrical parameters which separate pore throat from total porosity and effective from total hydraulic radius. The permeability of a pure shale is derived in a similar way but is strongly dependent on clay mineralogy. For the same porosity, there are 5 orders of magnitude of difference between the permeability of pure kaolinite and the permeability of pure smectite. The separate end-members' permeability models are combined by filling the sand pores progressively with shale and then dispersing the sand grains in shale. The permeability of sand shale mixtures is shown to have a minimum at the critical shale content at which shale just fills the sand pores. Pure shale has a slightly higher permeability. Permeability decreases sharply with shale content as the pores of a sand are filled. The permeability of sand shale mixtures thus has a very strong dependence on shale fraction, and available data confirm this distinctive shale-fraction dependence. In addition, there is agreement (within 1 order of magnitude) between the permeabilities predicted from our model and those measured over 11 orders of magnitude from literature sources. Finally, we apply our model to predict the permeabilities of shaly sand formations in the Gulf Coast. The predictions are compared to a data set of permeability determination made on side-wall cores. The agreement between the theoretical predictions and the experimental data is very good.

Permeability prediction based on fractal pore‐space geometry

Abstract: Estimating permeability from grain-size distributions or from well logs is attractive but difficult. In this paper we present a new, generally applicable, and relatively inexpensive approach which yields permeability information on the scale of core samples and boreholes. The approach is theoretically based on a fractal model for the internal structure of a porous medium. It yields a general and petrophysically justified relation linking porosity to permeability, which may be calculated either from porosity or from the pore-radius distribution. This general relation can be tuned to the entire spectrum of sandstones, ranging from clean to shaly. The resulting expressions for the different rock types are calibrated to a comprehensive data set of petrophysical and petrographical rock properties measured on 640 sandstone core samples of the Rotliegend Series (Lower Permian) in northeastern Germany. With few modifications, this new straight-forward and petrophysically motivated approach can also be applied to metamorphic and igneous rocks. Permeability calculated with this procedure from industry porosity logs compares very well with permeability measured on sedimentary and metamorphic rock samples.

Permeability of cracked concrete

Abstract: The goal of the research presented here was to study the relationship between cracking and water permeability. A feedback-controlled test was used to generate width-controlled cracks. Water permeability was evaluated by a low-pressure water permeability test. The factors chosen for the experimental design were material type (paste, mortar, normal and high strength concrete), thickness of the sample and average width of the induced cracks (ranging from 50 to 350 micrometers). The water permeability test results indicated that the relationships between permeability and material type differ for uncracked and cracked material, and that there was little thickness effect. Permeability of uncracked material decreased from paste, mortar, normal strength concrete (NSC) to high strength concrete (HSC). Water permeability of cracked material significantly increased with increasing crack width. For cracks above 100 microns, NSC showed the highest permeability coefficient, where as mortar showed the lowest one.

Effect of Cracking on Water and Chloride Permeability of Concrete

Abstract: The goal of this research was to study the relationship between cracking and concrete permeability and to support accounting for permeability and cracking resistance to other factors besides strength, as criteria to be considered in mix design to achieve a durable concrete. The effect of material composition [normal-strength concrete (NSC) and high-strength concrete (HSC) with two different mix designs] and crack width (ranging from 50 to 400 μm) on water and chloride permeability were examined. Cracks of designed widths were induced in the concrete specimens using a feedback-controlled splitting tensile test. Chloride permeability of the cracked samples was evaluated using a rapid chloride permeability test and the water permeability of cracked concrete was then evaluated by a low-pressure water permeability test. Uncracked HSC was less water permeable than NSC, as expected, but cracking changed the material behavior in terms of permeability. Both NSC and HSC were affected by cracking, and the water perm...

Gas permeability of concrete in relation to its degree of saturation

Abstract: Permeability, along with diffusion and absorption, is used to quantify durability characteristics of a concrete. The measured value of gas permeability of concrete depends strongly on its degree of saturation. Moreover, when the size of pores is of the same order of magnitude as the mean free path of molecules of the percolating gas, there is some molecular flow which violates the assumptions of the Darcy's law. As a result, the coefficient of permeability varies with the applied pressure. In an attempt to take these effects into account, we have tried to quantify different types of flow and we propose a method to calculate the apparent coefficient of permeability and hence the gas flow through concrete having any given degree of saturation and being under a given pressure difference across its extremities. For this purpose, we are characterizing concrete with an intrinsic permeability value. The variation of this intrinsic permeability and that of the contribution of non-viscous flow is studied, for a single concrete mix design in relation to the degree of saturation using a constant head permeametre named as CEMBUREAU and oxygen as the percolating gas.

Permeability and fluid flow in natural mudstones

Abstract: Abstract Mudstone permeabilities vary by ten orders of magnitude and by three orders of magnitude at a single porosity. Much of the range at a given porosity can be explained by differences in grain size; at a given effective stress, coarser-grained mudstones are more permeable than finer-grained mudstones, although the difference diminishes with increased burial. Pore size distributions illustrate why more silt-rich mudstones are more permeable than finer mudstones and also show that the loss of porosity and permeability with increasing effective stress is driven primarily by the preferential collapse of large pores. Pore size distributions can also be used to estimate permeability rapidly. None of the existing models are ideal and need to be adjusted and validated through the acquisition of a much larger permeability database of well-characterized mudstones. We also examine the role of faults and fractures as fluid conduits in mudstones. The occurrence of microscopic hydrofractures is inferred from the observation that fluid pressures in sedimentary basins rarely exceed minimum leak-off pressures. The extent to which microfractures enhance mudstone permeability, both instantaneously and over longer periods of geological time, is poorly constrained. Although fault zones in mudstones have generally low permeability, there is abundant evidence for episodic flow along faults in tectonically active regions. The role of faults as fluid conduits during periods of tectonic quiescence is less certain, and the timing and extent of any enhanced permeability and enhanced flow are not well known. In general, conditions conducive to fluid flow along muddy faults include an increase in the activity of the fault, high fluid pressures within the fault zone and the extent of overconsolidation and lithification of the mudstones.

Large-scale in situ permeability tensor of rocks from induced microseismicity

Abstract: Summary We propose an approach for estimating the permeability tensor using seismic emission induced by borehole hydraulic tests or by a fluid injection of an arbitrary nature. This approach provides a single estimation of the permeability tensor for the complete heterogeneous rock volume where the seismic emission was recorded. The approach is an extension of the method proposed by Shapiro et al. (1997) for the isotropic case. It is based on the hypothesis that the triggering front of the hydraulic-induced microseismicity propagates like a low-frequency second-type compressional Biot wave (corresponding to the process of pore-pressure relaxation) in an effective homogeneous anisotropic poroelastic fluid-saturated medium. The permeability tensor of this effective medium is the permeability tensor of the heterogeneous rock volume upscaled to the characteristic size of the seismically active region. We demonstrate the method using the microseismic data collected during the Hot Dry Rock Soultz-sous-Forets experiment (Dyer et al. 1994). These data show that the corresponding rock volume is characterized by a significant permeability anisotropy caused by oriented crack systems. The maximal principal component of the permeability tensor has a subvertical orientation. It is about seven times larger than the minimal subhorizontal principal component.

Primary Migration by Oil-Generation Microfracturing in Low-Permeability Source Rocks: Application to the Austin Chalk, Texas

Abstract: Fracturing of low-permeability source rocks is induced by pore-pressure changes caused by the conversion of organic matter to less dense fluids (oil and gas); these fractures increase the permeability and provide pathways for hydrocarbon migration. An equation for the pressure change is derived using four major assumptions. (1) The permeability of the source rock is negligibly small (0.01 µd; 10-20 m2) so that the pore-pressure buildup by the conversion is much faster than its dissipation by pore-fluid flow. (2) The stress state is isotropic so that horizontal and vertical stresses are equal. The source rock fails when the pore pressure equals the overburden pressure. (3) The properties of the rock, organic matter, and fluids remain constant during oil generation. This assumption is valid when the change in depth (i.e., pressure and temperature) is small. (4) Only two reaction rates are required for the conversions, a low-temperature reaction rate for the kerogen/oil conversion (E approx. = 24 kcal/mol, A approx. = 1014/m.y.) and a high-temperature reaction rate for oil/gas conversion (E approx. = 52 kcal/mol, A approx. = 5.5 ´ 1026/m.y.). The equations for generation rate and pressure change are applied to the Austin source rock by adjusting the several variables to fit geochemical data, core saturations, and observed levels of oil and gas production. This application demonstrates that the equations are easily applied in calculating depths of primary migration for low-permeability source rocks.

Radial flow permeability measurement. Part A: Theory

Abstract: This paper deals with permeability measurement in the context of Resin Transfer Moulding (RTM). A new approach to two-dimensional radial flow permeability measurement with constant inlet pressure is proposed. It allows principal permeability to be measured even if the experimental axes are not aligned with the principal direction. This part of the paper looks at the underlying theory of the new approach while part B reports on validation experiments. Formulae are derived which allow to calculate principal permeability and the orientation of the principal axes from flow front measurements in three directions. Transient effects of the developing flow front caused by the circular inlet are discussed and its influence on the measured permeability is illustrated. Numerical studies are performed which show that the shape of the flow front is dependent only on the size of the inlet diameter and the degree of anisotropy. This leads to the development of a formula for estimating the minimum required mould size for permeability measurement.

A mechanistic model for water seepage through thick unsaturated zones in fractured rocks of low matrix permeability

Abstract: The theme developed in this paper is that water seepage in thick unsaturated zones in fractured rocks of low matrix permeability cannot be understood by considering spatial and temporal averages We suggest that most such seepage proceeds in an unsteady, episodic manner in localized preferential pathways Mechanisms are proposed and explored which permit water to flow relatively freely in networks of interconnected fractures in the presence of strong suction pressures from an unsaturated rock matrix Imbibition into the rock matrix is reduced by limited wetted area, the episodic nature of flow, and possibly also by the presence of mineral coatings of low permeability on the fracture walls These effects and conditions combine to limit total imbibition rates to values less than saturated matrix hydraulic conductivity, leaving the rock matrix in partially saturated conditions even as water is flowing freely in portions of the fracture network The proposed conceptual model is elaborated and substantiated with analytical estimates and numerical simulation studies Mechanisms are demonstrated that can funnel distributed seepage into spatially localized preferential paths Subsequent horizontal broadening from dispersive mechanisms is found to be a slow process for fractures of high intrinsic permeability The presence of an unsaturated rock matrix provides constraints on fracture-matrix interface areas for imbibition and on the frequency of episodic deep seepage events Further studies are needed to test the proposed model

Linear stability of fluid flow down a porous inclined plane

Abstract: We investigate the linear stability of a fluid flowing down an inclined permeable plane by studying the evolution in time of infinitesimal disturbances of long wavelength. We assume that the flow through the porous medium is governed by Darcy's law, and determine the critical conditions for the onset of instability in the case when the characteristic length scale of the pore space is much smaller than the depth of the fluid layer above. The results reveal that increasing the permeability of the inclined plane destabilizes the flow of the fluid layer flowing above.

A Multiscale Network Model for Simulating Moisture Transfer Properties of Porous Media

Abstract: A multiscale network model is presented to model unsaturated moisture transfer in hygroscopic capillary-porous materials showing a broad pore-size distribution. Both capillary effects and water sorption phenomena, water vapour and liquid water transfer are considered. The multiscale approach is based on the concept of examining the porous space at different levels of magnification. The conservation of the water vapour permeability of dry material is used as scaling criterion to link the different pore scales. A macroscopic permeability is deduced from the permeabilities calculated at the different levels of magnification. Each level of magnification is modelled using an isotropic nonplanar 2D cross-squared network. The multiscale network simulates the enhancement of water vapour permeability due to capillary condensation, the hysteresis phenomenon between wetting and drying, and the steep increase of moisture permeability at the critical moisture saturation level. The calculated network permeabilities are compared with experimental data for calcium silicate and ceramic brick and a good agreement is observed.

Stress-dependent permeability: Characterization and modeling

Abstract: During the production lifecycle of a reservoir, absolute permeability at any given location may change in response to an increase in the net effective stress and a concomitant decrease in the value of in-situ permeability. This paper focuses stress dependent permeability in unconsolidated, high porosity sand reservoirs (offshore turbidites) and consolidated reservoirs (tight gas sands). Specifically we address: i) fundamental controls on stress dependent permeability, as identified through analysis of core samples, ii) rock-based log modeling of stress dependent permeability in cored and noncored wells, and iii) implications for production based on data from reservoir simulation. This work reveals a fundamental difference between the stress dependent permeability behavior of unconsolidated and consolidated sand reservoirs. In unconsolidated sand reservoirs, the greatest permeability reduction with stress occurs in the sands with the highest values of porosity and permeability. In cemented sandstone reservoirs, the opposite is the case: most of the reduction in permeability occurs in sandstones with the lowest values of porosity and permeability. This difference in behavior between unconsolidated and consolidated reservoir sands is controlled by pore geometry. We present a practical, rapid and cost efficient methodology to improve evaluation and enhance the productivity and management of stress-dependent reservoirs. The method is based fundamentally on the identification of Rock Types (intervals of rock with unique pore geometry). Thin section evaluation, together with integrated nuclear magnetic resonance and SEM-based image analysis of core material is used to quantitatively identify various Rock Types. Rock Type data is integrated with measurements of permeability at various levels of stress. Results demonstrate that, within a particular field, some Rock Types lose 90% of original permeability as a function of increasing stress. Rock Types are then identified using routine suites of wireline logs, allowing for field-wide determination of the net footage and distribution of each Rock Type in all wells and the foot-by-foot calculation of permeability at any value of net effective stress. Based on geological input, the reservoirs are divided into flow units (hydrodynamically continuous layers) and grid blocks for simulation. Several cases are presented of a conceptual, single well model of an overpressured, tight gas sandstone reservoir that include stress dependent permeability. Results of simulation analyses for varying conditions of reservoir stress demonstrate the importance of stress dependent permeability in more accurate forecasting of reserves and predicting optimum well bore producing conditions.

Seismic signatures of permeability in heterogeneous porous media

Abstract: In homogeneous poroelastic systems, the permeability tensor practically does not influence propagating seismic waves in the low frequency range (0–1000 Hz; see, e.g., Schmitt, 1989; Gelinsky and Shapiro, 1996). In this paper, we show that this situation changes in heterogeneous systems such as, layered or fractured sediments. Due to the heterogeneities of poroelastic structures, the attenuation of P-waves is influenced by the permeability in an enhanced way. We show, however, that such a “seismic permeability” can differ very strongly from the hydraulic permeability.

Network modeling of the evolution of permeability and dilatancy in compact rock

Abstract: Rock deformation and fluid transport are coupled together in many crustal settings. Nonhydrostatic stress can greatly affect pore structure and transport properties of a rock. When a compact rock is stressed to failure, dilatancy and permeability enhancement are generally observed whether the failure mode is brittle faulting or cataclastic flow. Laboratory data for the brittle faulting regime (in Westerly granite) and for the cataclastic flow regime (in Carrara marble and synthetic halite) are modeled. Dilatancy is simulated by incorporating an increasing number of stress-induced microcracks with similar geometric attributes in a random network model, thus enhancing permeability. The microcracks are represented by sheet-like conduits, with crack length and aspect ratio distributions constrained by microstructural data. Before the onset of dilatancy, a rock under overall compression has very low density of open cracks that occur in isolated clusters with relatively low connectivity. Nonhydrostatic loading induces damage in the form of extensile microcracks that gradually form a fully connected percolation network. Significant permeability increase of up to several orders of magnitude may occur in this percolative regime. Once the crack network is fully connected, the accumulation of additional cracks is not as effective in enhancing permeability, and our model predicts permeability and porosity changes to be linearly related in this fully connected regime. After accounting for the existence of fine cracks that are below the microscope resolution, our simulation results agree reasonably well with laboratory data.

Permeability anisotropy of consolidated clays

Abstract: Abstract Consolidation of clays tends to result in changes in particle orientation and pore size distribution as well as progressive reduction of porosity and permeability with increasing effective stress. Clay particles are expected to rotate normal to an axial load, thus decreasing flow path tortuosity parallel to the particle alignment direction and increasing tortuosity normal to the particle alignment. This results in the development of anisotropic permeability, such that the horizontal permeability of a consolidated sediment is greater than the vertical permeability at any given porosity. Within any uniform layer, levels of permeability anisotropy are modest. Typically, permeability anisotropy produced by consolidation of natural clays is in the range 1.1–3 and does not reach the high levels predicted by simple models of clay particle reorientation. The discrepancy arises from particle clustering and irregularities in particle packing. Although somewhat higher levels of anisotropy may exist as a consequence of lamination within individual beds, values > 10 that are known to exist on the formation scale are produced by strong contrasts between the permeabilities of interlayered beds. As argillaceous sediments have permeability ranges of many orders of magnitude, apparently subtle lithological layering in a shale unit may lead to a highly anisotropic flow behaviour.

Effects of groundwater flow on mineral diagenesis, with emphasis on carbonate aquifers

Abstract: This article provides a critical synopsis of the effects of groundwater flow on mineral diagenesis. Emphasis is placed on those aspects and processes that change porosity and permeability in carbonate aquifers, because they are of particular importance to human societies as sources of supplies of water for human consumption (drinking, irrigation) and of crude oil and natural gas. Diagenetic settings in carbonates as well as clastics are generally ill defined. This paper proposes a new comprehensive classification of diagenetic settings into near-surface, shallow-, intermediate-, and deep-burial diagenetic settings; hydrocarbon-contaminated plumes; and fractures. These settings are defined on the basis of mineralogy, petroleum, hydrogeochemistry, and hydrogeology. This classification is applicable to all sedimentary basins. Diagenesis is governed by various intrinsic and extrinsic factors that include thermodynamic and kinetic constraints, as well as microstructural factors that may override the others. These factors govern diagenetic processes, such as dissolution, compaction, recrystallization, replacement, and sulfate–hydrocarbon redox-reactions. Processes such as cementation, dissolution, and dolomitization require significant flow of groundwater driven by an externally imposed hydraulic gradient. Other processes, such as stylolitization and thermochemical sulfate reduction, commonly take place without significant groundwater flow in hydrologically nearly or completely stagnant systems that are geochemically "closed." Two major effects of groundwater flow on mineral diagenesis are enhancement and reduction of porosity and permeability, although groundwater flow can also leave these rock properties essentially unchanged. In extreme cases, an aquifer or hydrocarbon reservoir rock can have highly enhanced porosity and permeability due to extensive mineral dissolution, or it can be plugged up due to extensive mineral precipitation.

Determination of fluid transmissivity and electric transverse resistance for shallow aquifers and deep reservoirs from surface and well-log electric measurements

Abstract: . Fluid transmissivity (layer thickness times permeability) and electric transverse resistance (layer thickness time resistivity) are important parameter in groundwater and hydrocarbon exploration. Determination of these parameters provides a good knowledge of the potential of porous media, because they relate fluid flow to electric-current conduction, in terms of layer thickness, permeability and resistivity. In this study, both parameters were determined for shallow aquifers (Schleswig-Holstein, northern Germany) and deep reservoirs (Jeanne d'Arc Basin, offshore of eastern Canada), utilizing surface and well-log electric measurements. Direct relationships between both parameters, with coefficients of correlation of 0.99 (for the aquifers) and 0.94 (for the reservoirs), were obtained. The relationships suggest that an increase in both parameters indicate presence of zones of high fluid potential within the aquifers and the reservoirs.

An analysis of dynamic pseudo-relative permeability methods for oil-water flows

Abstract: The properties and limitations of six widely used dynamic pseudo-relative permeability methods are analysed for the case of incompressible, immiscible, two-phase flow. The example proposed by Stone is used to illustrate our findings. The analytical results are confirmed by numerical simulation.

An experimental study of the mechanical behaviour of a mortar and of its permeability under deviatoric loading

Abstract: This study, which presents the results of numerous tests on the sensitivity of mortar permeability under conventional triaxial loading, represents tests carried out over more than a year at various confining pressures under monotonic loading or with loading-unloading cycles. Permeability is measured on a sample, dried prior to the test, by injecting argon—a neutral gas. A description of the experimental method based on applying a permanent flow is given as well as the technique of test study used. In this work, we chose to measure permeability under loading at different levels of deviatoric stress and strain since this corresponds more closely to the real conditions under which the works are used. Preliminary tests showed that the permeability under load was susceptible to early appearance of microcracks and began to increase noticeably towards 75/80% of the stress peak. Further tests were then carried out with loading-unloading cycles for stress levels beyond the stress peak. The authors have shown that permeability inceases very rapidly, starting from this peak. This is irreversible as the measurements taken after unloading the sample prove. The different tests also prove that the damage level is insufficient to allow for a description of variation in permeability, as it clearly depends on the degree to which the microcracks have opened. Such a claim is supported by results where this variation is presented in relation to lateral strain.