Showing papers on "Effective porosity published in 2000"

Apparatus and Procedures for Assessing Inorganic Diffusion Coefficients for Geosynthetic Clay Liners

Abstract: The apparatus and procedure for performing tests to obtain inorganic diffusion coefficients for Geosynthetic Clay Liners (GCLs) are described, and the processes (diffusion, anion exclusion and osmosis) that can affect the interpretation of these tests are discussed. Results from several inorganic diffusion tests on GCL and bentonite specimens show that the diffusion coefficients deduced for sodium (Na+) and chloride (Cl−) are directly related to the final bentonite void ratio. It is shown that diffusion through GCLs (or thin bentonite layers) can be modeled using a bulk porous media diffusion coefficient (Dp) consisting of the total porosity, nt, and a deduced diffusion coefficient, Dt, (Dp = ntDt) without the need to establish the true effective diffusion coefficient () without the need to establish the true effective diffusion coefficient (De) and effective porosity (ne). However, for a longer bentonite plug sample it is shown that the true effective diffusion coefficient (De) and effective porosity (ne) must be established to predict the contaminant transport through the sample.

A Tracer Method to Determine Hydraulic Conductivity and Effective Porosity of Saturated Clays Under Low Gradients

Abstract: Based on the column tracer test, we developed a research method to determine the hydraulic conductivity and effective porosity of saturated clays under low hydraulic gradients or small flow rates. Derived from Darcy's law and the solute transport equation, this method evaluates the hydraulic conductivity through measuring solute concentrations rather than by measuring flow rates. And the effective porosity is determined by applying an analytical solution of the one-dimensional uniform flow equation. Two types of experimental data drawn from a review of the literature and four sorts of accuracy test data carried out in the laboratory are used to examine the proposed method. The reproducibilities of accuracy tests for hydraulic conductivity determination indicate a consistency within a 5.5% error margin. The experimental results further indicate that hydraulic conductivities determined using the tracer method are more precise than those from the conventional flowmeter method. In addition, with the assistance of the proposed tracer method, we argue that the effective porosities may be overestimated as shown in the drawn example cases due to mistaking total pore volumes as the effective pores. The results of the accuracy tests further indicate that the effective porosities of saturated clay specimens are significantly smaller than the total porosities, at least when the samples are allowed to swell freely.

Percolation theory and hydrodynamics of soil-peat mixtures.

Abstract: Great quantities of organic matter are added reconstituting soils in urban conditions. To evaluate the efficiency of organic matter on physical properties of reconstituted soils, we studied the effects of loading on intrinsic permeability of soils mixed with peat. Percolation theory associated with a statistical porosity approach was used to explain variation of permeability related to porosity. After compression, sample porosity was measured and the pore-space morphology described by image analysis. An ellipse with major axis (a ) and minor axis (b) was inscribed within each pore. All the pores appeared as lenses of different sizes which could be assumed to represent disk-shaped cracks characterized by a diameter d and an aperture e. The crack model was calculated by means of a percolation threshold, p c = 0.33, along with values d and e as well as the measured porosity. This allowed a determination of the crack interconnection factor f. During compression, the number of pores per unit area decreased. Increasing the loading closed off pores and modified flow pathways. Connected sites facilitating percolation became disconnected from each other and the flow was reduced. The effective porosity, which actually took part in flow, was determined for all the samples and was dependent on peat content.

Evolution of Porosity in the Process of Sinian Dolostone Diagenesis in Southwest Sichuan

Abstract: In South China, especially in Southwest China, Sinian dolostone is widely distributed and with great thickness,which is of favorable potential of oil/gas accumulation. But the formation is very old in geologic age and complicated in diagenesis, which results in the uneven distribution and apparent inhomogeneity of the reservoirs. In the area, the Sinian dolostone reservoirs are dominantly concentrated at the top and middle parts of the Dengying Formation, and pore spaces are completely dominated by pores, vugs and fractures with secondary origination.Study indicates that the formation and evolution of the porosity are controlled by diagenesis.The destructive diagenesis,which is unfavorable for the formation and evolution of porosity,are mainly compression, cementation of primary porosity and filling of chemical prceipitation in secondary porosity, and the constructive diagenesis,which stimulates the formation and resolution of porosity,include recrystallization, dissolution in hypergenesis and in burial diagenesis. Compression is the main factor of closing all the primary porosity in fine dolostone; the primary intergranular porosity in grain dolostone and algae bound dolostone and algae framework porosity were eliminated after cementation of three phase dolosparite with the shapes of fiber, silt fine crystal and coarse crystal, and only a little residual primary porosity was preserved.The chemical fillings in the secondary pores, vugs and fractures are the cause of the porosity being not able to be preserved. There are mainly three phases of chemical fillings.The first phase is characterized by fiber like dolomite with the shapes of grape lace, the second one takes shape of fine to course dolomite, and the third one megacrystal dolomite. They may reduce the secondary porosity by 20%～80%, or up to 100% locally. The cements and chemical fillings precipitated in different diagenesis environments differ from each other not only in shapes of the crystals occurrence and succession of formation,also in geochemical features. Recrystallization occurred in burial diagenesis is apparent in the dolostone with no or little insoluble residues.The recrystallization results in the dolomite crystals in original rocks growing bigger and more automorphic, and the outcome is that total porosity in original rocks is not increased,but the effective porosity and permeability are increased, which is favorable for the passing of soluble formation water during buried diagenesis.Moreover,a good basis is laid for burial dissolution and formation and evolution of porosity are stimulated. Dissolution is the main factor of forming the secondary porosity, the hypergenesis dissolution at the end of Sinian was related to Tong Wan Movement. The dissolution developed not only lots of inter breccia pores, vugs and residual grape lace like vugs, but made the porosity concentrate in the range of 5 to 75 meters under weathering boundary. There are two phases of dissolution in burial episode, the first one happened in mature stage, and the second one in over mature stage of organic matters,dissolution is tightly bound to the process of maturing and evoluting of organic matters within the formations. To sum up, the porosity in Sinian dolostone reservoirs in Southwest Sichuan is the end product of natural process of sedimentation, diagenesis and tectonics in as long as 0.6 billion years. Diagenesis turned the dolostone deposits dominated by primary porosity into the dolostone reservoirs completely dominated by secondary porosity.

Simulation of ground-water flow in steep basin with shallow surface soil

Abstract: A coupled ground-water/channel flow distributed model has been developed for continuous simulation in a 123-km 2 basin. The aim was to analyze the streamflow generation processes in natural vegetated environments. Finite-difference schemes have been used to solve conservation equations of the 2D saturated subsurface flow and the 1D kinematic surface flow. Because of the high hydraulic conductivity of the surface soil, only the saturation excess mechanism of runoff production has been considered. Parameter sensitivity analysis showed the overriding influence of soil storage capacity and conductivity. A grid discretization >100 m produces a hydraulic conductivity greater than physically meaningful, which considerably increases as the space-grid step increases. Results indicate that the model can satisfactorily simulate the water-flow behavior of the catchment after fitting the three parameters of surface hydraulic conductivity, effective porosity, and evapotranspiration losses. These are done after calculating the conductivity as a function of the height of the water table. The simulation efficiency has varied from 87% in the first 5-year calibration period to 85.8% in the subsequent 5-year validation period.

Computer Modeling of Porosity and Lithology for Complex Reservoirs Using Well-Log Measurements

Abstract: The high degree of heterogeneity, saturation of multiphase fluids, and presence of clays in complex reservoirs make each of the three porosity logs (sonic, density, and neutron), if used independently, generally record inaccurate porosity. For such reservoirs, combining different logs gives accurate results of porosity. The reservoirs of Terra Nova and Hibernia (Jeanne d?Arc Basin), offshore of the eastern coast of Canada, are saturated with multiphase fluids, enriched with clays, and made of compacted and heterogeneous rocks, in terms of the lithological and mineralogical composition, and the size and shape of the grains and pores. In this study, the porosity and the rock constituents were determined for both reservoirs using a computer technique in which the iteration process was applied. That was done by developing and using various computer programs and models, and utilizing numerous data from several logs analyzed at 0.2-m sampling-depth intervals. The more the number of logs and iterations used in c...

Particles, Pores, and Permeability

Abstract: This chapter deals with the particles, sediment aggregates, properties of pores, and permeability. The porosity of a rock is the ratio of its total pore space to its total volume. There are two main genetic groups of pore types: primary porosity and secondary porosity. Primary porosity is formed when sediment is deposited. It includes intergranular or interparticle porosity, which is characteristic of sands, and intraparticle porosity found in skeletal carbonate sands. Secondary porosity forms after sedimentation by diagenetic processes. Permeability is the ability of a fluid to flow through a porous solid. Permeability is controlled by many variables: the effective porosity of the rock, the geometry of the pores, including their tortuosity, and the size of the throats between pores, the capillary force between the rock and the invading fluid, its viscosity, and pressure gradient. The chapter summarizes various methods for analyzing particle shape, size, sphericity, and roundness. It also describes the direct and indirect methods for measuring both porosity and permeability.

An Improved Empirical Approach for Prediction of Formation Water Saturation and Free Water Level for Uni-modal Pore Systems

Abstract: Minimizing the uncertainty associated with the prediction of formation water saturation can be achieved through the integration of formation evaluation (log analysis), capillary pressure data, and pressure gradient information. To reduce the uncertainty, it is often necessary to determine porosity, permeability and water saturation using integrated methods. The magnitude of uncertainty reduction associated with predicted petrophysical parameters is a function of the inherent quality of the various data acquisition devices. Thus, reduced uncertainty in hydrocarbon pore volume can often be achieved by combining explicit probabilistic formation evaluation and a quantitative capillary pressure model. An empirical quantitative capillary pressure-water saturation model has been developed that relates the capillary pressure and height above the free water level, FWL to the wetting phase saturation. This relationship generalizes the correlation for all existing rock types that exhibit uni-modal pore geometries. The model constants are expressed as continuous functions of the interval speed that is defined as the square root of core measured permeability divided by porosity (√k/Φ) data. The water saturation is calculated continuously as a function of the height above the FWL using the log predicted permeability-porosity ratios. In the absence of a clear fluid contact, the new model provides an excellent tool for describing the fluid contacts by iterating on saturation. The FWL is adjusted until a good agreement exists between a base water saturation and the predicted water saturation from the empirical capillary pressure relationship. This paper describes the new saturation model and its application for a siliclastic layered reservoir. The model parameters are tuned based on a statistically significant number of measured high-pressure mercury injection capillary pressure curves. Furthermore, the model parameters are calibrated to data from a key well. The results of the model are then used to predict formation water saturation in neighboring wells.

Research on the Law of Porosity of Rock with Depth

Abstract: Porosity is an important parameter characteristized physical, mechanical and seepage properties of rock. In general, when seepage problem of rock is studied in shallow underground rock in natural condition, porosity of rock is taken as a constant. However, porosity of rock changes with depth, geostress and engineering forces. Through analysis of triaxial test of rock samples, it is discovered that there is a quartic polynomial relation between porosity of rock and depth and porosity of rock decreases with depth increase. Porosity of rock approximates to a constant under 6000 m～7000 m depths.

Integrated Interpretation of 3D Seismic and Wireline Data to Delineate Thin Oil-Producing Sands in San Jorge Basin, Argentina

Abstract: Hydrocarbon-bearing sand units in San Jorge Basin, Argentina exhibit thicknesses between 0.5 and 15m, with a mean value of 1.5m. These sand units are associated with ephemereal and laterally heterogeneous fluvial architectures embedded within much thicker shale units of lacustrine and flood-plain origin. Tuffaceous laminations originated from pulses of volcanic activity are also present in the sedimentary column. In view of the wide range of spatial variability and size of porous sands, a typical well is planned to intersect vertically as many sand units as possible. The best producing sands have average porosities of 10% and permeabilities of a few millidarcies. There are several problems faced by the petrophysicist in assessing whether or not a given sand unit should be perforated, namely, (a) discrimination between oil- and waterbearing sands is not trivial because of very low salinity water, (b) there exist substantial vertical variations of effective porosity within an individual sand because of shale laminations, and (c) it is often impossible to assess lateral extent away from the well. We have successfully addressed most of these difficulties using an interpretation procedure centered about the full 2D inversion of wireline array induction data. Full 2D inversion of array induction data is necessary to accurately estimate shallow and deep resistivities, as well as invasion radii in light of significant shoulder bed effects. This procedure has been complemented with the use of borehole NMR data to provide estimates of effective porosity within individual sand units. Finally, we have made use of geostatistical inversion of 3D post-stack seismic data to estimate the lateral extent of hydrocarbon-bearing sands laterally away from the well. We present several application examples that yield results consistent with borehole testing and production data.

Effective-Porosity and Dual-Porosity Approaches to Solute Transport in Fractured Tuff of the Saturated Zone at Yucca Mountain: Implications for Repository Performance Assessment

Abstract: The effective-porosity approach and the dual-porosity approach are examined as two alternative conceptual models of radionuclide migration in fractured media of the saturated zone at Yucca Mountain. Numerical simulations of one-dimensional radionuclide transport are performed for the domain relevant to repository performance assessment using the two alternative conceptual approaches. The dual-porosity solute transport modeling produces similar results to the effective-porosity model for fracture spacing of less than approximately 1 m and greater than about 200 m, corresponding to values of effective porosity equal to the matrix porosity and the tiiicture porosity, respectively. For intermediate values of fracture spacing, the dual-porosity approach results in concentration breakthrough curves that differ significantly fi-omthe effectiveporosity approach and are characterized by earlier first arrival, greater apparent dispersion, and lower concentrations at later times. The effective-porosity approach, as implemented in recent pefiormance assessment analyses of saturated zone transport at Yucca Mountain, is conservative compared to the dual-porosity approach from the perspective of both radionuclide concentrations and generally for travel times.