Showing papers on "Effective porosity published in 1998"

A comparison of estimated and calculated effective porosity

Abstract: Effective porosity in solute-transport analyses is usually estimated rather than calculated from tracer tests in the field or laboratory. Calculated values of effective porosity in the laboratory on three different textured samples were compared to estimates derived from particle-size distributions and soil–water characteristic curves. The agreement was poor and it seems that no clear relationships exist between effective porosity calculated from laboratory tracer tests and effective porosity estimated from particle-size distributions and soil–water characteristic curves. A field tracer test in a sand-and-gravel aquifer produced a calculated effective porosity of approximately 0.17. By comparison, estimates of effective porosity from textural data, moisture retention, and published values were approximately 50–90% greater than the field calibrated value. Thus, estimation of effective porosity for chemical transport is highly dependent on the chosen transport model and is best obtained by laboratory or field tracer tests.

Evaluation of chloride and pesticide transport in a fractured clayey till using large undisturbed columns and numerical modeling

Abstract: Saturated groundwater flow and tracer experiments using fluorescent dye, chloride, and the herbicides mecoprop and simazine were carried out in the laboratory using three large-diameter (0.5 m) undisturbed columns of fractured clayey till. Hydraulic conductivity of the columns ranged from 10−5 m/s in the shallowest column (1 m dept)) to 10−7 m/s in the deepest column (4 m depth) and were similar to field-measured values for these deposits. Results of the tracer experiments are consistent with a conceptual model of advective transport along the fractures combined with diffusion into the fine-grained matrix between the fractures. Arrival of the chloride tracer in the effluent was highly retarded relative to fracture flow velocities calculated on the basis of the cubic law and measured values of fracture spacing and hydraulic conductivity. The herbicides were more strongly retarded than the chloride at low flow rates, but at higher flow rates the herbicides arrived with the chloride, indicating the influence of nonequilibrium sorption of the herbicides to fracture walls and the matrix solids. The columns were dismantled following the tracer experiments and mapping under UV light showed that nearly all of the visible, weathered fractures (and the few root holes in the case of the shallowest sample) were active transport pathways, with the dye appearing mainly on the fracture surfaces and as a “rim” in the adjacent matrix. Concentration profiles measured perpendicular to the fracture surfaces showed that the herbicides had also moved into the matrix, apparently by diffusion. Simulations of solute transport with a discrete fracture flow/matrix diffusion model showed that the simulations could be “fit” to the data if all of the visible fractures were hydraulically active, but could not be fit if all or most of the flow was channelled through just the primary fractures (defined by prominent oxidation stains). Simulations with an equivalent porous media (EPM) model could not fit the data using the measured total porosity as the effective porosity. The simulations could likely be fit with a smaller value of effective porosity, but this would limit applicability to field situations because fitted effective porosity is expected to change with physical scale and residence time of the solute in the soil.

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.

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...

Simulations of Single-Fluid Flow in Porous Media

Abstract: Several results of lattice-gas and lattice-Boltzmann simulations of single-fluid flow in 2D and 3D porous media are discussed. Simulation results for the tortuosity, effective porosity and permeability of a 2D random porous medium are reported. A modified Kozeny–Carman law is suggested, which includes the concept of effective porosity. This law is found to fit well the simulated 2D permeabilities. The results for fluid flow through large 3D random fibre webs are also presented. The simulated permeabilities of these webs are found to be in good agreement with experimental data. The simulations also confirm that, for this kind of materials, permeability depends exponentially on porosity over a large porosity range.

Hydraulic parameters and solute velocities in triple-porosity karstic-fissured-porous carbonate aquifers: case studies in southern Poland

Abstract: Rock and flow parameters of three karstic-fissured-porous aquifers in the Krakow-Silesian Triassic formations were measured using various methods and compared. Though cavern and fissure porosities are shown to be very low (cavern porosity below 0.5% and fracture porosity below 0.2%), they contribute dominantly to the hydraulic conductivity (from about 1.3×10–6 to about 11×10–6 m/s). Matrix porosity (2–11%) is shown to be the main water reservoir for solute transport and the main or significant contributor to the specific yield (<2%). Though the matrix porosity is shown to be much larger than the sum of the cavern and fissure porosities, its contribution to the total hydraulic conductivity is practically negligible (hydraulic conductivity of the matrix is from about 5×10–11 m/s to about 2×10–8 m/s). On the other hand, the matrix porosity (for neglected cavern and fissure porosities) when combined with tracer ages (or mean travel times) is shown to yield proper values of the hydraulic conductivity (K) by applying the following formula: K≅(matrix porosity×mean travel distance)/(mean hydraulic gradient×mean tracer age). Confirming earlier findings of the authors, this equation is shown to be of great practical importance because matrix porosity is easily measured in the laboratory on rock samples, whereas cavern and fracture porosities usually remain unmeasurable.

Method for improving the accuracy of NMR relaxation distribution analysis with two echo trains

Abstract: The total porosity is calculated by summing over the effective porosity and the CBW porosity. With the correction described in this patent, an overcall of the total porosity from some part of the clay bound water signal being counted twice is avoided. The effect will be more significant in shaley sand where CBW porosity is abundant. The correct T2 distribution is essential to BVI and SBVI estimation. The same data correction concept can be applied to make a meaningful comparison of logs or laboratory core NMR data that are obtained with different sampling periods TE.

Geologic Framework and Hydrogeologic Characteristics of the Edwards Aquifer Recharge Zone, Bexar County, Texas

Abstract: In Bexar County, residential and commercial development on the Edwards aquifer recharge zone is increasing. The aquifer possibly can be contaminated by spills, leakage of hazardous materials, or runoff from the rapidly developing urban areas that surround, or are built on, the intensely faulted and fractured, karstic limestone outcrops characteristic of the recharge zone. Furthermore, some of the hydrogeologic subdivisions that compose the Edwards aquifer have greater effective porosity than others. The areas where the most porous subdivisions crop out might provide efficient avenues for contaminants to enter the aquifer. The Edwards aquifer recharge zone has relatively large permeability resulting, in part, from the development or redistribution of secondary porosity. Lithology, stratigraphy, diagenesis, and karstification account for the effective porosity and permeability in the Edwards aquifer outcrop. Karst features that greatly enhance effective porosity in the outcrop area include sinkholes and caves. Hydrogeologic subdivision VI, the Kirschberg evaporite member, appears to be the most porous and permeable subdivision within the Kainer Formation. Hydrogeologic subdivision III, the leached and collapsed members, undivided, is the most porous and permeable subdivision within the Person Formation. Hydrogeologic subdivision II, the cyclic and marine members, undivided, is moderately permeable, with both fabric- and not-fabric-selective porosity. The faults in northern Bexar County are part of the Balcones fault zone. Although most of the faults in this area trend northeast, a smaller set of cross-faults trend northwest. Generally, the faults are en echelon and normal, with the downthrown blocks typically toward the coast.

Application of time domain reflectometry to determination of volumetric water content in rock

Abstract: Time domain reflectometry (TDR) is a widely used technique for measuring soil water content. To examine its applicability to rock, the authors applied TDR to a variety of rocks in the laboratory. The obtained relationships between the apparent dielectric constant and the volumetric water content showed a behavior that can be partly explained by the effect of gaps between the TDR probe and rock. For rocks with an effective porosity smaller than that of soil, the well-known model presented by Topp et al. [1980] for soil tended to overestimate the water content. For such rocks it was determined that an individual calibration was needed for accurate determination of water content. If no experimental calibration is possible, frequently used mixing models may be used to establish the calibration function. Our investigations suggest that TDR showed the potential for measuring the apparent dielectric constant of rock and, further, for determining water content in rock.

Integrating Geophysics, Geology And Petrophysics: A 3D Seismic AVO And Borehole/logging Case Study

Abstract: Introduction Upper Mannville aged strata in southern Alberta consist of a complex lithologic assemblage of narrow shoestring like “Glauconite” and “Lithic” valley-systems trending roughly north-south and variably cutting down into older regional Glauconite, Ostracod and Basal Quartz strata. These valley systems are commonly filled with hydrocarbon-bearing sandstones with varying reservoir quality. In the study area, the Glauconite valley systems are up to 35 meters deep, 2 kilometres wide and several tens of kilometres long. The younger “Lithic” valleysystems generally follow the same trend as the Glauconite valleys and are up to 40 meters deep, 3 to 4 kilometres wide and also several tens of kilometres long. In 1996, a 3D seismic survey was acquired over the valley trends to image reservoir quality sands. It is often difficult to seismically distinguish between Glauconite sand and Ostracod shale by using poststack amplitudes. Improved techniques from recent prestack seismic work suggest that AVO (Amplitude Versus Offset) might be an effective way to extract rock properties and detect gas (Smith and Gidlow, 1987; Gidlow et al., 1992; Fatti et al., 1994, Goodway et al., 1997). Smith and Gidlow (1987), Gidlow et al(1992) and Fatti et al. (1994) proposed a method to create a display which highlights the Vp/Vs ratio anomaly (often caused by the presence of gas) by using the so called “fluid factor stack”. Goodway et al. (1997) proposed a new improved fluid detection and lithology discrimination indicator using petrophysical parameters and where and are Lame’s constants (incompressibility and rigidity) and is density. The geological and geophysical effort in this area is to determine where the two valleys are present, and to differentiate sands from shale. Geology and Reservoir Quality Log analysis and core data, where available, indicate that the Glauconite sandstones generally have good to excellent reservoir quality whereas the Lithic sandstones are generally moderate to poor quality rocks. For example, the Glauconite sandstones in well B (Figures 1 & 2) have core porosities ranging from 18 to 26% and permeabilities ranging from 948 to 4900 md.. In contrast, the Lithic sandstones in well C (Figure 1) have core porosities ranging from 10 to 14% and permeabilities from 1 to 40 md.. Glauconite channels in the study area produce both oil and gas whereas Lithic channels produce primarily gas. The differences in reservoir quality between the Glauconite and Lithic channels are related to major differences in sandstone framework grain composition. Sedimentological interpretations from available core indicate that the Glauconite and Lithic channels consist predominantly of fine to medium grained fluvial sandstone at the base and grade upwards into finer grained marine influenced (estuarine) deposits at the top. Typical regional, non-channel, deposits in this area can be seen in wells A & D in Figure 1. The Ostracod interval, which consists of shale and limestone is a significant regionally extensive stratigraphic marker. Absence of the Ostracod is one of the key correlation indicators for downwards incision by overlying Glauc and Lithic channels. Petrophysical Analysis and Lame Parameters Petrophysical parameters for “complex” lithology, porosity, and fluid saturations were modeled. A rather complete log-data set including density, neutron, sonic, gamma ray and magnetic resonance logs permitted a comprehensive model to be developed and correlated to core lithology, porosity and saturation data. This “complex” lithology model is required to account for bitumen, gas, irreducible water and intraparticle porosity evident in the log, core and thin section data. Magnetic resonance data enables meaningful evaluation of bitumen and irreducible water in this lithology. Model parameter selection is constrained by comparison of model results to core lithology, porosity and fluid saturations. The petrophysical model illustrated by figure 2 for well B shows a clean quartz rich sand lithology having a maximum effective porosity of 0.26 (core porosity overlaid in red). The porosity match deviates in intervals where intraparticle porosity is not included in the core porosity. Effective porosity is reduced by shaliness at the top and by bitumen at the base of the sand. Modeled hydrocarbon saturation is overlain by core oil saturation (in red). The saturation curves correspond in intervals containing residual oil and deviate in gas bearing intervals.

Geologic Investigation of Cross-Well Seismic Response in a Carbonate Reservoir, McElroy Field, West Texas

Abstract: Cross-well seismic data from McElroy field, a Permian dolomite reservoir in west Texas, demonstrate that high-resolution velocity and reflection images are obtainable in this carbonate reservoir. Our geologic "ground-truthing" results suggest that cross-well data, when integrated with porosity models based on log facies, add value to reservoir characterization. The cross-well data added information at the interwell scale that we could get no other way. Reservoir quality in the portion of McElroy field that we investigated does not obviously relate to core-based lithofacies due to a complex diagenetic overprint, primarily cementation by gypsum and anhydrite. The coincidence of S-wave reflections on the cross-well data with decreases in porosity or gypsum cement from whole-core analysis suggests that total porosity and mineralogy dominantly influence velocity. Overall, the vertical location of layers generated by reflection imaging correlates fairly well with major log variations. In particular, positive events on the S-wave images correspond almost exactly with increases in sonic velocity, increases in resistivity, increases in bulk density, and decreases on the neutron porosity log from high porosity (or gypsum) to low porosity (or gypsum). Both the log and cross-well data respond to the same diagenetic overprint and its resulting petrophysical characteristics; therefore, we group log data into log facies using multivariate statistical techniques, such as cluster analysis, rather than using core data for correlating reservoir flow units and relating them to the cross-well images. Many of the positive-amplitude events on the S-wave profiles correspond to transitions, in a vertical sense, between the "best" reservoir cluster and less porous reservoir clusters, which indicates the strong relationship between velocity and porosity. In addition, lateral variations in many of the positive-amplitude events can be tied to changes in porosity and differences in the clusters between the wells. Comparing geostatistical porosity models directly to the S-wave images suggests that the S-wave reflection images appear to be resolving lateral changes in porosity of less than 56 m (185 ft) but more than 15 m (50 ft).

Application of Petrophysically Derived "Flow Facies" for Reservoir Characterization and Simulation: Wara Reservoir, Greater Burgan Field

Abstract: In fluvially dominated delta plain reservoirs, such as the Wara formation in the Greater Burgan Field, characterizing a reservoir's flow properties accurately is essential in developing a sound reservoir model. This is easier said than done. Typically, lithofacies identified in cores are correlated to multiple log suite characteristics. These are then used to help define simulation flow properties in wells. In Greater Burgan, with over fifty years of production, much of the field development occurred before modem diagnostic logging tools became available. Therefore, direct correlation of core lithologies and corresponding lithofacies description to multilog character is not possible in the majority of wells. Relationships discovered between shale volume (V SH ) ranges and effective porosity (o eff ) to permeability transforms allowed us to apply unique rock properties to flow units or facies defined by the V SH -porosity ranges. These flow facies eliminated the difficult task of trying to predict changing lithologies and lithofacies in wells with limited log traces and no core.

Saturated conductivity and effective porosity of the forest soil

Abstract: Soil samples were taken from different soil depth of different forest soil story in the Natural Reserve of Changbai Mountain, and their saturated conductivity and effective porosity were measured. The variation of saturated conductivity and effective porosity with different soil depth were studied by regressive analysis and a logarithmic model. The results were compared with the exponential model (Beven 1982). The results of comparison showed that the logarithmic model was more accurate and reasonable than the exponential model for forest catchment.