Showing papers on "Effective porosity published in 1989"

Evaluation of spatial distribution of hydraulic conductivity using effective porosity data

Abstract: The use of spatial distribution of effective porosity (Φe) to estimate the distribution of saturated hydraulic conductivity (K2) is evaluated on five new soils and on a combination of soils. The K2 is related to Φe by a generalized Kozeny-Carman equation. This equation is then combined with scaling

Sandstone diagenesis and porosity modification during basin evolution

Abstract: The properties of sandstones as potential reservoirs and shales as source rocks depend on primary facies relationships and diagenesis. Porostiy loss due to mechanical compation and pressure solution is essentially a function of grain parameters (sorting, packing and composition) and net overburden stress. The porosity loss can be predicted to a certain extent. The importance of secondary porosity caused by dissolution of framework grains and cements has been fully recognized. The discussion has focused on the processes causing such dissolution and to what extent it can cause net increase in porosity. The most critical factor in clastic diagenesis is the nature of porewater flow and the degree of mass transfer taking place as a result of this. In the North Sea reservoir rocks, petrographic and geochemical evidence suggest that most of the leaching of feldspar and mica resulting in the formation of kaolinite occurred early during fresh wather flushing. Recent calculations indicate that »acids« derived from source rocks are inadequate to explain the secondary porosity observed in reservoir rocks. Mathematical modelling suggests that thermal convection is of limited importance in sedimentary basins, except where there are high lateral changes in geothermal gradients. Evidence from porewater geochemistry suggests that porewaters in sedimentary basins are often stratified or compartmentalized in a way which is inconsistent with large scale convection or compactional flow, making it necessary to assume that diagenetic reactions are relatively isochemical during deeper burial. A better understanding of the diagenetic reactions will help us to improve our predictions about porosity/depth relations, pore size, and pore geometry distribution in reservoir rocks. Porosity depth trends from offshore Norway and published data from other basins are discussed. Empirical linear best fit lines are found to illustrate the relationship quite well for depths between one and five km. Within a specific region, the linear porosity gradient is a function of mineral composition and of temperature and pressure gradients. Primary porosity tends to be best preserved in sandstones with high proportions of stable grains (e. g. in quartz arenites) down to about 3 or 4 km. At greater depth, porosity loss is accelerated due to increased pressure solution. Secondary and primary porosity adjacent to feldspar grains then tends to be selectively preserved relative to primary pores between quartz grains.

Bacteriophage Transport in Sandy Soil and Fractured Tuff.

Abstract: Bacteriophage transport was investigated in laboratory column experiments using sandy soil, a controlled field study in a sandy wash, and laboratory experiments using fractured rock. In the soil columns, the phage MS-2 exhibited significant dispersion and was excluded from 35 to 40% of the void volume but did not adsorb. Dispersion in the field was similiar to that observed in the laboratory. The phage f2 was largely excluded from the porous matrix of the two fractured-rock cores studied, coming through 1.2 and 2.0 times later than predicted on the basis of fracture flow alone. Because of matrix diffusion, nonsorbing solutes were retarded by over a factor of three relative to fracture flow. The time for a solute tracer to equilibrate with the porous matrix of 6.5-cm-diameter by 25-cm-long cores was measured in days. Results of both granular-medium and fractured-rock experiments illustrate the inability of a solute tracer to provide estimates for dispersion and effective porosity that are applicable to a colloid. Bacteriophage can be used to better estimate the maximum subsurface transport rate of colloidal contaminants through a porous formation.

Diagenesis and Preservation of Porosity in Norphlet Formation (Upper Jurassic), Southern Alabama

Abstract: Sandstones of the Norphlet Formation (Upper Jurassic) in the vicinity of Lower Mobile Bay, Alabama, have porosities greater than 20% and permeabilities up to about 1 darcy at depths of more than 20,000 ft (6,096 m). Typically, pre-Tertiary sandstones buried to such depths have approximately 6% (±2%) porosity; thus, deep Norphlet sandstones contain up to 14% "excess" porosity. This porosity is largely intergranular and appears to be preserved primary porosity. When the diagenetic scenario for the Norphlet is reconstructed in a time-temperature framework, it is apparent that porosity preservation is not the result of any single diagenetic event, but is due to a continuous series of diagenetic conditions that overlapped in time. The most important of these circumstances were the following. (1) The lack of pervasive early calcite or anhydrite cements found in the large eolian dunes (10-300 ft or 3-91 m high) that comprise the best Norphlet reservoirs. This lack of cement may have set the stage for subsequent events by keeping open pathways through which fluids were later able to efficiently contact the sandstone. (2) Early grain-coating clay/iron oxide rims reacted to form chlorite in the 176°-248°F (80°-120°C) thermal wind w. The grain-rimming chlorite may have inhibited subsequent quartz cementation. Up to 10.5% porosity may be due to the porosity preserving effect of chlorite. (3) Migration of hydrocarbons and development of geopressures, beginning at temperatures as low as 230°F (110°C), retarded further cementation. Up to 4.5% porosity may be due to the combined effects of hydrocarbons and geopressures. (4) A series of CO2-generating reactions occurred, including decarboxylation of organic acids, thermal cracking of liquid hydrocarbons, and thermochemical sulfate reduction. As a result of these reactions, late carbonate cements were not abundant. Thermochemical sulfate reduction (275°-356°+F or 135°-180°+C) has been of particular importance in the deep Norphlet. Reaction of hydrocarbons with anhydrite has resulted in the local removal of nodular anhydrite and has also affected gas quality (i.e., H2S content). The preserved porosity in Norphlet sandstones is the result of a combination of circumstances. The fact that these conditions overlapped in time-temperature space may be as important as any single factor.

Laboratory estimation of gas diffusion coefficient and effective porosity in soils

Abstract: This paper describes an improved numerical method to estimate the gas diffusion coefficient (D) of a finite soil sample, giving as a by-product an estimation of the effective porosity (e). This method is applied to an earlier apparatus used to measure the rate of transfer of tracer krypton-85 through a finite soil sample: the soil sample is enclosed by two gas cells, and the concentration in each gas cell is regularly measured after gas injection using β radiation count rates from the test gas. The proposed numerical approach combines the finite-element method to solve Fick's second law of diffusion, and a nonlinear, iterative procedure to find the estimated parameters &OV0429; and ê, where the residual differences between the measured and simulated count rates at opposite ends of the sample are minimized. An error analysis that takes into account the random process of β emission of tracer is numerically simulated: the gas diffusion coefficient is shown to have a low sensitivity to this source of error, whereas the affective porosity has a larger sensitivity. The proposed method is applied to a large range of soils, including (1) wet and dry undisturbed field soil cores, (2) compacted, water-saturated aggregates, (3) dry and wet textural soil cores. The parameters &OV0429; and ê were fitted satisfactorily to the measured data on each soil sample: the diffusion coefficient estimates are compared with existing estimation methods, whereas the estimated effective porosity is approximately the same when using the iterative proposed approach or a mass balance method applied to the experimental apparatus. Furthermore, it is shown that there is no simple and unique relationship for all the porous media between the calculated gas diffusion coefficient and soil sample air-filled porosity: such a relationship is likely to be highly dependent on the physical characteristics of the pore space, such as pore continuity, tortuosity, and morphology.

Solute advection in stratified formations

Abstract: Advection-dominated solute movement in stratified formations is investigated using a Lagrangian interpretation of particle motion. A probability density function (pdf) for particle position quantifies the expected depth-integrated resident concentration. A pdf for particle arrival time quantifies the expected depth-integrated rate of mass arrival, from which the flux-averaged concentration can be defined. The difference between the flux-averaged and resident concentrations is shown to be significant for the variability in the hydraulic conductivity that is commonly encountered in field applications. The influence of porosity variations on the advection-dominated solute movement in stratified porous media is shown to be notable only for large variability in the effective porosity.

Scaling the saturated hydraulic conductivity of an alfisol

Abstract: Alfisols exhibit a high degree of spatial variability in their physical properties As a result, it is difficult to use information on physical parameters measured at one location to model larger-scale hydrologic processes In this study, the saturated hydraulic conductivity, Ks, of an Alfisol was determined on 109 undisturbed monoliths using the falling-head permeameter method The model developed by Arya & Paris (Soil Science Society of America Journal 45, 1023-1030, 1981) was used to calculate the pore volume from sand and clay fractions Scaling factors were calculated from the measured Ks, sand pore-volume, clay pore-volume, clay content and effective porosity, using the similar media concept Prediction of Ks of gravelly Alfisol using clay pore-volume is confounded by high gravel content which, when discounted, improves the prediction remarkably The scaled mean saturated hydraulic conductivity K* for all horizons of the alfisol was approximately 10 × 10-5 m s-1

A Model for the Relationship between the Hydraulic Conductivity and Primary Sedimentary Structures of Till

Abstract: An investigation has been made of the relationships between saturated hydraulic conductivity, porosity and micro-structure of undisturbed lodgement till samples. 35 measurements with a constant head laboratory permeameter are the basis for the discussion. All the measurements were made on undisturbed till samples. In order to minimize the effects of soil forming processes and to have the most homogeneous conditions all sampling were made from the C -horizon. The samples have a size of about 300 cm 3 . Porosity data were derived from capillary pressure curves. A model for how flow direction and long-axis orientations of elongated grains relate to the saturated hydraulic conductivity is presented. For an unsorted sediment such factors as grain size are concluded to be of minor importance for the hydraulic conductivity. The structural properties seem to be a more important factor. This effect can be explained in two ways. Either due to directional relations between sorted lenses and bands which have higher permeability and the flow route through the sample. The other explanation is due to a more continuous pore pattern parallel to the grain orientation. The hydraulic conductivity takes on a directional property, being smaller in directions normal to the structural long-axis orientation than in directions parallel to the orientation. A study of the effective porosity Versus hydraulic conductivity exhibits weak correlation.

On the field determination of effective porosity

Abstract: Effective porosity of geologic materials is a very important parameter for estimating groundwater travel time and modeling contaminant transport in hydrologic systems. Determination of a representative effective porosity for nonideal systems is a problem still challenging hydrogeologists. In this paper, some of the conventional field geophysical and hydrological methods for estimating effective porosity of geologic materials are reviewed. The limitations and uncertainties associated with each method are discussed. 30 refs., 8 figs.

Seismic properties of tuffs from the epithermal system of Creede, Colorado

Abstract: Ultrasonic P wave velocity, effective porosity, grain and bulk density, residual water saturation and petrography of 46 tuff samples from the epithermal system of Creede, Colorado, were analyzed with stepwise multiple curvilinear regressions to predict dry bulk density, P wave velocity (Vp), and acoustic impedance (I). Changes in porosity best predict variations of all three parameters; residual water saturation is significantly correlated with Vp and I for coarse-grained matrix tuffs; and high aspect ratio cracks have a small but significant ability to predict Vp and I. The importance of porosity suggests that seismic and gravity work could be used to outline porosity-related structures of crucial importance to our understanding of Creede's epithermal system. In areas where lithologies are comparable to our samples, seismic velocity or acoustic impedance measured remotely with seismic surveys may yield estimates of porosity and provide important quantitative information on rocks that cannot be studied directly through a comprehensive drilling program.

Compaction of basin sediments as a function of time-temperature history

Abstract: Processes that affect burial diagenesis are dependent on time-temperature history (thermal maturity). Therefore, the porosity loss of sedimentary rocks during burial may often be better treated as a function of time-temperature history than of depth. Loss of porosity in the subsurface for sandstones, carbonates, and shales can be represented by a power function /phi/ = A(M)/sup B/, where /phi/ is porosity, A and B are constants for a given sedimentary rock population of homogeneous properties, and M is a measure of thermal maturity such as vitrinite reflectance (R/sub 0/) or Lopatin's time-temperature index (TTI). Regression lines of carbonate porosity and of sandstone porosity upon thermal maturity form an envelope whose axis is approximated by /phi/ = 7.5(R/sub 0/)/sup /minus/1.18/ or, equivalently, by /phi/ = 30(TTI)/sup /minus/0.33/. These equations are preliminary generic relations of use for the regional modeling of both carbonate and sandstone compaction in sedimentary basins. The dependence of porosity upon time-temperature history incorporates the hypothesis that porosity-reducing processes operate continuously in sedimentary basins and, consequently, that compaction of basin sediments continues as long as porosity exists. Calculations indicate that subsidence due to loss of porosity through time (with depth held constant) can produce a second-stage passively formed basinmore » in which many hundreds of meters of sediments can accumulate and which conforms with the structure of the original underlying basin. Such sediment accumulation results from the thermal maturation of thick sequences of sedimentary rocks rather than from global sea level change or tectonic subsidence.« less