Showing papers on "Effective porosity published in 1992"

Sandstone porosity loss; a big picture view of the importance of compaction

Abstract: Proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones, studies of mass transport during diagenesis, and the modeling of basin fluid flow. Quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point-count data on cement and pore space abundance. This method is discussed and its sources of error are evaluated. Error in the method arises mainly from uncertainty in initial sandstone porosities, and the amount of local grain dissolution during burial. Data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45% will produce small average errors in the calculation of compactional porosity loss for clean sandstones. Grain dissolution, whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement, will cause an underestimation of the significance of compaction. A large database of point-count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss. It is shown that the global importance of compactional porosity loss has generally been under-appreciated. In fact, compaction (mechanical and chemical) is probably the dominant mechanism of porosity loss in most sandstones. The extent of compactional porosity loss is related to sand grain composition and basin thermal structure.

Estimation of saturated hydraulic conductivity of soils from particle size distribution and bulk density data

Abstract: Since laboratory and field measurement of soil hydraulic properties is time consuming and subject to large error, numerous models have been proposed to predict soil hydraulic properties from easily measurable soil properties such as particle size distribution, bulk density, effective porosity and carbon content. In this study a multiple linear regression model was developed to predict the saturated hydraulic conductivity of soils from their particle size distribution and bulk density data. Published data from 350 soil core samples of varying soils from different sources were used to develop the model. Stepwise regression selected the best model for prediction of soil hydraulic conductivity (R2 = 0.68, P < 0.0001) from the independent parameters of silt, clay, and bulk density. Additional field measured data were collected to test and validate the model using several statistical evaluation procedures. Based on the statistical evaluation criteria, the model performed fairly well and gave a satisfactory validation versus the field measured data.

Effects of freezing on hydraulic conductivity of compacted clay

Abstract: A compacted clay is subjected to five cycles of freezing and thawing at constant water content. The test specimens are then permeated in flexiblewall permeameters at a relatively low effective stre...

Application of the Green-Ampt Infiltration Equation to Watershed Modeling

Abstract: A computer model was developed, based on the Green-Ampt infiltration equation, to computed rainfall excess for a single precipitation event. The model requires an estimate of parameters related to hydraulic conductivity, wetting front section, and fillable porosity of the soil layers. Values of parameters were estimated from soil textural averages or regression equations based on percent sand, percent clay, and porosity. Average values of effective porosity and wetting front suction were largely acceptable due to the relatively low variability and low model sensitivity to the parameters. Hydraulic conductivity was the most erratic constituent of the loss rate computation due to the high variability and the high sensitivity of the computed infiltration to the parameter. The performance of the Green-Ampt infiltration model was tested through a comparison with the SCS curve number procedure. Seven watersheds and 23 storms with precipitation of one inch or greater were used in the comparison. For storms with less than one inch of rainfall excess, the SCS curve number procedure generally gave the best results; however, for six of the seven storms with precipitation excess greater than one inch, the Green-Ampt procedure delivered better results. In this comparison, both procedures used the same initial abstractions. The separation of rainfall losses into infiltration, interception, and surface retention is, in theory, an accurate method of estimating precipitation excess. In the second phase of the study using nine watersheds and 39 storms, interception and surface retention losses were computed by the Horton equations. Green-Ampt and interception parameters were estimated from value sin the literature, while the surface retention parameter was calibrated so that the computed runoff volumes matched observed volumes. A relationship was found between the surface retention storage capacity and the 15-day antecedent precipitation index, month of year, and precipitation amount.

A new method to measure the effective porosity of clays

Abstract: The effective porosity is a required parameter for the mass flow rate calculations in groundwater hydrogeology. It is also a useful parameter in geotechnical engineering. In this paper, a methodology based on the tracer analysis technique is proposed to measure the effective porosity of clays. A test apparatus was developed and operational conditions were optimized to measure the effective porosity in the laboratory. Then the effective porosity of three clays under different chemical environments was measured. The measured effective porosity values showed that there is no unique effective porosity value for a given clay when tested under different chemical environments. The ratio of effective to total porosity values for silty and low plastic clays decreased while those for high plastic clay increased with the increase in degree of contamination.

Determining pore pressure while drilling a borehole

Abstract: Pore pressure Ppore of a subsurface formation is determined, while drilling a borehole, by measuring formation strength FS, resistivity RES and natural gamma ray activity GR. The measurement FS includes contributions of clay volume Vcl, non-clay mineral strength, and a porosity, the measurement RES includes contributions from a porosity, non-clay mineral volume Vsilt and pore fluid conductivity and the measurement GR includes contributions from clay and non-clay mineral, and the sum of Vcl, Vsilt and PHI eff ist 1. The contribution to the measurement of clay volume Vcl, non-clay mineral volume Vsilt, clay porosity PHI clay and effective porosity PHI eff are derived for the formation being drilled, the clay porosity PHI clay and effective porosity PHI eff being related by PHI eff = PHI clay(1 - Vsilt). The pore pressure Ppore is derived from the relationship PHI eff = PHI clay(0)[1-Vsilt(Z)]exp[-aPpore(Z)], wherein a=b DIVIDED (Pb-Pw)g and b = compaction coefficient at depth of interest Pb = density of shale Pw = density of pore fluid g = gravitational acceleration, 9.81 ms PHI eff = effective porosity PHI clay = clay porosity at zero depth of burial Vsilt = volume of non-clay mineral (silt) at depth of interest.

On-line method for measurement of the dispersion coefficient of radionuclide migration using a column method

Abstract: An on-line method to obtain breakthrough curves from a conservative tracer generated in crushed rock columns has been introduced. The breakthrough curve can be used to evaluate some important hydrologic parameters for studying radionuclide migration in groundwater system. These parameters include the dispersion coefficient, average flow yelocity, effective porosity, and retardation factor of the columns tested. A conservative radiotracer,131I, was used to generate the breakthrough curves, and linear regression analysis was applied to obtain the optimum value of dispersion coefficient. The effects of the injected volume of radioactive tracer, average flow velocity, and effective diameter of packed material on the dispersion coefficient as well as the stability of the packed material, and their in-situ application are discussed.

Shaly Sand Analysis as an Indicator of Hydrocarbon Production Potential in the Olmos Formation, Las Tiendas Trend, South Texas

Abstract: The sandstones of the Olmos Formation in the downdip Las Tiendas trend of Webb County, Texas originated on a storm-dominated shelf seaward of a major deltaic complex. Stratigraphically trapped gas is produced along a northwest-southeast trend 40 mi long and up to 8 mi wide. The reservoir comprises thin-bedded sandstones separated by non-productive thin siltstones and stacked into two to six, 20- to 50-ft-thick productive intervals. Porosity and permeability sufficient for gas production are preserved in the bases of individual storm-event sandstones below the depth to which bioturbation has completely homogenized sand with overlying silt and clay. Due to the presence of chlorite and illite-smectite mixed-layer clays in the Olmos sandstones, shaly sand analysis was required to determine accurate hydrocarbon pore-feet thicknesses. Because of the diversity of logging suites, the same types of porosity logs were not available from all wells and effective porosities had to be calculated in different wells using a variety of porosity-log types. In addition effective water saturations had to be calculated by both the Fertl and the Automatic Compensation methods. A volume of clay cutoff of less than 30% and an effective porosity cutoff of 10% were used to determine net pay intervals. Hydrocarbon pore-feet thicknesses were determined using shale-corrected effective porosities and water saturations. Cross plots of hydrocarbon pore-feet thickness versus cumulative gas production have a correlation coefficient of +0.79. The cross plots also reveal that wells that do not contain net pay in both major productive intervals, the Olmos A2 and Olmos B2 zones, and that have hydrocarbon pore-feet thicknesses less than 1.5 ft, typically have cumulative productions of less than 0.5 Bcf. Notwithstanding the diverse logging suites that were run on wells in the Las Tiendas trend, hydrocarbon pore-feet thicknesses can be related to well performance using detailed shaly sand analysis.