Showing papers on "Effective porosity published in 1978"

Porosity gradients in North Sea oil-bearing sandstones

Abstract: The relationship between porosity and burial depth of sandstone reservoirs is complex. It depends on primary porosity, which is related to grain-size and depositional process and also on the porosity gradient (the change in porosity with depth) which, in most cases, is linear. This relationship may be expressed as: φ d =φ p − G.D, where φ d is the porosity at depth D below surface, φ p is the primary depositional porosity, and G is the porosity gradient in % porosity per 1000 ft (305 m). Porosity gradients decrease with increasing quartz content, abnormal pressures, and in the presence of hydrocarbons. They increase with increasing geothermal gradient. There are at present insufficient data to quantify these relationships. Empirically derived porosity gradients for North Sea Rotliegendes, Jurassic and Palaeocene reservoir sandstones are 2.2, 2.6 and 2.9 per cent per 1000 ft (305 m) respectively. When calibrated with the porosity spectra of the various facies, graphs can be drawn showing the depth: porosity windows within which economic reservoirs may be found.

Petrographical factors relating to porosity and permeability in the fell sandstone

Abstract: Samples of Fell Sandstone were taken from approximately every 3 m from the core material left at Shirlawhope Well, Longframlington, Northumberland. Most of the core remains and it extends to a depth of 94 m. Various analyses were made at each 3 m depth interval. These included a modal analysis, particular attention being given to the cement and matrix material, as well as the void space; and a petrographic analysis of grain packing. Grain size analyses were also carried out at each depth to determine the individual median size and sorting coefficients. The absolute and effective porosities were obtained for each depth interval, together with permeabilities of intact specimens. The results of the individual parameters were compared statistically by means of correlation coefficients and Student9s t tests in order to evaluate the significance of their relationships. Porosity and permeability were shown to be influenced by void percentage and grain packing. Particle size distribution, however, does not influence porosity, although it has some influence on permeability. Cement-matrix content has little influence on porosity. A highly significant relationship was found to exist between effective porosity and permeability. Both porosity and permeability decrease with increasing depth from the surface.

Use of Laboratory Data to Predict Sulfate Sorption During Artificial Ground-Water Recharge

Abstract: Sulfate sorption data determined from laboratory investigations were used to predict movement of sulfate during a field experiment of artificial ground-water recharge in a basin near Lubbock, Texas. Laboratory studies confirmed that sulfate sorption is controlled in the system by surface area, content of iron oxides and hydroxides, and pH. Predicted sulfate distribution in the unsaturated zone was made by assuming constant one-dimensional flow. Where these assumptions were met, predictions were generally good at shallow depths and for short times. At greater depths and longer times, these assumptions combined with other factors, such as changes in effective porosity, variable mineralogy, changing hydrodynamic dispersion coefficients, and variable infiltration rates contributed to the failure of the predicted values to match the observed data. It appears that sulfate distribution in the unsaturated zone during artificial recharge can be predicted by using easily derived laboratory data if the flow conditions in the field can be described adequately.

Petrography and Diagenesis of the Hosston Sandstone Reservoirs at Bassfield, Jefferson Davis County, Mississippi

Abstract: Bassfield Field, discovered in 1974, is a gas accumulation in Lower Cretaceous Hosston Formation sandstone reservoirs above a deep-seated salt structure. Since 1974, it has served the Industry as an analog for later Hosston discoveries. Production at Bassfield is obtained from the Booth and Harper sandstone members of the Hosston Formation, which consist of a network of fluvial channel sandstones. The coarser-grained lower parts of the sandstone are the principal reservoirs, with porosities ranging from 8 to 16 percent and averaging 12 percent, and permeabilities ranging from 2 to 300 md and averaging 20 md. The finer-grained upper parts of the sandstones are generally tight with little or no effective porosity or permeability. Secondary quartz cement and compaction through pressure solution of grains are the principal causes of porosity reduction. Early stages of localized dolomite cementation and scattered kaolinite cementation are also instrumental to some degree in reducing porosity. A minus-cement porosity exercise suggests that oil moved into the porous lower parts of channels when they had been buried to a depth of about 6,000 feet, which may have prevented further diagenesis. The upper, tighter parts of the sandstone bodies were not invaded by oil and cementation and compaction proceeded to near completion here. Bottom-hole temperatures in the formation approach 300°F (150°C), the approximate threshold temperature of the illite zone of burial metamorphism. Scanning electron microscopy reveals the presence of small amounts of illite apparently forming from kaolinite. Such illite could seriously impair Hosston reservoirs which have undergone temperatures in excess of those at Bassfield.

Porosity Loss in Sandstone by Ductile Grain Deformation During Compaction

Abstract: As exploration for hydrocarbons moves to greater depths, attention must be paid to the amount of porosity lost in sandstones by compaction. Deformation of ductile grains is the chief cause of porosity reduction during compaction except in nearly pure quartz sandstones. Shale clasts and micaceous rock fragments (argillite, slate, schist, and phyllite) are the chief ductile grains in sandstone, but glauconite and fecal pellets are locally abundant. The amount of porosity loss by deformation of these grains depends on their abundance and amount of overburden. It is common for sandstones to lose from 5 to 15% porosity this way. Some lithic arenites and glauconitic sands have lost their entire porosity during compaction. Knowledge of sandstone composition is helpful in predicting the amount of porosity lost by ductile-grain deformation.

Density-neutron Crossplot Analysis For Shaly Gas Sands Using Hand-carried Calculators

Abstract: A program for hand-carried calculators has been developed for shaly gas sand crossplot analysis. This program processes data obtained from a log suite consisting of FDC-CNL-GR and a deep-investigation resistivity log, and prints out values for effective porosity, shale content, and water saturation. The program uses an analytical method that describes the shaly-sand crossplot technique. This technique is capable of handling cases where the density porosity at the clay point is either positive or negative. The program executes the crossplot analysis accurately and quickly, taking less than 30 seconds for each data point.

Analysis of Pump Test Data for Partial Penetrating wells

Abstract: The solutions of unsteady phreatic flow toward a partially penetrating well in an aquifer of finite thickness are described. Firstly the solution for a confined aquifer is shown. In this case,three methods of analyzing field data with partially penetrating well are given, that is, "Log-Log Method, Log-Log Distance Drawdown Method and Jacob's Method Ajusted for Partial Penetration". By using these methods the hydraulic conductivities and the specific storage of the aquifer may be determined. Secondly the solution for an,unconfined aquifer is shown. In this case, also two methods of analyzing field data with partially penetrating well are given. By using these methods, the anisotropic permeability and the storage coefficient (effective porosity) of the aquifer may be determined. Moreover in each case, the effects of partial penetration are discussed and the limits of adapting the Theis' and Jacob's methods are setted. From these analytic results, some cosiderations are added to determine the anisotropy of permeability and to evaluate the storage coefficient.