Showing papers on "Effective porosity published in 1979"

Statistical properties of etched nuclear tracks

Abstract: Statistical distributions of quadratic holes of uniform shape on a planar surface are investigated Analytical calculations give expressions for the effective porosity, and the probability and areal dispersion of multiple holes of multiplicity n ≤ 2 as function of nominal porosity Computer simulations give a simple expression for the effective porosity due to hole overlap, further the probability of multiple holes of multiplicity n ≲ 50, and the areal dispersion of holes of multiplicity n ≲ 7, taking into account closed rings of multiple holes forming islands The results apply directly to nuclear tracks of rhom-boidal cross section as found in mica They give a good approximation for convex holes with inversion symmetry as eg circular holes They can be interpreted in terms of the multiple hit theory in radiation biology The range of porosity investigated reaches beyond the realm of mechanically stable nuclear track filters into the realm relevant for surface micromachining

Secondary Reservoir Porosity in the Course of Sandstone Diagenesis

Abstract: Secondary porosity plays an important role in the diagenesis of sandstones. The volume of secondary porosity equals or exceeds that of primary porosity in the sandstones of many sedimentary basins worldwide and a significant percentage of the world's reserves of natural gas and crude oil are contained in secondary sandstone porosity. The main geological and economic significance of secondary sandstones porosity is that it extends the depth range for effective sandstone porosity far below the depth limit for effective primary porosity. Generation and primary migration of hydrocarbons occurs mainly below the range of effective primary porosity, and therefore the path of primary migration and the site of accumulation of hydrocarbons are commonly controlled by the distribution of secondary porosity. In this paper, secondary porosity in sandstone diagenesis is discussed in detail. Genetic-textural classes of secondary sandstone porosity are given along with recognitition criteria, geological occurrence and diagenetic origins of secondary porosity.

The Central Role Of Qv And Formation-water Salinity In The Evaluation Of Shaly Formations*

Abstract: A new evaluation approach, covering both the fluid saturation and producibility aspects of shaly formations, is presented. The approach is based on the combined use of the total and effective porosity systems linked via the parameters Q/sub v/ (cation-exchange capacity per unit total pore volume) and formation-water salinity. The quantity Q/sub v/ is calculated from the FDC/CNL on the basis of clay-mineralogical data obtained from cores or sidewall samples. Q/sub v/ is then used to calculate both the total water saturation, via the Waxman-Smits equation, and the amount of clay-bound water (CBW), via the Hill, Shirley and Klein equation. The latter parameter, in turn, facilitates estimation of the effective porosity and, if desired, the effective water saturation, correction of air-mercury capillary-pressure curves for CBW, conversion of air permeabilities into brine permeabilities. In this approach all input parameters required for the evaluation are obtained from the interval under study; the use of clay parameters obtained from adjacent shale beds (with all inherent uncertainties of such approaches) is thus avoided. Furthermore, all quantities playing a role in the evaluation are interdependent via the parameters Q/sub v/ and formation-water salinity. This enhances internal consistency and reliability of the results obtained.

Importance of secondary leached porosity in lower Tertiary sandstone reservoirs along the Texas Gulf Coast

Abstract: Secondary leached porosity is common to dominant in near surface to deep subsurface lower Tertiary sandstone reservoirs along the Texas Gulf Coast. This secondary porosity is in the form of leached feldspar grains, volcanic rock fragments, carbonate cements, and carbonate-replaced grains. Leached porosity occurs in sandstones with compositions ranging from volcanic litharenite and lithic arkose to quartzose sublitharenite and quartzose subarkose. A generalized diagenetic sequence indicates that leaching is a multi-staged phenomenon occurring at or near surface, at burial depths of 4000 to 6000 ft, and at burial depths of 7000 to 10,000 ft. Feldspar grains are dissolved during the first stage, whereas grains, cements, and replacement products are dissolved during the last two stages. Intensity of leaching in each stage varies in different formations and in different areas. Plots of secondary porosity as a percent of total porosity versus burial depth show that secondary porosity is dominant beneath 10,000 ft, ranging from 50 to 100 percent of total porosity. Above 10,000 ft more than half the samples have secondary porosity as the dominant type. Similarly, individual plots for the Wilcox, Yegua, Vicksburgs, and Frio sandstones all demonstrate the predominance of secondary leached porosity. Primary porosity is destroyed by compaction and cementation with increasing depth of burial. If this were the only porosity type, no deep, high-quality reservoirs would exist. Leaching, however, restores reservoir quality after primary porosity has been reduced. Most productive lower Tertiary sandstone reservoirs, especially deep reservoirs, along the Texas Gulf Coast exist only because of secondary leached porosity.

Importance of Secondary Porosity in Sandstones to Hydrocarbon Exploration: ABSTRACT

Abstract: Terrigenous sandstones in many basins owe their reservoir quality to secondary porosity that developed by the dissolution of detrital framework grains (chiefly feldspar) and cement minerals (chiefly calcite and evaporite minerals). This dissolution event is responsible for changing tight sandstones to porous and permeable sandstones slightly prior to the major episode of hydrocarbon migration. Dissolution of most non-evaporite minerals is accomplished by formation water containing carbon dioxide generated during the thermal or bacterial breakdown of hydrocarbons. Dissolution porosity is commonly well developed at 6,000 to 9,000 ft (1,829 to 2,743 m), but gradually is lost during deeper burial stages by recementation (chiefly by ferroan calcite, ferroan dolomite, and kaoli ite). During sandstone burial, variations in the simple scheme of cementation ^rarr decementation ^rarr recementation is complicated in basins with complex "plumbing" systems and those that experience uplift. For example, dissolution porosity in some uplifted sandstones develops during invasion by meteoric water flowing downdip. Dissolution porosity in sandstones can be suspected from certain log responses and water-saturation characteristics, but is best identified from clues visible in thin sections made of dyed, epoxy-impregnated perm plugs. Clues to secondary porosity in thin section include: (1) oversized pores formed where framework grains have been dissolved; (2) patchy distribution of carbonate or evaporite cement; (3) honeycombed feldspar grains; (4) fossil molds; (5) grains whose margins were etched by previous cement; (6) broken silicate grains that formed when rapid compaction followed removal of cement minerals; and (7) quartz grains that have been reduced to shards when calcite, which invaded quartz along hairline fractures during cementation, was dissolved. Secondary porosity is not likely to d velop good reservoir quality in sandstones whose primary porosity was lost chiefly by compaction. Sandstones with abundant clay clasts, fecal pellets, glauconite, or micaceous rock fragments can lose all effective porosity by ductile grain deformation. A knowledge of sandstone composition is important to predicting reservoir quality. End_of_Article - Last_Page 747------------

An analysis of the pressure transient testing of a man-made fractured geothermal reservoir *

Abstract: Pressure-transient testing of a hydraulically fractured geothermal reservoir in low-permeability crystalline basement rock has involved constant rate injection and pressure buildup tests under a wide range of field conditions for a number of fractured regions. Following conventional reservoir analysis methods, data are treated in terms of a transient diffusion equation that relates fluid flow and pressure levels in the main fracture system, associated joints, and the matrix permeability. Pressure-flow data are compared to type curve solutions of the diffusion equation for various flow geometries. The following points are considered in detail: (1) the limits on the fracture geometry, aperture and diffusing areas as determined from the diffusion parameters; (2) the parameters (flow impedance, diffusivity) of the flow-through systems are related to those governing the pressure inflation of the main fractures; (3) the relationship of the rock properties to the reservoir compressibility, effective porosity and permeability are discussed. In particular, laboratory experiments show that the flow properties of all sizes of cracks from large single fractures to the microstructure are pressure dependent if the fluid pressure is near the confining stress; and (4) the competition of flow into the various types of porosity (main fractures, joints, and microstructure) and the effectmore » on the interpretation of type curves are discussed.« less