Porosity, limestone

Zero-Porosity Limestone Zero-Porosity Dolomite... 

Rasmus (1983) states that the ratio of fracture porosity to total porosity is critical for the amount of resistivity decrease in low porosity carlxMiates. For fracture porosity, m values are less than the common matrix value of 2.0 in a low porosity limestone and can approach values as low as 1.0. If a secondary vuggy porosity is present without any fractures, a resultant m value of at least 2.0 is recommended. Depending on the amount of non-connected vug porosity, Rasmus (1983) recommends values exceeding 2.0. 

There is considerable variance in the porosity of limestones, thus the bulk densities generally are 2000 2800 kg/m (125-175 Ib/ft ). The density of some chalk limestones are even less. Dolomitic stones average 2—3% higher densities than high calcium ones. Depending on the physical size of the quicklime particles and their divergent porosities, bulk densities are 770 1120 kg/m (48-70 Ib/ft ), and densities of their hydrates 400 640 kg/m ... 

Dolomitization is the process by which limestone (CaCO,) is transformed into dolomite CaMg (CO,). During the transformation (which occurs under pressure), crystal reorientation occurs, which results in porosity in dolomite. 

The typical value of porosity for a clean, consolidated, and reasonably uniform sand is 20%. The carbonate rocks (limestone and dolomite) normally exhibit lower values, e.g., 6-8%. These are approximate values and do not fit all situations. The principal factors that complicate intergranular porosity magnitudes are uniformity of grain size, degree of cementation, packing of the grains, and particle shape. 

A high neutron porosity and low density porosity occur in the shale zones 2256-2274 and 2356-2388 ft. In the clean zone, around 2280-2290 ft, the cleanest part reads = 18% and = 13%. Plotting the point in the CNL chart of Figure 4-303, the rock matrix appears to be a dolomitized limestone and the true porosity is 17%. 

CNLcor, NEUTRON POROSITY INDEX (pu) (Apparent Limestone Porosity)... 

Filter. A filter is used in some cases when coagulation and sedimentation do not completely separate the solids from the liquid waste in areas where sand and sandstone formations are susceptible to plugging. Filters with a series of metal screens coated with diatomaceous earth or cartridge filters are typically used.27 Where limestone formations with high solution porosity are used for injection, filtration is usually not required. 

Other wells in the same field had shown a limestone reservoir of fair permeability, with major tectonic faults and some associated fissures. Well data were provided by the operator. The low value of the wellbore storage demonstrates no coupling of the well with secondary porosity. However, the operator suspected the presence of some kind of fault not far from Well B, whose production test was unusual. He decided to acidize in an attempt to establish communication. 

We then pick up the fluid from the previous step as a reactant and define a system representing the limestone and its pore fluid. We specify that the rock contains 3000 cm3 of calcite, implying a porosity of about 25% since the extent of the system is 1 kg (about 1 liter) of fluid. The pore fluid is similar to the reactant fluid, except that it contains less magnesium. The procedure is... 

Properties and extraction processes Tight-formation gas is natural gas trapped in low-porosity (7 to 12%), low-permeability reservoirs with an average in-situ permeability of less than 0.1 millidarcy (mD), regardless of the type of the reservoir rock tight gas usually comprises gas from tight sands (i.e., from sandstone or limestone reservoirs) and shale gas. Sometimes tight gas also comprises natural gas from coal and deep gas from reservoirs below 4500 m. Shale gas is produced from reservoirs predominantly composed of shale rather than from more conventional sandstone or limestone reservoirs a particularity of shale gas is that gas shales are often... 

Limestones vary greatly in color and texture, the latter ranging front dense and hard limestone, e.g.. marble or travertine, which can be suwed and polished, to soft, friable forms, e.g., chalk and marl. Chalk is a very fine-grained white limestone, while marl is an impure deposition product that contains clay and sand. Texture, hardness, and porosity appear to be functions of the degree of cementation and consolidation during the formation of these materials. Color variations arise from the presence of impurities. Some impurities, such as sulfur and phosphorus, make limestone unattractive for metallurgical uses. 

Lens-Type Traps. These form in limestone and sand. In this type of trap the reservoir is sealed in its upper regions by abrupt changes in the amount of connected pore space within a formation. A trap formed in sand is shown in Fig, 7(a). An example is the Burbank Field in Osage County, Oklahoma. This type of trap may occur in sandstones where irregular deposition of sand and shale occurred at the time the formation was laid down. In these cases, oil is confined within the porous parts of the rock hy the nonporous parts of rock surrounding it. A lens-type trap formed in limestone is shown in Fig. 7(b). In limestone formations there are frequent areas of high porosity with a tendency to form traps. Examples of limestone reservoirs of this type are found in the limestone fields of West Texas. 

Secondary porosity results from the dissolution of carbonates in the subsurface environment. It can occur both in limestones and in sandstones where carbonate cements of original labile detrital minerals are dissolved. Because the formation of secondary porosity can substantially enhance the reservoir properties of sediments, it has received considerable attention from the petroleum industry. 

Cement stratigraphy" Many ancient limestones contain little porosity... 

Martin G.D., Wilkinson B.H. and Lohmann K.C. (1986) The role of skeletal porosity in aragonite neomorphism — Strombus and Montastrea from the Pleistocene Key Largo Limestone, Florida. J. Sediment. Petrol. 56, 194-203. 

The decrease in molar volume during calcination of CaC03 to CaO has the effect of increasing the porosity of the sorbent which is important to the efficient utilization of limestone. The gas-solid reaction between S02 and limestone sorbent consists of a number of steps (2.) diffusion of gaseous S02 through the pores of the calcined limestone, reaction of 02 with CaO to form calcium sulfate (CaSO,), and diffusion of S02 through the calcium sulfate product layer to react with additional CaO in the particle. Unfortunately, the sulfation reaction ... 

The decrease in molar volume associated with the conversion of MgC03 to MgO Increases the porosity of the stone. Although the MgO is relatively inert toward reaction with S02, the increased porosity of the stone provides a greater surface area of CaC03 for the direct reaction with S02 described by Eq. (3). There is an optimun temperature for maximum sulfur retention in the range of 800-850 C which is more pronounced for limestone than it is for dolomite (15,19-21). 

Water is stored in rocks mainly in pores. The effective porosity of a rock is the volume percentage of the rock that may contain water in pores. The values (Table 2.1) are high for nonconsolidated granular rocks—soils, clays, silts, and gravels. The porosity is low for crystallized rocks such as limestone, dolomite, and most igneous rocks. Movement of water through... 

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