Sand grains

The main component of sandstone reservoirs ( siliciclastic reservoirs ) is quartz (Si02). Chemically it is a fairly stable mineral which is not easily altered by changes in pressure, temperature or acidity of pore fluids. Sandstone reservoirs form after the sand grains have been transported over large distances and have deposited in particular environments of deposition. 

The pores between the rock components, e.g. the sand grains in a sandstone reservoir, will initially be filled with the pore water. The migrating hydrocarbons will displace the water and thus gradually fill the reservoir. For a reservoir to be effective, the pores need to be in communication to allow migration, and also need to allow flow towards the borehole once a well is drilled into the structure. The pore space is referred to as porosity in oil field terms. Permeability measures the ability of a rock to allow fluid flow through its pore system. A reservoir rock which has some porosity but too low a permeability to allow fluid flow is termed tight . 

In the internal gravel pack shown, carefully sorted sand grains, called gravel, are placed between a wire wrapped screen and the perforations with the objective of stopping... 

Grain size distributions for sediments and soils are used to determine the amount of sand, silt, and clay present in a sample. For example, a grain size of 2 mm serves as a boundary between gravel and sand. Grain size boundaries for sand-silt and silt-clay are given as 1/16 mm and 1/256 mm, respectively. 

Mechanical attrition is used to remove most of the spent binder. First, dry attrition or abrasion processes crush lumps to grain size. Mechanical abrasion is then used to separate the binder from the sand grains. Sometimes, sand is pneumatically propelled against a metal target plate. The impact of the sand on the plate scrubs off the clay and resin coating from the sand grains. Fines are separated and removed by dry classification. 

Another method of sand control is use of a silicon halide which reacts with water at the surface of sand grains forms SiO which can bond the grains together (55). Reducing the cost of sand consolidation could be very useful since the applicability of gravel packing methods is limited by the bottom hole circulating temperature and the limited temperature stability of polysaccharide polymers. 

Figure 1. Schematic of the bubble-flow regime in porous media. Open space corresponds to bubbles, dotted space is the aqueous surfactant solution, and cross-hatched areas are sand grains.

Dunes and sand plains form where strong winds carry sand grains in saltation over short distances. Particles finer than sand are transported in suspension and over greater distances until they settle as loess , predominantly in the steppe regions adjacent to the desert zone. 

Wind transport. Wind-blown components are carried away over a more or less important distance as a function of wind velocity and particle size of the material. Wind speeds up till 6.5 m/sec transport dust and fine sand with a diameter of less than 0.25 mm sand grains up to 1 mm diameter are uplifted at wind speeds of 10 m/sec. At 20 m/sec also particles of 4-5 mm may be removed. Based on these physical laws, the transportation of coarse fragments, in casu the sand fraction, occurs over-relatively short distances from the deflation zones. These sand grains settle then in more or less continuous layers and either become progressively mixed with the underlying soil layers, or concentrate in dune formations. 

This special form of granular iron hydroxide consists of sand grains coated with iron hydroxides. This material has very good mechanical properties and a high content of the active phase iron hydroxide (RFA 56 % Fe203). 

Figure 4. SEM photomicrograph of characteristic surfaces of sand grains from a Venezuelan soil profile. Samples from a) 90 cm deep, b) 40 cm deep. (Scale bar 2.5 microns).

Some green clay minerals occur as ovoids, probably having formed within fecal pellets or casts of forams. They also occur as films or stains on shells, sand grains, and phosphate nodules. They are finmd in highest density in sediments of the outer continental shelves and slopes where waters are shallow (20 to 700 m) and mildly suboxic. Slow sedimentation rates are necessary to prevent burial as green clays form at very slow rates. A notable example are the green muds found on the Blake Plateau. 

Almeida MB, Alvarez AM, de Miguel EM, del Hoyo ES (1983) Setchenow coefficients for naphtols by distribution method. Can J Chem 61 244-248 Amirtharajah A, Raveendran P (1993) Detachment of colloids from sediments and sand grains. Coll Surfaces A, 73 211-227... 

A method of coating Si02 sand grains with various Fe oxides for use in percolation experiments, was developed by Scheidegger et al. (1993) (cf Schwertmann and Cornell, 2000). 

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