Graveling

Well completions are usually tailored to individual wells, and many variations exist. The following diagrams show a completion with a gravel pack, designed to exclude sand production downhole, and a dual completion, designed to allow controlled production from two separate reservoirs. 

Figure 9.15 Gravel pack completion and dual completion...

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... 

The bare foot completion, which leaves an open hole section below the previous casing, is cheap, simple and suitable for consolidated formations which have little tendency to collapse. The slotted liner s an uncemented section of casing with small intermittent slots cut along its length, which prevents the hole from collapsing, but allows no selectivity of the interval which will be produced. The cased and cemented horizontal completion does allow a choice of which intervals will be perforated and produced. None of these examples provides any effective sand exclusion it this is required a gravel pack or a pre-packed liner can be used. 

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. 

Several types of aggregate-bed filters are available which provide in-depth filtration. Both gravel and particle-bed filters have been developed for removal of dry particulates but have not been used extensively. Filters have also been developed using a porous ceramic or porous metal filter surface. 

Grades, Quality Control. Fluorspar is marketed ia several grades metallurgical fluorspar (metspar) is sold as gravel, lump, or briquettes. The minimum acceptable assay is 60% effective calcium fluoride. The effective value is determined by subtracting from the contained calcium fluoride 2.5% for every percent of Si02 found ia the complete analysis apparently based on the following stoichiometry (1) ... 

Fig. 1. Global distribution of seabed mineral deposits, where x represents chromite + barite titanium, zirconium, hafnium, and thorium tin I gold, platinum, and silver 3 sand and gravel shell, calcium carbonate gems marine polymetaUic sulfides phosphorites Cl cobalt cmsts S sulfur and B...

Paring is the surface preparation in a processing faciUty which may be concrete, asphalt, gravel, cmshed shell, or brick. 

The nitrate deposits are made up of several layers (Fig. 1). The ore bodies are very heterogeneous and variable in size, thickness, composition, and hardness. The overburden may include chuca a layer of unconsoHdated sand, silt, and clay, andpanqueque a layer of semiconsoHdated and porous material poody cemented by salts over poody cemented gravel. The ore composition has degraded considerably since the eady days of the industry, when it was reported that ores of up to 50% sodium nitrate were mined. There are stiU reserves that can be commercially mined well into the twenty-first century (1). 

Fig. 1. Schematic of a nitrate ore bed, detailing the various overburden layers. The overburden thickness can vary from 0 to 2.5 m, where chuca = 0.1 0.5 m, panqueque = 0.1 0.4 m, and poody cemented gravel = 1.5-2.5 m. CaUche ranges from 0.8 to 8 m.

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