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Weight %

Specific

Gravity

Volume %

Mud Density: lb/gal

Specific

Gravity

Weight %

Volume %

Mud Density: a/cm

Figure 11-1. Effect of specific gravity of weighting material on the solids con centration of weighted mud.

the surroundings. Consideration of these factors, along with chemical inertness and specific gravity, has made barite the only mineral now used in significant quantities in the United States to raise the density of muds.

Barite

Characteristics. Pure barite (barium sulfate, BaS04) contains 58.8% barium and has a specific gravity of 4.5. Commericial barite, sometimes called "heavy spar" or "tiff," is of lower specific gravity because other minerals (such as quartz, chert, calcite, anhydrite, celestite, and various silicates) are included. In addition, it ususally contains several iron minerals, some of which may increase the average specific gravity of the product.

Barite is virtually insoluble in water, and does not react with other components of the mud. Calcium sulfate, sometimes present as gypsum or anhydrite associated with barite, is objectionable as a contaminant of lightly-treated, fresh water muds. Sulfide minerals, such as pyrite and sphalerite, if present, may undergo oxidation with the formation of soluble salts that adversely affect the mud performance. The dark-gray-to-black barite produced from mines in Arkansas, California and Nevada contains a small amount of organic matter and gives off the odor of hydrogen sulfide when the ore is broken, but the odor does not persist in the finished product.

Barite occurs in many geological environments in sedimentary, igneous and metamorphic rock. Commercial deposits of barite occur as vein or cavity-filling deposits, residual deposits, and bedded deposits.

The United States is the world's leading producer of barite and, by far, the leading consumer. The oil drilling industry uses over 90% of domestic production and almost all of the imported ore. In recent years, Nevada has become the major source of barite (80% of the total); second in production is Missouri. Other states producing barite are Alaska, Arkansas, California, Georgia, Idaho, Illinois, Montana, and Tennessee. Barite is imported from many countries. The principal sources of imported barite are Peru, Ireland, Chile, Morocco, Mexico, and Thailand.

Because of the economic importance of barite, its occurrence and com-merical exploitation have been reported in numerous publications. Fifty-four references are listed by Brobst12 in his review of barium minerals.

Mining and Processing. After a barite deposit has been found, an extensive evaluation program must be carried out on the prospect. Samples are collected from trenches, test pits, and by coring. Beneficiation tests are conducted to make certain that an economically feasible product which meets specifications can be made. Numerous developmental and marketing factors must be considered in evaluating a barite deposit.

In addition to production from barite deposits, barite is also produced as a by-product in the exploitation of some other minerals, although not in significant amounts. Vein and cavity-filling deposits occur in limestone, dolomite, sandstone and shale, and in geologic ages from Precambrian to Tertiary. Residual deposits are formed by differential weathering of preexisting deposits, leaving lumps of barite in clay produced by weathering of limestone or dolomite. Commercially valuable bedded deposits of barite arc generally gray to black in color, and may extend for many acres, ranging in thickness up to more than 100 feet (30 m). The barite in these deposits is fine grained, with fine-grained quartz as the main impurity. Small amounts of clay minerals and pyrite are common.

Mining of barite may be open pit or underground, or both methods may be used on the same deposit (as at Magnet Cove, Arkansas). Open pit methods are usually less expensive than underground, and are used extensively in all types of deposits. In residual deposits, the barite-bearing residium is dug with power shovels and hauled by truck to the washer plant. Rotary breakers, using water at high pressure, separate the softer barite from the harder gangue, which is discarded. The barite and the remaining gangue move on to the log washers, where the clay is separated. A secondary rotary breaker or trommel screen discards more oversize gravel material, and the barite material goes to jigs for the final gravity separation of the barite from the remaining fine gravel gangue. The ore is then ready to be dried and ground,

Barite from some deposits (principally from veined and bedded deposits) cannot be separated from the gangue by jigging. If economically practical, the ore is beneficiated by wet grinding in ball mills. The barite is then concentrated by flotation, filtering and drying. Probably less than 10";, of the barite used in drilling mud in 1978. was produced by flotation.

Barite that meets API specifications13 must meet the requirements listed in Table 11-2.

Barite in Drilling Mud. In California, barite was first used in a well being redrilled with cable tools in 1923.14 According to the report, density of the mud was raised to 90 lb/ft3 (1.44 g/cm3) to control gas inflow and to stop caving. In the same report, another barite use—pulling of dry pipe is mentioned.

One other function of barite has developed—the preparation of a temporary high-density plug formed from a slurry of barite in water (2.65 SG). Such a slurry contains the maximum concentration of barite that is

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