Table 112 Barite Requirements for API Specification

Specific gravity: 4-20, minimum

Wet screen analysis:

Residue on U.S. Sieve (ASTM) no. 200: 3.0% maximum Residue on U.S. Sieve (ASTM) no. 325: 5.0% minimum

Soluble alkaline earth metals as calcium: 250 ppm, maximum

Courtesy API Specification for Oil-Well Drilling-Fluid Materials, API Spec. 13-A, seventh edition. Mav. 1979.

used—about 750 lb/bbl (2100 kg/m3). The minimum concentration of barite might be as low as 10 lb/bbl (28 kg/m3), although usually it would be substantially higher. The quantity of barite required to raise the density of a given volume of mud a specific amount can be readily calculated (as pointed out in Chapter 1) from the relation, in consistent units:


= original mud density PH = barite density Vo = original mud volume V« = barite volume v0 • Po + VB£>B

The quantity of barite consumed annually has increased each year since 1971, as shown in Fig. 11-2,3,13 Also shown in the figure are the number of wells and the total number of feet drilled each year from 1968 through 1978.16 A rough correlation is evident between barite consumption, number of wells, and total footage drilled, although the barite usage over the interval has increased more than might be anticipated from the drilling record.

iron oxides. Natural iron oxides (principally hematite, Fe203) were among the first materials used to increase the density of muds (see Chapter 2). The use of iron oxides was discontinued in the early 1940's because an ample supply of barite was available at a lower production cost. Furthermore, drilling crews objected to the staining of their skins and clothing by the dark red iron oxide. Now that the supply of good grade barite is dwindling, iron oxide is being increasingly used. It has the advantage of having a higher density than barite (5.1 for hematite compared to 2.5 for barite). The higher density enables faster rates of penetration to be obtained because the volume of weight material in a hematite mud is less than that in a barite mud of the same weight.228 22b It is much harder than barite (5-6 Mohs compared to 3 Mohs) which gives it the ad vantage that attrition is low, hence fewer fine particles are generated in the course of drilling and thus there is less increase in viscosity. On the other hand, it is much more abrasive than barite, and wear on pumps and bits may be severe, especially with water base muds. Abrasion is limited when iron oxides are used to increase the density of low grade barite, but the resulting abrasiveness should be checked in the laboratory using the API standardized procedure.22c

Other disadvantages of iron oxide are wetting and foaming arising from the surfactants used in processing the ore.22d e

A micaceous (specular) hematite, known as itabirite, from Brazil is a major source of iron oxide. It is called micaceous because it splits easily into thin, flat flakes, making it easily friable. Abrasion is minimized if a maximum particle size of 45 microns is specified. Thus limiting particle size has the additional advantage that 120 or 150 mesh screens can be used on the shakers without excessive loss of weight material.22f Field experience has shown that the use of finer screens results in faster rates of penetration because fewer drilled solids are incorporated into the system.

In Germany, an iron oxide weighting material is made from the residue of pyrite roasting process for sulfuric acid manufacture. The residue is quenched, neutralized, leached, and dried. The product is classified to a particle si/e below 75 microns and to a particle size distribution such that not more than 50% is below 10 microns.10 Advantages claimed for the product include: specific gravity of 4.7; low abrasion and low magnetic susceptibility compared to natural iron ores; 85% soluble in hydrochloric acid; and reactive to hydrogen sulfide with the formation of noncorrosive, insoluble iron poly sulfides.'0,21,22

As discussed under the subject of corrosion control in Chapter 9, an iron oxide, described as a synthetic iron oxide (mainly Fe304) is used as a scavenger of hydrogen sulfide.23'24 The material is ferromagnetic, and the particles (as shown by electron microscope) are spherical and porous. Although the product's cost precludes its use as a weighting material, the specific gravity (4.5 to 4.6) is adequate for such an application.

Siderite. Siderite consists of ferrous carbonate (FeC03). The mineral usually contains small amounts of iron oxides (with which it is found), dolomite, calcite and quartz. Siderite is readily soluble in hot hydrochloric acid, and it also dissolves in formic acid, a property considered desirable in completion muds. Present domestic production is from East Texas.

Siderite has a higher specific gravity (3.7 to 3.9) than does calcite. Consequently, a higher density mud having the same solids content can be made. Either water or oil muds can be weighted with siderite to 19 lb gal (2.28 g/cm3).25 Compositions suitable for workover muds and gravel placement can be made from brines, polymers, and mixtures of calcite and siderite,26 or siderite alone can be used as the weighting material. Tests on sandstone cores showed satisfactory return permeabilities after acidizing :5


Ilmenite, Fe.Ti02, SG 4.5 to 5.1, hardness 5 to 6 Mohs, promotes a fast drilling rate in the same manner as itabirite does and has the same advantages and disadvantages of hardness. Blomberg26® found that specifying that not more than 3 % of the particles be larger than 45 microns reduced abrasion to an acceptable level. Ilmenite reacts with sour gases, and is suitable for use in workover fluids because it is acid soluble. Supplies are abundant.

Calcium Carbonate

Calcium carbonate was proposed as a weighting material because the filter cake that forms on the productive formation can be removed by treatment with hydrochloric acid.27 Calcium carbonate is readily available as ground limestone or oyster shell.28 Calcium carbonate is dispersed in oil muds more readily than is barite. Its low specific gravity (2.6 to 2.8) limits the maximum density of the mud to about 12 lb/gal (1.4 g/cm3). Shell flour or ground limestone is frequently used in workover muds.

High-filtration slurries carrying graded marble or limestone particles in suspension have been found effective in overcoming loss of circulation2^ (see Chapter 9). The particle-size distribution of the crushed rock was designed to produce a pack of maximum density from the larger particles. The very small particles (-200 mesh) were eliminated in order to maintain a high filtration rate. The suspending slurry (consisting of water, salt, attapulgite, diatom-aceous earth, and barite) had a rapid filtration rate. The compositions of slurries ranging in density from 12 lb/gal (1.4 to 2.4 g/cm3) were devised from laboratory tests and field experience.

Consumption of limestone and oyster shell in 1978 drilling is estimated to have been 10,000 tons.


Galena, PbS, with a specific gravity of 7.4 to 7.7, is used only in preparation of the extremely heavy muds sometimes needed to control abnormally high pressures.30 Galena is expensive; consequently, barite is used with it in preparing muds to a density of about 30 lb/gal (3.6 g/cm3). Mud having a density of 32 lb/gal (3.8 g/cm3) can be prepared with galena alone as the weighting material. About 1,200 lb of galena is needed to make one barrel (3400 kg/m3) of such mud.

Galena is not a normal component of weighted muds. A supply of galena is maintained in the Gulf Coast area for use in an emergency.


The term clay is used in three ways: (1) as a diverse group of fine-grained crystalline minerals; (2) as a type of rock, and (3) as a particle-size term.31 Clay is a natural earthy, fine-grained material largely composed of crystalline substances that make up the group of clay minerals. The clay minerals are hydrous aluminum silicates usually containing alkalies, alkaline earths, and iron in appreciable quantities.

From the standpoint of drilling fluids technology, the behavior of clays in water, rather than their chemical composition as such, is of paramount importance. This behavior is critical to the successful drilling and completion of most wells. Consequently, the mineralogy and colloid chemical characteristics of clays have received detailed attention in Chapter 4. This section will be limited to the application of clays in drilling muds.

Prior to the introduction of the filter press as a means of evaluating mud performance, local surface clay deposits were extensively used for making mud. The extremely poor filtration characteristics shown by many of these products caused the abandonment of such deposits, while other "low yield" clays continued to be used for raising the density of muds to as high as 10.5 lb/gal (1.26 g/cm3). Laboratory and field studies in the mid 1950s began to show the objectionable effect of clay solids on drilling rate and this feature has led to the virtual elimination of so-called "drilling clays" from the list of mud components. Bentonite is the only commercial clay now used in significant amounts in fresh water muds.


Characteristics. The term bentonite was derived from the location of the first commercial deposit in the United States. In 1897, Knight32 reported that since 1888 William Taylor of Rock Creek, Wyoming, had been selling a "peculiar clay" found in the Fort Benton shale of Cretaceous age. Knight proposed the name "taylorite." On learning that the term "taylorite" was already in use, Knight33 suggested "bentonite" for the "soft, greasy to touch" clay that had been sold to eastern companies for uses unknown. Only $810 had been paid for 150 tons shipped in 1897, a reduction from the original price of $25 per ton. Uses (reported later34) had been as a component in toilet soap and, after leaching, as an adulterant in candy.

Geologists35'36 concluded that bentonite was formed by the devitrification and chemcial alteration of volcanic ash, and the mode of origin of the clay was included as part of the definition for this rock.37 Such a definition is inadequate because it excludes deposits of bentonite in many countries that did not originate from volcanic action.31 Thus, Grim and Niiven's definition is preferred: "Any clay which is composed dominantly of a smectite clay mineral, and whose physical properties are dictated by this clay mineral.' Grim and Nüven have described the worldwide geologic and geographic occurrence of bentonites, and have examined the variations in the mineral and chemical composition of the smectrites. Bentonite has also been defined as consisting of fine-grained clays that contain not less than 85"., montmorillonite.

In mud parlance, bentonite is classified as sodium bentonite or cu/ciiun bentonite, depending on the dominant exchangeable cation. Correspondingly, in terms of performance, bentonite is classed as "high yield" and "low yield," and in terms of geographic origin as "Western" and "Southern" because of the superior mud-making qualities of bentonite supplied from the Wyoming, South Dakota, and Montana deposits. This does not imply that these terms refer to distinct compositions.

Although Western bentonite is recognized as the highest quality clay for use in mud, the colloidal properties vary, even in the same deposit.' 4,1 Variations in properties within a deposit near Colony, Wyoming, as related to the thickness of the overburden, are shown in Fig. 11-3.40 The specific conductivity and the excess salts are highest at the weathered outcrop, as is the viscosity (yield) and gel development of the muds. The effect of drying temperature on the yield (bbl per ton of 15 cp mud) is also noticeable.

The heterogeneous nature of bentonite was shown by X-ray diffraction patterns and cation-exchange data for several samples of Wyoming bentonite separated into three fractions by centrifuging the suspensions.41 A correlation was observed between the plastic viscosity and gel strength properties as affected by the surface area and the exchangeable cation in centrifuged fractions.42 The relatively coarse fractions (least surface area) contained mainly calcium as exchangeable cation and showed distinctly lower viscosity and gel strength than the finer fractions in which sodium was dominant.

These laboratory studies have been cited to show the necessity for a bentonite supplier to conduct adequate sampling of deposits; maintain large stockpiles; assure thorough mixing of the stockpiles, and carefully control drying, in order to produce a consistent product .

Figure 11 -3. Variation in properties of bentonite with thickness of overburden. (From Williams, Elsley, and Weintritt.49)

Mining and Processing. The economic importance of bentonite has prompted state and federal geological surveys to publish numerous reports on its occurrence, its processing and its use. Only a sampling of these publications are cited here.43 44,45,46 47,48 The references listed in these papers and the more extensive reviews30,38,49 will provide additional sources of information.

After a potentially commercial bentonite deposit has been located, auger or core drills are used to collect samples for evaluation. If results are favorable, pits are laid out, with particular consideration given to quality and tonnage of recoverable bentonite, ratio of required over-burden removal to recoverable bentonite, drainage, and reclamation.

Table 11-3

Bentonite Requirements for API Specification

Moisture, as shipped from point of manufacture:

Wet screen analysis, residue on U.S. Sieve (ASTM) no. 200:

lö/ft maximum 4% maximum

Properties of a suspension of 22.5 g of bentonite (as received) in 350 cm3 of distilled water; stirred 20 minutes; allowed to stand overnight; restirred 5 minutes.

Viscometer dial reading at 600 rprn: 30 minimum

Yield point, lb/100 ft2: 3 x plastic viscosity, maximum

I ilirate: 13.5 cm3, maximum

Tests to be made as stated in API RP-13B, "Standard Procedure for Testing Drilling

* Courtesy API Sped/nation for Oil-Well Drilling-Fluid Materials, API Spec. 13-A, seventh edition. May, 1979.

Minable bentonite beds vary in thickness from a minimum of two feet. The maximum stripping depth is about sixty feet. After the overburden has been removed, the bed may be resampled on closer spacing. Based on test results, the pit is marked for selective mining.

A common practice is to expose the clay to air for several months, during which time the bed may be plowed or ripped. This practice promotes drying and improves the quality of the clay.

The bentonite areas of differing quality are mined separately and hauled to stockpiles located at the processing plant. From the stockpiles, the selected bentonite is passed through a slicer or cutter for sizing, and then into a dryer where the moisture content is reduced from 15 -35% to 8 10%. The dried bentonite is ground in roller mills. Based on the results of performance tests on samples of the clay, small amounts of polyacrylates (maximum of 2 lb/ton) may be added to the mill feed. Cyclone collectors extract the minus-200-mesh product, which is transferred to storage in silos pending bagging or loading into hopper cars.

Bentonite furnished to API specifications13 must satisfy the requirements listed in Table 11-3.

Bentonite in Drilling Mud. Bentonite is added to fresh water or to freshwater muds for one or more of the following purposes: (1) to increase hole cleaning capability; (2) to reduce water seepage or filtration into permeable formations; (3) to form a thin filter cake of low permeability; (4) to promote


hole stability in poorly cemented formations, and (5) to avoid or overcome loss of circulation. The amount to be added will, of course, vary with specific conditions but approximate quantities are suggested in Table 11-4.

Bentonite continues to rank second in quantity only to barite as a mud additive, as shown in Fig. 11-2.14 In spite of the partial replacement of bentonite by polymers in some muds, about 1,160,000 short tons (1,050,000 tonnes) of swelling-type bentonite, with a well-site cost of roughly $120,000,000, was used in 1978.

Southern Bentonite. Bentonite in which calcium predominates as the exchangeable cation is mined in Texas, Mississippi, Alabama, Oklahoma and Louisiana.30 As much as 30,000 to 40,000 tons per year of bentonite mined in Texas is treated with sodium carbonate50 and polyacrylates51 to meet API specifications. About an equal quantity of Southern bentonite treated with sodium carbonate is sold as "drilling clay" and is largely used only in drilling operations near the source of the clay.

Beneficiated, Super-Yield Bentonite. Terms such as peptized, bénéficia ted and extra-high yield describe bentonites to which organic polymers (¡and sometimes also soda ash) have been added during processing. These products

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