Thinners Mud Conditioning Agents

The substances discussed in this section have traditionally been called thinners. Thinners are added to mud to reduce flow resistance and gel development, and that was the original purpose in applying these materials. Experience has shown, however, that some of these so-called thinners perform other important functions, frequently of greater significance than improving the flow properties of the mud. Specifically, some of these substances are used to reduce filtration and cake thickness, to counteract the effects of salts, to minimize the effect of water on the formations drilled, to emulsify oil in water, and to stabilize mud properties at elevated temperatures. Thus the term mud-conditioning agent is often more appropriate than thinner.

Thinners typically have a relatively large anionic component which is adsorbed on positive sites of the clay particles, thereby reducing the attractive forces between the particles without affecting the hydration of the clay. The mechanism of thinner action was more fully discussed in Chapter 4 in dealing with the subject of flocculation/defloceulation.

Materials commonly used as thinners for clay-water muds can be broadly classified as: (1) plant tannins, (2) polyphosphates, (3) lignitic materials, and (4) lignosulfonates. Other important functions of these substances also will be reviewed in this section.

Tannins

Composition and Source. The word tannins is a collective term for a group of complex astringent substances, made up of carbon, hydrogen and oxygen, and, in some cases, containing small amounts of nitrogen and phosphorous. Characteristically, they form insoluble compounds with gelatin and gelatin-yielding tissues—their principal use is to convert skin into leather. They are very weak acids. Their molecular weight is in the range of 300 to 1,000. They are readily soluble in alkalies and are precipitated by heavy metals.

Tannins are broadly classified chemically as hydrolyzable tannins, consisting of esters of one or more polyphenolic acids, such as gallic acid, or condensed tannins, consisting of phenolic nucleii from which catechol or protocatechuic acid can be derived. The hydrolyzable tannins are also called gallotannins, or pyrogallol tannins, and the condensed tannins are called flavotannins, or catechol tannins.

Tannins occur in many plants, and are extracted from bark, wood or fruit. The commercial sources are limited by tannin content and availability. Sources include the barks of wattle, mangrove and eucalyptus, the woods of quebracho and chestnut, and the fruits of myrabolans and divi-divi. At present, the only tannin material used as a mud thinner in significant quantity in the U.S. is quebracho extract.

Quebracho. Quebracho extract was one of the first mud thinners used in the United States1 77 ,1 78 and at one time was an important component of red muds and lime muds described in Chapter 2.

Several factors led to a decline in the use of quebracho. World War II restricted the supply and caused a search for domestic substitutes.179 The introduction of calcium lignosulfonates180 decreased the need for quebracho in lime muds, and the almost complete replacement of lime muds by muds treated with chromelignosulfonate181 .brought about a major reduction in quebracho usage in the late 1950's.

Quebracho tannin is extracted by hot water from the wood of certain dense hardwood trees (quebracho means "axe breaker") of the genus Schinopsis which grow in northern Argentina and western Paraguay. The red wood, containing 15-25% tannin, is chipped, and extracted with water at a temperature of about 260° F (125 C). The water is then removed in evaporators and by spray drying. The finished product is ground and bagged.

Quebracho contains 60-65% tannin, as determined by the hide-powder test. It is classed as a catechol tannin.

Quebracho does not dissolve readily in cold water. Tannin is usually dissolved with caustic soda in proportions ranging from equal parts by weight to 5 parts of tannin to 1 part caustic soda. A dried reaction product which dissolves easily has been prepared.182 To prevent caking and to reduce the cost, quebracho may be ground with dry lignite.

Current use of quebracho as a thinner is limited to shallow holes and fresh water muds to which it is added as a solution in caustic soda. Quebracho alone may be added to counteract cement contamination. Concentration ranges between 0.5 and 2 lb/bbl (1.5 -6 kg/m3).

Modified Tannins. Quebracho can be made cold-water soluble by treatment with sodium bisulfite. Further modifications can be made to produce metal complexes of sulfoalkylated tannins that are easily dissolved in water, and are more effective in thinning and in reducing filtration than is the original tannin.1 83,1 84,185 A typical method of preparing sulfomethylated tannin involves adding formaldehyde and sodium bisulfite to a solution of quebracho and caustic soda, and heating the mixture. The reaction product may be recovered by evaporating the water, or it may be further reacted in solution with chromium chloride to form the chromium complex before drying the product. The dried sulfomethylated quebracho may be mixed with a dry chromium salt, such as sodium chromate, and the resulting mixture added to the mud. Improved heat stability is claimed for the mixture.184

Consumption of quebracho tannin, both as extract and as modified product, is estimated at 2,000 tons in 1978.

Polyphosphates

Characteristics. The term polyphosphates is applied to those phosphates in which two or more phosphorous atoms are joined together by oxygen atoms, for example, sodium tetraphosphate,

Na Na Na Na o o o o till NaO — P — O— P --O- P — O— P ONa

Polyphosphates may be crystalline or glassy. The glassy polyphosphates are amorphous mixtures of sodium polyphosphates with varying chain lengths, and the formula can be expressed as the ratio of Na20 / P2Os. The general formula for the crystalline polyphosphates is (Na,H)n + 2Pn03n+

The sodium polyphosphates are produced by heating hydrogen-containing orthophosphates to specific temperatures for the removal of water, or by melting together proper proportions of the anhydrous components. Crystalline polyphosphates are formed in the temperature range of 300°F to 1,150°F (150°C to 620°C). Glassy polyphosphates are fused at higher temperatures and are rapidly chilled. The products are crushed, producing a coarse white powder.

The sodium polyphosphates are very effective deflocculants for clays in fresh water and were among the first thinners for mud186,1871KH,18g (see Chapter 2). They are not effective in salty muds ( > 10,000 ppm chloride). The glassy polyphosphates effectively soften hard water by forming soluble complexes with calcium and magnesium ions. This action, called sequestration, is applied in dispersing bentonite for filtration reduction. The reversion of the polyphosphates to orthophosphates, which may cause thickening of the mud, occurs rapidly as the temperature approaches the boiling point of water. This reversion limits the use of polyphosphates to relatively shallow drilling. Another factor that has caused a marked decrease in the use of polyphosphates in shale drilling is their tendency to promote disintegration and dispersion of shale cuttings, thereby increasing the solid content of the mud.

Commercial Products. Three products are now being sold as thinners: sodium acid pyrophosphate, sodium tetraphosphate, and sodium "hexa-metaphosphate1'.

Sodium acid pyrophosphate, SAPP, Na2H2P207, can be made by heating sodium dihydrogen orthophosphate (thus, 2NaH2P04-+Na2H2P207 + H20). A solution of SAPP has a pH of about 4.2 and consequently is effective in overcoming cement contamination of fresh water muds.

Sodium tetraphosphate, Na6P4013, can be made from the orthophosphates by heating (2Na2HP04 + 2NaH2P04-+Na6P4013 + 3H20) or by the reaction of soda ash and phosphoric acid, in the ratio of 3Na20 /' 2P2Os, and rapidly cooling the melt. Sodium tetraphosphate is the most frequently used polyphosphate. Its solution has a pH of about 7.5.

Sodium "hexametaphosphate(NaPO,)6, is a glass, not a definite compound, having the ratio of approximately 1 Na.,0/ 1 P.,05. It can be prepared by fusion and rapid cooling of the melt (NaH2P04-+NaP03 f HjO). Its solution has a pH of about 7.

The sodium polyphosphates are normally used in concentrations of 0.1 to 1 lb/bbl (0.3 to 3 kg/m3). Estimated consumption in 1978 was about 1,800 tons.

Organic Phosphates and Phosphonates. A deflocculating agent composed of monodibutyl orthophosphate and similar substances, or di-isooctyl acid pyrophosphate, has been proposed as a means of dispersing siliceous materials.1"0

Phosphonic acid compounds have been tested as mud thinners. Alkylene diphosphonic acids or salts (for example, tetrasodium methylene diphos-phonate) compare favorably with the polyphosphates.191 A product that is more stable to temperature and pH changes than are the polyphosphates is a related amino phosphate, penta sodium amino tri (methyl phosphonate), having the formula N-(CH2P03Na2)2(CH2P03H Na).192 This substance has had limited field use.

Lignite: Leonardite

Characteristics. Variously called lignite, leonardite, mined lignin, brown coal, and slack, this substance has become an important drilling fluids constituent. In 1947, it was introduced as a partial substitute for quebracho extract.0 As limitations were placed on quebracho imports,1"0 lignite use increased. Lignite is no longer regarded simply as a thinner for water muds, but now serves for filtration reduction, oil emulsification, and stabilization of properties against high temperature effects. Additionally, it is a raw material from which products applicable in both water muds and oil muds are made.

The terms lignite and brown coal are often used interchangeably for low heat valve coals (less than 8,300 Btu/lb, 4,610 K cal/kg).

Leonardite is a naturally oxidized lignite, resulting from prolonged weathering. The substance was named for Dr. A. G. Leonard, North Dakota's first state geologist, a pioneer in the study of lignite deposits.19 3 1" + Leonardite is not a single mineral species. The name is applied to the lignite oxidation products having a high content of humic acids.105,10c',]°"

Humic acids are closely related phenolic, carboxylic compounds, probably associated by hydrogen bonding. Molecular weights range from 300 to 4,000. The term humic acid is sometimes broadly applied to soil organic matter which is soluble in alkalies and insoluble in mineral acids and alcohol. Humic acids have been studied both as important soil constituents and as intermediate substances in the conversion of vegetation to coal. Because of the complexity of the source materials, it is not surprising that the details of the structure of humic acid have not been agreed upon.198,199 One of the possible models is based on a hydroxyquinone structure.200

Leonardite is the lignitic material used in drilling muds. Distinguishing features are the higher oxygen and moisture contents of leonardite than lignite. In the deposits, three grades of leonardite are recognized.19 5 These are: (1) A mixture of lignite and leonardite, containing about 45% humic acids, occurring at broken seam tops and along stream courses; (2) black, colloidal material, swelling in water, identified in Dana's System of Mineralogy as "native humus acid," containing about 80% humic acid, and (3) fine grained, reprecipitated "calcium humates," intermixed with gypsum, containing about 10% humic acids. Moisture content of leonardite in the deposit ranges from 30% to 60%. Solubility in alkalies can be measured as an indication of available humic acid content.

Lignite (and leonardite) contains the pollen and spores of the flora in existence at the time of its deposition. Consequently, polynologists examining drill cuttings from which lignite-treated mud may not have been completely removed must take care to avoid confusion as to the origin of the specimens.201

Source and Preparation. North Dakota is the principal source of leonardite used as a mud additive. Minor sources include lignite deposits in South Dakota, Montana, New Mexico, and Texas. The numerous reports of state and federal geological surveys on lignite deposits are examples of available information.

As a preliminary to mining, a deposit is sampled, usually with an auger drill. A simple field test for humic acid content can be made by comparing the color of a caustic soda extract of the sample with the color of a reference solution. The overburden, usually only a few feet thick, is removed, and the soft, moist leonardite is moved to stockpiles where some drying, and possibly some oxidation, takes place. The moisture content is then reduced to 15% to 20% by passing the material through a drier; after drying, the product is crushed and bagged.

Lignite (Leonardite) and Derivatives in Drilling Muds. Humic acid was cited as a mud thinner in an early patent178 but extensive use of leonardite came only with restrictions on quebracho in World War II. Lignite is less acidic than quebracho. Therefore, less caustic soda, often in the ratio of 1:5, is used with lignite than with quebracho. Soluble dry reaction products are made by evaporating a solution of lignite in caustic soda202 and by grinding lignite and caustic soda together.203 Causticized lignite in most cases is not as effective as quebracho in thinning fresh water muds on a weight basis, but may be more economical because of its lower cost. Lignite is not a satisfactory thinner for calcium-contaminated muds, although it can be used to counteract the effect of cement by removing the calcium as a precipitate and lowering the pH by its acidic groups. Lignite is not a thinner for salty muds.

Leonardite has greater temperature stability than other thinners. It shows exceptional performance in preventing solidification of lime muds at temperatures near 300°F (150°C).204 Lignite maintains stable filtration rates in drilling hot holes.159'20S For years, lignite has been an important constituent of muds used in geothermal drilling.200 "Brown coal" is a major component of a mud composition devised especially for geothermal drilling.207 This mud showed only a moderate increase in filtration after being aged 24 hours at 500°F' (260°C). Lignite is a component of an aerated or foam composition proposed for use in geothermal drilling.208 (As pointed out earlier in this discussion, the terms lignite and brown coal are used interchangeably in drilling fluids publications for the substance identified as leonardite.)

A major application for lignite is in conjunction with chromelignosul-fonates (see Chapter 2). Addition of sodium chromate to alkali-solubilized lignite provides a convenient product for improving the filtration properties and thermal stability of muds treated with chromelignosulfonates.209 210

Leonardite is a versatile material for emulsion stabilization. When solu-blized by caustic soda, it is an emulsifier for oil in water.211 A lignite derivative, used in oil muds primarily to reduce filtration, improves the stability of water-in-oil emulsions.212

For reducing the gel strength of bentonite mud, advantages are claimed tor a mixture of a calcium compound, leonardite, and a polyphosphate,21 ' and for a mixture of potassium salts, leonardite, and a polyphosphate.214

An alkaline potassium lignite product restricts water imbibition by shales, reduces filtration into permeable rocks, and affords a supplemental source of potassium ions in a mud composition designed for shale drilling.25 5

The phenolic and carboxylic units in humic acid are reactive groups in lignite which make possible many derivatives. Water soluble sulfomethylated lignite216 and lignite sulfonates '11 which are analogous to the quebracho derivatives can be prepared. A reaction product of sulfonated lignite and a salt of iron, chromium, manganese, or zinc gave stable mud properties for 16 hours at 300°F (150°C).218 A composition consisting of sulfonated lignite and water-soluble sulfomethylated phenolic condensation products afforded satisfactory stability to muds heated at 350°F (177 C) for 16 hours.210 Such compositions were effective in maintaining satisfactory filtration and flow properties in salt-saturated mud heated at 250 F (120 C) for 16 hours.

A zinc complex can be formed by adding zinc sulfate to a solution of leonardite in alkali. The dried reaction product is added to mud for filtration reduction, particularly at elevated temperatures.2 20

Leonardite is used in drilling fluids in a wide range in concentration from 1 to 40 lb/bbl (3 to 120 kg/m3). The many and varied uses of leonardite make a reliable estimation of the total consumption difficult, but the amount is thought to be about 55,000 tons in 1978.

Lignosulfonates

Source and Composition. Lignosulfonates are byproducts of the sulfite process for the separation of cellulose pulp from wood. Wood's cell-wall tissue is a complex mixture of polymers. The polysaccharides (called holocellulose) make up 70 to 80% of the tissue, and lignin is the remainder. Lignin is the bonding agent that gives rigidity to plants. It serves also to restrict loss of water and to protect the plant from microbial attack. Holocellulose consists of cellulose and hemicellulose. Hemicellulose is a mixture of relatively short-chain polymers made up of sugar-related components. In separating the cellulose (about one half of the wood, dry basis) by the sulfite process, the lignin and the hemicellulose are degraded and dissolved by hot acid sulfite solution. The bisulfite may be calcium, ch,oh ¿H

ch2oh ch ch2oh l ch

+1 0

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