Figure 11-14. Lignin and lingosulfonate polymers are derived from the primary units of: (A) p-coumaryl alcohol, (B) coniferyl alcohol, and (C) sinapyl alcohol.

magnesium, sodium or ammonium, although calcium bisulfite is most common. The spent sulfite liquor contains about 10% solids, of which about half came from lignin. The remainder of the solids are wood sugars, organic acids, and resinous materials.

No attempt will be made here to summarize the complicated chemistry of lignin and the lignosulfonates. A few features will be touched upon that may indicate the variety and versatility of these substances which have assumed such importance in drilling fluids. Monographs can be consulted for details of the extensive research on lignin chemistry.221 — .<>.224,225,226 Xhere is general agreement that the structure is complex and that lignin is a collective term applied to a group of similar, very large molecules, having phenyl-propane as the basic monomer unit. The primary units, shown in Figure 11-14, are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol.226 The insoluble lignin in wood is believed to be a highly crosslinked or branched network of phenylpropane chains. On treatment of the lignin with acid sulfite, soluble fragments of lignosulfonic acids and their salts are produced.

Even though the lignosulfonates are simpler compositions than lignin, their structures have not been defined. The spent sulfite liquor contains polymers having different degrees of sulfonation and varying molecular weights, ranging from about 1,000 to 20,000. Sulfonic groups are attached to the three-carbon side chain (see Figure 11-14) but not all reactions proceed at the same rate, so that, in general, about one sulfonic group exists for every two phenylpropane units. Hydrophilic properties are supplemented by secondary and tertiary alcohol groups.

Browning,227 in discussing emulsion stabilization by lignosulfonate, suggests a possible structural arrangement for a section of the polymeric lignosulfonic acid, as being distinctly different from that of a soap. HeJ " visualizes the lignosulfonate particle structure as a rigid ellipsoid from the surface of which short chains containing sulfonic and hydroxyl groups protrude. Such a structure is very hydrophilic and is capable of hydrogen bonding, but swelling is limited because of the tightness of the particle structure. Further research appears to confirm this conclusion.229 The chemistry of lignosulfonates, particularly regarding reactions with chromic ion, are discussed by Browning and Perricone.:'"

Preparation of Mud-Conditioning Agent. The problem of disposal of the spent sulfite liquor led to recovery of the materials from which today's mud-conditioning agents are produced. In the sulfite process for wood pulp production, wood chips are cooked under pressure with acid sulfite solution. After the pulp has been separated, the residual dilute solution of lignosulfonates, wood sugars, organic acids, and other minor constituents must be disposed of. A method of fractional precipitation carried out by adding increasing amounts of lime reduced the disposal problem.231 The fraction consisting mainly of calcium lignosulfonate could be further purified, and other metal lignosulfonates could be prepared by double decomposition reactions involving the precipitation of calcium sulfate.232 Calcium lignosulfonate was introduced as a deflocculant in the preparation of lime-treated muds180 around 1947, and became known as the "break-over chemical.": '3 Lignosulfonate-stabilized emulsions came into use in 1949.227

Numerous products have since entered the drilling fluids market to perform several functions, as will be discussed later. These products are made possible by the chemical reactivity of lignin,230 and, in most instances, make use of the spent sulfite liquor as their raw material. The most valuable of the lignin derivatives are the chrome lignosulfonates which were introduced in 1955 (see Chapter 2). The details of preparing lignosulfonates of chromium, iron, aluminum, and copper are described in two basic patents.231 : " In brief, the patents disclose various modifications for improving the effectiveness of spent sulfite liquor components for mud treatment through oxidation and formation of metal salts. The varied courses which can be followed in the modifications are indicated in Figure 11 15.

One method of preparing a ferrochrome lignosulfonate can be outlined as follows: Sugars are removed by fermentation of the spent liquor from calcium bisulfite sulfonation. Solids content is raised to about 50% by evaporation of water. Sodium hydroxide is added to raise the pH to 8 and the solution is digested for 8 hours 195'F (90 C). Ferric sulfate is added to precipitate caclium sulfate which is removed by centrifugation. Sodium dichromate is added to the liquid to oxidize the lignin derivatives. Time is allowed for reducing all of the hexavalent chromium to trivalent chromium. The final product is recovered by spray drying.

The source of the spent sulfite liquor solids significantly affects the properties of the lignosulfonate. For example, hardwood lignins differ from softwood lignins in their average molecular weights and in the mud-

equal amounts of calcium lignosulfonate and lime—up to 6 lb/bbl (17 kg/m3)—were added in the breakover. Additions were made thereafter as needed for maintenance. Some decrease in the amount of starch required for filtration control was reported. Stable emulsions were formed when oil was added to muds treated with calcium lignosulfonate, including salt-water muds.258

Browning227 studied the mechanism of emulsion stabilization and concluded that lignosulfonates are adsorbed at the oil-water interface, forming a semirigid film which has greater mechanical and chemical stability than interfacial layers formed by solid powders. Lignosulfonates are not soluble in oil, although they are wetted by it. Hydrolytic dissociated of lignosulfonate contributes electrokinetic charge effects which promote stabilization. Lignosulfonate-stabilized emulsions can be prepared in salt solutions without other filtration control materials. Calcium lignosulfonate is one component of a mixture used to prepare clay-free, brine-base drilling fluids.151

The mechanism of the deflocculation of clays by lignosulfonates has been studied in detail,259'260-261,262 and was reviewed in Chapter 4. In brief, thinning of clay muds is believed to result mainly from the adsorption of the negative lignosulfonate micelles on the edge surfaces of the clay particles. Because an adsorption layer of finite thickness is formed, higher concentrations of lignosulfonates are needed than with polyphosphates, which affect the electrokinetic double layer.25<J Usually 4 to 6 lb/bbl (II to 17 kg/m3) of lignosulfonates are needed to thin a fresh water mud, compared to 0.2 to 0.5 lb/bbl (0.6 to 1.4 kg/m3) of polyphosphate. The polyphosphate is ineffective in salty mud, while the lignosulfonates serve both as thinner and emulsifier in saturated salt water mud.

Adsorption of lignosulfonate on clay surfaces reduces clay swelling and cleavage, thus promoting hole stabilization and recovery of undispersed cuttings.230 According to Browning and Perricone,26J "The multi-layer adsorption film of the lignosulfonates that surrounds clay particles forms a semi-permeable membrane which acts to retard fluid invasion, thus minimizing clay particle disintegration when penetrating soft dispersible formations." Reference was made in Chapter 2 to the general applicability of chrome lignosulfonate—chrome lignite in mud conditioning and the widespread use of CL-CLS muds.

Mud's temperature stability became a serious problem as exploratory wells were drilled to greater depths in the late 1950's. The addition of sodium chromate or dichromate to the commonly-used lignosulfonate-treated muds was found to be effective in prolonging the thinning action of the lignosulfonate.

The origin of this practice is obscure. According to Kelly,264 field personnel noticed that following the addition of sodium chromate for corrosion control, daily usage of chrome lignosulfonate decreased. Whatever the origin, field experience demonstrated an improvement in flow properties when sodium chromate was used with ferrochrome lignosulfonate in drilling a hot South Texas well.205

Polarigraphic analysis was used in studying the rate of reduction of chromate by chrome lignosulfonate.265 The reaction rate is slow at 75 F (24 C). but becomes very rapid at 350°F (177°C). The rate is increased by raising the ratio of lignosulfate to chromate. Chromate in low concentrations is rapidly reduced by lignosulfonate, particularly at higher temperatures.

In studying the thermal degradation fo chrome lignosulfonates, Kelly2''4 noted that degradation, as indicated by a decrease in thinning effectiveness on the heated and aged mud, was apparent at temperatures as low as 250 1' (120°C). Filtration rate, however, increased only slightly until the temperature was raised to about 350°F (177°C). In these tests, the muds were alternately heated and cooled, and the total time at the high temperature was only about two hours. Sodium chromate restored thinning action at 300 F (15Q°C), but did not materially improve mud properties on heating at 350 F (177°C).

Failure of high-strength tubing in several wells in which chrome lignosulfonate muds had been left in the casing-tubing annulus led to several studies of thermal degradation of lignosulfonates. Skelly and Kjellstrand,266 after heating muds containing chrome lignosulfonates for two hours in vessels pressured to 400 psig with nitrogen, found that degradation began at 330" F (165°C). Severe degradation was evident at 450 F (232 C), as measured by the gaseous decomposition products released.

Carbon dioxide was the first gas produced by this decomposition. Hydrogen sulfide increased markedly at temperatures above 410 F (210 C). The residual material appeared to have undergone some polymerization reactions, forming insoluble compounds. Variations in composition of the test muds, such as barite content, affected the release of gaseous products.

Bush et alZfi7 observed the degradation of alkaline chrome lignosulfonate solutions after seven days aging at 325 °F (190°C). Prestressed roller bearings aged in the solutions showed corrosive attack by hydrogen sulfide. Decrease in pH gave evidence of acidic substances being formed. At temperatures above 400 F (205 C), at a pressure of 10,000 psig, sulfide formed in 16 hours.

In contrast to these observations of corrosion, Chesser26* reported that stressed, high-strength steel coupons exposed to muds containing chrome-treated lignosulfonate did not undergo stress corrosion cracking, even in the presence of soluble sulfides, after 30 days exposure at 400 F (205 C). Lignosulfonates in alkaline solutions are oxygen scavengers. Chesser observed formation of a film, inhibitive against sulfide-stress cracking, on steel heated in contact with lignosulfonate-treated mud.

In summary, field experience confirms laboratory observations of materially decreased effectiveness of chrome lignosulfonates as bottom hole temperature approaches 400 F (205 C). Chromate addition usually im proves performance. Numerous factors in addition to temperature affect mud's stability. As down-hole temperature increases, frequent pilot tests should be made to determine the effect (including corrosion) that aging has, at temperatures substantially above the current bottom hole temperature, on the mud's properties. Results of such tests should establish the need for m ud program modification.

Even a casual review of the patents cited here will show the great variety of lignosulfonate compositions that are available as mud conditioning agents. Field experience has shown differences in performance, sometimes depending on local conditions or product quality, but more often being governed by the skill of the mud engineer.

The varied uses of the lignosulfonates cover a concentration range from 1 to 30 lb/bbl (3 to 85 kg/m3). Total consumption in drilling fluids in 1978 is estimated to have been about 65,000 tons.

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