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> \'OTI": SSCC Milfido 'tre^s corrosion cracking produce* r,tpid brittle lailure* ni hielt streneih meint i usually li*<l itnnls or bilM.

the formation of excessive amounts of soluble carbonates causes high viscosities. In such cases, calcium hydroxide may be used to neutralize the acid, but the resulting calcium carbonate precipitate tends to form scale, thus setting up corrosion cells. This tendency may be offset by the use of scale inhibitors, and by cleaning the pipe during round trips.

Hydrogen Sulfide. Hydrogen sulfide may massively contaminate mud by a sudden inflow of sour gas, or gradually by the gradual degradation of ligno-sulfonates either by sulfate-reducing bacteria or by high temperatures. The thermal degradation of lignosulfonates starts at about 330°F (165°C) and increases gradually until a major decomposition occurs at 450°F (232°C).76 Reaction products are hydrogen sulfide, carbon dioxide, and carbon monoxide.

Molecular hydrogen sulfide is a poisonous gas, and every possible precaution must be taken to protect rig personnel when it is encountered, even in small quantities. It is weakly acidic when dissolved in water, and attacks iron to form iron sulfides, as approximately described by the equation:

These reversible reactions are a function of pH, as shown by Figure 9-59.78 It may be seen that the sulfide is in the form of H2S up to about pH 6; as HS from pH 8 to 11, and as S~~ above pH 12. Since sulfide stress cracking is caused by the atomic hydrogen formed together with HS" in the first ionization stage (Equation 9-11), it follows that maintaining the pH between 8 and 11 is not a viable means of control. The formation of atomic hydrogen is suppressed when the pH is above 12, but maintaining such high alkalinities is not desirable because it involves the accumulation of S~~ in the mud (Equation 9-12). Should the pH fall because of a sudden inflow of more hydrogen sulfide, or for any other reason, the ionization reactions would reverse, and large amounts of atomic hydrogen or, possibly, hydrogen sulfide gas, would be generated. A high pH is, of course, also undesirable in high temperature wells because of the aforementioned degradation of clay minerals. It is preferable, therefore, to combat hydrogen sulfide by the addition of a scavenger rather than by maintaining a high pH.

Formerly, copper salts were used to scavenge hydrogen sulfide until it was realized that these salts caused bimetallic corrosion77 (i.e., the process shown in Figure 9-53). To prevent bimetallic corrosion, the cation of the scavenger must be higher on the electromotive series than iron. Zinc meets this qualification,

The sulfides are deposited on the pipe as a black powder. Hydrogen sulfide ionizes in two stages, viz:

Figure 9-59. Equilibrium of the aqueous system H2S HS~, S =, relative concentrations versus pH. {From Garrett, et al.?e Copyright 1978 by SPE-AIME.)

and basic zinc carbonate is now commonly used. Care must be taken to maintain the pH between 9 and 11. At higher or lower pH, the solubility of zinc carbonate increases sharply and the zinc ion flocculates clay suspensions.78 Figure 9-60 shows the consequent increase in yield point, gel strength, and filtration rate. It is evident that optimum properties are at pH 9, or thereabouts.

Flocculation by the zinc ion may be avoided by the use of a zinc chelate.79 The zinc is held by coordinate bonds in the chelating agent so that a very low level of zinc ions is maintained in solution, but the zinc is readily available to react with the sulfide as required.

Powdered iron minerals also act as scavengers for hydrogen sulfide. For example, hydrogen sulfide reacts with iron oxides to form insoluble iron sulfides. The reaction takes place at the surface so that the efficiency of the material depends on the surface area exposed. A synthetic form of magnetite, Fe304, which has a high specific surface because of its porous nature, is commercially available. The reaction product is pyrite, but the chemistry involved is complex, and depends on a number of variables such as pH, mud shear rate, and temperature.80 Reaction time is fastest at low pH, and the material is therefore most effective in neutralizing sudden large influxes of hydrogen sulfide.81 The ability to operate at low pH is also an advantage in high temperature wells. Another great

Figure 9-61. Effect of temperature and salinity on corrosion rate. (From Cox.8®)

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