The clarity or solid free nature of brines is the most important aspect of their use as completion fluids. Brine clarity is measured by the quantity of the total suspended solids in a brine.
Briefly, two parameters can be determined which relate to clarity: (1) turbidity and (2) solid content in milligrams per liter (mg/l) or parts per million (ppm). These two parameters do not always correlate well. If absolute solid content is required, calibration curves must be determined in order to relate turbidity units to solid content in mg/l, ppm, or percent by volume. Procedures to prepare these curves are detailed in “API RP 13J: Recommended Practice for Testing Heavy Brines.”
In certain situations, turbidity is used as a relative measure of clarity. For example, turbidity measurements can be taken upstream and downstream from a filter unit and then compared to one another. If the turbidity of the downstream (effluent) sample is lower than that of the upstream (influent) sample, then the general conclusion is that the effluent sample contains fewer solids. Test filters may also be used for this purpose. If the effluent sample filters more quickly through a test filter than the influent sample does, the conclusion is that the effluent contains less solids.
Standards or specifications for clarity are difficult to set. Ideally, a brine should have a near zero turbidity or a solid content of near zero mg/l. A common means of measuring turbidity is by the light scattering ability of suspended solids. Turbidity measurement is reported in nephelometric turbidity units (NTUs). A value of 20 NTUs is considered the upper limit of acceptable brine clarity.
Clarity Problems—Contaminants and the Formation of Solids
The presence of contaminants, either solid or gaseous, can have a serious impact on the potential for localized corrosion, especially on the probability that environmentally assisted cracking (EAC) could initiate a catastrophic cracking event. The presence of solids or scale can lead to the establishment of a concentration cell, or crevice corrosion, which may initiate pitting or cracking of the metal. Additionally, acidic gaseous contaminants, like CO2 and H2S, can induce pitting or cracking, especially when CRA metallurgies are used. As a consequence, the impact of the following contaminants on both general and localized corrosion, given susceptible combinations of specific fluids and metallurgies, must be considered.
Iron. When additives, like corrosion inhibitors are introduced into fluids, turbidity will often increase even though no solids have been added. Also, meta-stable dissolved solids, such as ferrous chloride (FeCl2) or ferric chloride (FeCl3), may not show up on turbidity or suspended solid tests. However, these compounds may precipitate as insoluble solids when the brine chemistry changes. More significantly, these contaminants or the presence of iron, in general, may lead to precipitation of solids, owing to the ease with which hydrated iron species are formed.
The formation of these hydrated species leads to a lowering of the pH (increase in acidity). This is especially true with ferric compounds. As a consequence, the issue of enhanced corrosion may have to be addressed. Once formed, these hydrated iron species at pH levels of ~3 or higher can yield gelatinous hydrated oxide, ferrous Fe(II), or ferric Fe(III) hydroxides that precipitate depending on the pH.
Carbon Dioxide. The introduction of carbon dioxide into divalent brines (calcium or zinc) can lead to increased turbidity and solid formation, owing to the formation of calcium carbonate (CaCO3) or zinc carbonate (ZnCO3), both of which are insoluble in aqueous media.
Dissolved bicarbonate ions, present in most naturally occurring sources of water, will react with calcium or zinc ions in brines to form calcium or zinc carbonate. Precipitation of carbonate salts will lower the pH of the fluid, raising corrosion concerns.
Hydrogen Sulfide or Sulfides. The presence of the sulfide ion from hydrogen sulfide (H2S) can lead to turbidity or solid deposition problems due to three different phenomena:
- the formation of insoluble iron sulfide salts,
- the formation of insoluble zinc sulfide salts (in zinc brines), and
- the formation of free elemental sulfur by the slow oxidation of the sulfide ion.
Invariably, the metal sulfides that precipitate in the presence of hydrogen sulfide are really polysulfides (MSx) in which elemental sulfur is chemically bound to the sulfide.
Like CO2, H2S is a weak acid in water. As a consequence, formation of insoluble metal sulfide salts will lower the fluid pH and have the effect of accelerating corrosion.
H2S itself is a strong promoter of hydrogen embrittlement, which can be manifested as EAC. See “Sulfide Stress Cracking.”
Bacteria. Although normally bacteria do not survive in basic media or highly concentrated brines, the potential danger of certain bacteria in dilute brines must be recognized. Sulfate reducing bacteria can produce H2S from trace amounts of the sulfate ion, which can lead to increased turbidity and/or insoluble matter.
Seawater. If a CBF containing calcium comes into contact with seawater, the levels of bicarbonate ions and sulfate ions that are routinely present in seawater may result in the formation of solid calcium carbonate and calcium sulfate. In addition, the high chloride concentration in most brines will result in the formation of sodium chloride crystals. Seawater is a rich source of bacteria and other microorganisms.
Formation of Solids
Sulfide Precipitation. As in the case of carbonate precipitation, the formation of insoluble sulfides leads to an increase in acidity. This is illustrated below in the formation of hydrobromic acid (HBr) from the reaction of H2S with zinc bromide (ZnBr2).
The increased acidity with the introduction of a large amount of H2S at some point redissolves the formed zinc sulfide (ZnS), i.e., the reaction becomes reversed. This process lessens the amount of solid formed, but, clearly, it also may have health, safety, and environmental consequences due to a greater release of H2S.
Calcium or Zinc Hydroxide Precipitation. With an increase in pH, divalent brines may form hydroxides and increase turbidity, since these species are rather insoluble. As a consequence, the practice of increasing the alkalinity of calcium or zinc brines to reduce the brines’ susceptibility to corrosion must be undertaken with great care. Otherwise, filtration and fluid weight up will be needed, owing to the density loss which will occur as a result of the precipitation of calcium hydroxide (Ca(OH)2) or zinc hydroxide (Zn(OH)2).
|Contact a TETRA fluids specialist if you have concerns about zinc precipitation. TETRA has developed a number of solutions to address this problem.
Sodium Chloride Precipitation. The precipitation of sodium chloride from brine mixtures containing high concentrations of sodium and chloride ions can lead to solid control problems. Being soluble only to an extent of ~26%, sodium chloride (NaCl) is the least soluble of the common brine salts. Furthermore, in contrast to most other soluble salts, its solubility does not increase significantly with a rise in temperature, and it decreases in the presence of other salts. As a consequence, care needs to be exercised when calcium chloride (CaCl2) is added to solutions containing sodium bromide (NaBr).