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Wells – Chemicals

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Chemicals Used in Fracturing

The identities of chemicals incorporated in fracturing fluids were probably the first thing sensationalized about fracturing. The movie “Gasland” created quite a stir with the statement that a “cocktail” of several hundred toxic chemicals were “potentially” used in fracturing. The grain of truth was that there are many chemicals in additives sold for incorporation in fracturing; however; the fact is that most fracs use only a dozen or so major chemicals, some of which are food-grade additives and many are in parts per million concentration. About half of fracturing jobs are “slick water” fracturing fluid that often use low concentrations of two to five chemicals. Many claims of chemical usage also include trace amounts of chemicals at the edge of detection and most well below the EPA’s strictest limits. Analysis of drinking water, for comparison, has shown arsenic, lead, chromium, solvents, gasoline, pesticides, prescription drugs, and a myriad of household products as the most common contaminants – none from fracturing. The upside to this commentary is that public concerns have moved chemical manufacturers to make and operators to use safer chemicals and less overall chemicals. Many companies have moved toward biocides with less residual activity, mechanical biocides such as ultraviolet light and the use of chemicals on the US EPA’s Safer Choice chemicals (formerly Designed For Environment or DfE) or UK North Sea’s OCNS Hazard rating of Gold Band (lowest possible hazard quotient). These listed materials meet requirements of rapid biodegradation and minimum harm to environments.

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Friction reducer, the largest volume chemical in slick water fracs, is polyacrylate, a polymer whose main use is in baby diaper absorbent and as a drinking water purifier that adsorbs heavy metals. A cross section of chemicals used in fracturing, the volumes used and some alternate uses helps explain oil field fracturing chemical usage. Chemicals such as diesel, benzene and proven carcinogens, mutagens and endocrine disruptors are not used in modern safe fracturing fluids. The CAS number identifies exact identity (no “trade secret” identities).

One of the most impactful problems from fracturing in Pennsylvania was the use of local water treating plants to treat water produced from oil and gas wells before disposal into Pennsylvania rivers. The practice was evidently instituted in Pennsylvania decades prior to the shale drilling boom in the Marcellus when volumes of water flowed from conventional wells was very small and natural salt contents were low. Dilution of locally severe acid mine drainage in some creeks by the produced water was expected to be beneficial; however; large volumes of produced water from fracturing in the shales with high salinity and ions such as bromine and barium proved too problematic for such a disposal method. This practice, although allowed by law in Pennsylvania until about 2010, has been forbidden by law in nearly all western states since the 1950’s.

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Chemicals Used in Production Operations

Producing oil and gas with the associated salt water from hydrocarbon bearing formations creates corrosion potential, flow restriction deposits such as mineral scales of calcium or barium and challenges in separating oil from water. Corrosion remains one of the biggest deterioration problems in the oil industry (a large problem in other industries as well). Scales may precipitate in tubulars until they restrict flow. Paraffins (wax) are longer carbon chain components of oil and can deposit anywhere in the well as temperatures cool and pressure declines. Mixing of salt water, oil, gas and a small amount of solids such as sand, rust or even ice can produce emulsions, froths and foams that must be separated before the oil and gas can be sold and the salt water can be recycled or properly re-injected into the hydrocarbon producing formation. A wide variety of specialty chemicals, often at part per million (ppm) concentration, can be used, but only a handful of products are typically selected after laboratory testing. Using minimum amounts of the best additives reduces cost and risk in transport or storage.

Any chemical usage may be frightening to some people and there are definitely chemicals that should not be used, particularly where contamination or airborne emissions are possible. By using chemicals proven safe for specific uses, all elements of potential pollution are reduced. Even when the chemicals will never be disposed of in the environment outside of oilfield containment, the safe chemical route minimizes impact in the event of a spill or leak.
Note: BTX (Benzene, Toluene, Xylene) content in many additives is steadily declining but some operators have not phased the products out completely. Many companies are reviewing product offerings for the BTX or other troublesome materials and choosing alternatives. Although BTX is often reported in wells as if they were part of a chemical additive, the most likely source is in the produced oil. BTX and diesel range oil components are a natural part of many produced oils.

What is Oxygen Scavenger

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Is oxygen corrosive?

Yes. Oxygen is a highly reactive gaseous element. In the presence of steel, the corrosion rate of oxygen doubles for each 30°F rise in temperature. For example, in a boiler system operating at 400 psig and 444°F, the corrosion rate for oxygen is 256 times more reactive than at room temperature.

How does oxygen attack metal surfaces?

Oxygen forms localized corrosion areas referred to as “pits.” This distinctive formation is readily distinguishable from acid attack, caustic gouging or chelate corrosion. Oxygen pits can rapidly “drill” through metal surfaces, leading to metal fatigue and failure.

What is the oxygen corrosion mechanism?

Oxygen corrosion is an electrochemical process similar to a simple battery. Iron dissolves at the anode and releases electrons which are subsequently consumed by oxygen at the cathode. (See Figure below) Pitting is the result of this localized mechanism.

Anode: Fe0 ® Fe+2 + 2e–

Cathode: 2e– + H2O + 1/2O2 ® 2OH–

Where can oxygen corrosion occur in a boiler system?

Because oxygen is a gas, it will “flash” into the condensate system, turbines, and other steam operated equipment. Every metal surface of the boiler system is vulnerable to oxygen attack.

Why is oxygen corrosion a serious problem?

As oxygen corrodes the boiler metal, it dissolves the iron surface. This weakens the metal site, but more importantly, sends dissolved iron into the boiler. This dissolved iron can deposit onto boiler tubes, causing overheating and tube failure.

How can oxygen corrosion be minimized?

Oxygen control is generally both a mechanical and chemical process. The majority of oxygen in the boiler feedwater is typically reduced to less than 20 parts per billion (ppb) by heating the water to reduce its solubility and releasing it out of the system via venting (deaeration).
Since even very low levels of oxygen will cause corrosion, a chemical scavenger is used to supplement mechanical deaeration to reduce the level to zero.

Can an oxygen scavenger be used if the plant currently does not deaerate the feedwater?

Yes. Oxygen scavengers are effective at high oxygen levels, but mechanical deaeration is more efficient and cost-effective in the long run. Oxygen corrosion cannot be completely inhibited by chemicals alone. There is no real substitute for good mechanical deaeration. Chemicals are most effective when used only to polish a properly operating system.

Should oxygen scavengers be fed with other products?

No. Oxygen scavengers should not be mixed with other chemicals because this may result in a complete loss of product activity. The addition of water to a liquid oxygen scavenger may also result in a loss of product activity.

Dilution water contains oxygen, which is consumed by the scavenger and reduces the overall effectiveness of the product. All liquid oxygen scavengers should be fed neat (undiluted).

What types of sodium sulfite are available?

Sodium sulfite is a highly effective oxygen scavenger and is available in several forms:

Uncatalyzed sulfite — The reaction of sulfite with oxygen is very slow at temperatures below 200°F.

Without a catalyst, it takes up to ten minutes to reduce the oxygen content of water from the saturation point to 70% with sodium sulfite at room temperature. This type of product is recommended only when the application cannot tolerate the presence of a cobalt catalyst (catalyst beds, food applications, etc.).

Catalyzed sulfite — The catalyst, usually a cobalt salt, speeds up the reaction rate of sulfite at low temperatures (<240°F). A fully saturated oxygen solution can be completely deoxygenated in as little as 30 seconds. A potential drawback is that the catalyst can be deactivated or precipitated by improper feed application, such as water dilution or mixing with caustic.

Decharacterized sulfite — This is a term used by the USDA. It refers to specific chemicals which must be removed from sodium sulfite for the product to be USDA and FDA approved. Sulfite which is not decharacterized has the ability to give meat a red or “fresh” appearance.

What are the disadvantages of using sodium sulfite?

Several costly disadvantages are inherent with using sodium sulfite as an oxygen scavenger. These problems occur regardless of whether the sulfite is catalyzed or uncatalyzed.

  • Adds solids to the boiler — Additional solids to the boiler contributes to an increased blowdown requirement. Blowdown is literally “money down the drain.” It is a direct loss
    of energy and increases water treatment costs.
  • Cannot be used for spray attemperation — The solids contribution of sodium sulfite would cause superheater and turbine deposits. Also, since chemical feed must occur
    after the attemperation water take-off point, all equipment upstream of that point would be subject to oxygen corrosion and costly repairs.

Cannot be used at boiler pressures above 900 psig. Sodium sulfite begins to decompose at approximately 600 psig and is complete at 900 psig. The decomposition products are H2S and SO2, both highly corrosive gases, which can cause catalyst poisoning and severe corrosion on steam operating equipment.

What is metal passivation?

Metal passivation has traditionally been considered to be the reduction of hematite to magnetite in iron-based boiler tubes. Actually, it is a process by which bare metal surfaces form a protective oxide film. The passive film is very thin and dense. It is distinguishable from the base metal by the coloration.

In the case of carbon steel, this protective film is magnetite (Fe3O4) and is black in color.

Can oxygen scavengers enhance metal passivation?

Yes. Hydrazine has traditionally shown passivation properties and Nalco has developed several proprietary products which effectively enhance metal passivation.