Although many acid compounds are available to the oil industry, only the following types have been proven economically effective in oil well stimulation:
Inorganic Acids (Strong).
· Hydrochloric Acid (HCl).
· Hydrofluoric Acid (HCl:HF).
Other inorganic acids include Sulphamic, Sulphuric and Nitric acids.
Organic Acids (Weak).
· Acetic Acid and Glacial Acetic Acid.
· Acetic Anhydride.
· Citric Acid.
· Formic Acid.
Hydrochloric Acid (HCl).
Hydrochloric acid is an inorganic acid and is the most commonly used acid in oil well stimulation. Hydrochloric acid has many advantages in its application as follows:
· Low cost and availability.
· Easily inhibited to prevent attack on oil-field tubulars.
· Surface tension can be controlled to aid in :
– Wetting properties.
– Exhibit detergency.
– Reducing friction pressure.
· Can be emulsified for slower reaction rate.
· Exhibit de-emulsification properties for rapid clean up.
· Most reaction products are water soluble and easily removed.
· Additives to minimise or eliminate insoluble reaction products can be applied.
It has long been recognised that hydrochloric acid is the best field acid for most applications. It is however, not without limitations. Hydrochloric acid is quite reactive; therefore, it will spend quite rapidly on some formations. It is essential with hydrochloric acid to size acid treatments and pump rates to optimise this property.
read more about Acidizing Different Formations
The reaction rate also dictates the selection of additives that will perform their functions during the relatively short spending time. These same additives must survive the spending process and function in the spent acid. Certain materials are soluble in hydrochloric acid but not necessarily in the spent acid water. For example, calcium sulphate can be partially solubilised by hydrochloric acid, but will crystallise out as scale when the acid spends. Iron oxide will dissolve in hydrochloric acid but will re-precipitate, as the acid spends, at about a pH of 2.0. These properties require
the selection of additives that will circumvent these problems.
Hydrochloric acid is normally pumped in concentrations ranging from 3.0% to 28%.
The low concentration acids are used for the removal of salt plugs and emulsions. The high concentration acids are selected to achieve longer reaction times and to create larger flow channels. By far the most frequently used strength is 15%, for the following reasons :
· Less cost per unit volume than stronger acids.
· Less costly to inhibit.
· Less hazardous to handle.
· Will retain larger quantities of dissolved salts in solution after spending.
In addition to the above advantages 15% hydrochloric acid will also provide other specific properties such as emulsion control and silt suspension. The general uses for hydrochloric acid are as follows :
· Carbonate acidizing – Fracture and Matrix.
· Sandstone acidizing – Matrix only.
· Preflush for HCl:HF mixtures.
· Post-flush for HCl:HF mixtures.
· Acidizing sandstones with 15% to 20% carbonate content.
· Clean-up of acid-soluble scales.
· Perforation washes.
Pure hydrochloric acid (muriatic acid) is a colourless liquid, but takes on a yellowish hue when contaminated by iron, chlorine, or organic substances. It is available commercially in strengths up to 23.5° Bé (Baumé scale) or 38.7% percent by weight of solution.
Some processes dictate that hydrochloric acid is not the most suitable acid to use. In these cases, alternatives, such as organic acids (acetic and formic) may be used.
These acids are used because of their inherently retarded nature, their ability to be used at higher temperatures and their solvation ability in “dirty” formations. The primary objection to the use of organic acids is their cost and their lack of effectiveness in removing limestone.
Other acids are also used in limited quantities. An example is citric acid, which can be used both alone, or as a component of an acid blend, or for use as a stabiliser, buffer and iron control agent. Also sulfamic acid has been used in the oil industry on a “do it yourself” basis. Its usage is recommended because of its low corrosivity,
although it is limited by its ability to strip chrome from chrome pumps and by its relatively high cost.
Hydrofluoric Acid (HF).
Hydrofluoric acid, another inorganic acid, is used with hydrochloric acid to intensify the reaction rate of the total system and to solubilise formations, in particular sandstones. In general hydrofluoric acid is used as follows :
· It is always pumped as an HCl:HF mixture.
· Ensure that salt ion contact is prevented.
· Sandstone matrix acidizing.
· Removal of HCl insoluble fines.
· Normal concentrations 1.5% to 6.0%.
· One gallon of 12:3 HCl:HF will dissolve 0.217 pounds of sand.
Hydrofluoric occurs as a liquid either in the anhydrous form (where it is fuming and corrosive), or in an aqueous solution (as used in well stimulation). Hydrofluoric acid attacks silica and silicates, (glass and concrete). It will also attack natural rubber, leather, certain metals such a cast iron and many organic materials.
In well stimulation, hydrofluoric acid is normally used in combination with hydrochloric acid. Mixtures of the two acids may be prepared by diluting mixtures of the concentrated acids with water, or by adding fluoride salts (e.g. ammonium bifluoride) to the hydrochloric acid. The fluoride salts release hydrofluoric acid when dissolved in hydrochloric acid.
Hydrofluoric acid is poisonous, alone or in mixtures with hydrochloric acid, and should be handled with extreme caution.Other Inorganic Acids.
Some consideration has been given to using sulfuric and nitric acids; however, these acids are not used extensively in the oil industry today. The reasons for the lack of use are; sulfuric acid will form insoluble precipitates, and nitric acid often forms poisonous gases during its reaction with certain minerals.
These acids are used in well stimulation basically because they have a lower corrosion rate and are easier to inhibit at high temperatures than hydrochloric acid. Although mixtures of organic acids are considered corrosive to most metals, the corrosion rate is far lower than that of hydrochloric or hydrofluoric acid, therefore, organic acids are used when long acid-pipe contact time is required. An example of this is when organic acid is used as a displacing fluid for a cement job. The organic acids is left in the production string. and is subsequently used as the perforating fluid.
Organic acids are also used when metal surfaces of aluminium, magnesium, and chrome are to be contacted, such as in trying to remove acid-soluble scales in wells with downhole pumps in place. They can also be used as iron control agents for other acid systems. Many organic acids are available, but the four most commonly
used are :
· Acetic Acid.
· Acetic Anhydride.
· Citric Acid.
· Formic Acid.
Acetic Acid (CH3COOH).
Acetic acid is a colourless organic acid soluble in water in any proportion and in most organic solvents. Although mixtures of acetic acid with water are considered corrosive to most metals, the corrosion rate is far lower than that of hydrochloric and hydrofluoric acids. Acetic acid is easy to inhibit against corrosion and is used frequently as a perforating fluid where prolonged contact times are required. With this ability, the acid is sometimes used as a displacing fluid on a well cementing job, where the contact time may be hours or days before perforating takes place. This ability is beneficial in three ways:
· Reduces formation damage. The first fluid two enter the formation will be an acid or low pH fluid which will react with carbonate or the calcareous materials of a sandstone formation.
· Reduces clay swelling.
· Can be used where aluminium, magnesium or chrome surfaces must be protected.
The relation of dissolving power of one gallon of a 15% concentration of acetic acid compared to that of hydrochloric acid and formic acid at the same volume is listed in Table 1, page 3. The cost of acetic acid per unit, based on dissolving power, is more expensive than either hydrochloric acid or formic acid.
Normally, acetic acid is used in small quantities or with hydrochloric acid, as a delayed reaction, or retarded acid. The general uses and properties of acetic acid are as follows:
· Acetic acid is relatively weak.
· Normal concentrations of 7.5% to 10% when used alone.
· Mainly used in hydrochloric acid mixtures.
· Used as an iron control additive.
· Carbonate acidizing.
· Perforating fluid.
· Retarded acids.
Commercially available acetic acid is approximately 99% “pure”. It is called glacial acetic acid because, ice-like crystals will form in it at temperatures of approximately 60° F (16° C) and will solidify at approximately 48° F (9° C). When glacial acetic acid is mixed with water, a contraction occurs. For this reason, the amount of acetic acid and the amount of water normally total more than the required volume.
Care should be exercised when handling acetic acid. This solution in concentrated form can cause severe burns and fume inhalation can harm lung tissue
Acetic Anhydride Acid.
Acetic anhydride is the cold weather version, for use instead of acetic acid due to its lower freezing point of 2.0° F (-17° C). The properties of acetic anhydride are the same for those of acetic acid, the only changes are those in relation to volumes used.
A comparison of acetic anhydride to acetic acid shows that one gallon of acetic anhydride mixed with 0.113 gallons of water is equivalent to 1.127 gallons of acetic acid. Expressed alternatively one gallon of acetic acid is equivalent to 0.887 gallons of acetic anhydride mixed with 0.101 gallons of water.
When mixing acetic anhydride always add it to water or dilute acid. If water or dilute acid is added to acetic anhydride, an explosion will occur due to a rapid increase in temperature caused by the chemical reaction.
As with acetic acid, care should be exercised when handling acetic anhydride as this solution in concentrated form can cause severe burns and fume inhalation can harm lung tissue.
Citric Acid (C6H8O7)
Iron scales are normally found in the casing and tubing in wells and sometimes as the mineral deposits in the formation rock itself. When hydrochloric acid solutions come into contact with these scales or deposits, the iron compounds are partially dissolved and are carried in solution as iron chloride. As the acid becomes spent,
the pH rises above 2.0, allowing the iron chloride to undergo chemical changes and re-precipitate as insoluble iron hydroxide. This re-precipitation can reduce formation permeability and injectivity.
Citric acid (Ferrotrol 300) is a white granular organic acid material. It is used to “tie up” dissolved iron scales and prevent re-precipitation of dissolved iron from spent hydrochloric acid solutions. Normally, citric acid (often referred to as a sequestrant or sequestering agent), is used with X-14 to make the effects of suspension more
Citric acid is not used alone as an acid treating solution itself but is used in hydrochloric acid solutions known as sequestering acids (SA-systems) for the control of iron.
The amount of citric acid added to the hydrochloric acid system depends upon the amount of iron that is present. The first 50 pounds of citric acid added to 1000 gallons of acid, will sustain 2000 parts per million (ppm) of iron in solution (SA-2).
Each additional 50 pounds of citric acid added will increase its sequestering property by an additional 2000 ppm
Formic Acid (HCOOH)
Formic acid is the simplest of the organic acids and is completely miscible (capable of being mixed) with water. Formic acid is stronger than acetic acid yet weaker than hydrochloric acid. Formic acid is used in well stimulation, most frequently in combination with hydrochloric acid as a retarded acid system for high-temperature wells. The percentage of formic acid used in such applications is commonly between 8.0% and 10%. Formic acid can be easily inhibited, but not as effectively as with acetic acid at high temperatures and long contact times. The properties and uses of formic acid parallel those of acetic acid as stated below:
· Formic acid is relatively weak.
· Seldom used alone.
· Mainly used in hydrochloric acid mixtures.
Corrosion inhibitor aid.
· Hot wells.
· Retarded acids.
Acetic acid, acetic anhydride and formic acid are used when exceptionally retarded acid is needed because of extreme temperature or very low injection rates. At high temperatures, blends of organic and hydrochloric acid are much more successfully inhibited by organic inhibitors, than when hydrochloric acid is used alone. This property minimises the danger of hydrogen embrittlement of steel associated with hydrochloric acid treatments in high-temperature wells. Organic acid concentrations of up to 25% by weight are required, making acid treatment costs increase. Organic acids do not give as much reacting capability as hydrochloric acid treatments.