Introduction to Acidizing

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Organic Acids

Organic acids offer the benefit of relatively low corrosivity and easy inhibition at elevated temperature. They are particularly applicable, therefore, to high-temperature well applications.

Acetic (HC₂H₃O₂) and formic (HCHO₂) acids are the two principal organic acids used in acidizing. These acids are weaker than mineral acids; they react more slowly and are also less damaging. Acetic, for example, is the only acid available that will not damage chrome plating or aluminum and magnesium surfaces. In some cases, organic acids are mixed with HCl or HF to provide deeper acid penetration into the formation.

Acetic Acid

Acetic acid was the first organic acid to be used in appreciable volumes in acidizing. It is easy to inhibit against corrosion and commonly used as a 10% by weight solution of acetic acid in water. At this concentration, the reaction products are generally soluble in spent acid. Acetic acid functions well as a perforating fluid or low-corrosivity acid when in contact with metals that corrode easily.

Commercially available “pure” acetic acid is approximately 99% acetic acid. It is called glacial acetic acid because icelike crystals form at temperatures around 60° F and it solidifies near 48° F. When glacial acetic acid is mixed with water, a contraction occurs. For this reason, the amounts of acetic acid and water normally total more than the required volume when making dilute solutions.

Acetic acid reacts with limestone to form calcium acetate (Ca(C₂H₃O₂)₂), water and carbon dioxide:

$2HC_2H_3O_2 + CaCO_3 \rightarrow Ca(C_2H_3O_2)_2 + H_2O + CO_2$ $\quad$ (6)

As shown, 2 moles of acetic acid reacts either 1 mole of CaCO₃ to produce 1 mole each of calcium acetate, water and carbon dioxide. The molecular weights of these compounds are as follows:

Compound	Molecular Weight
HC₂H₃O₂	60
CaCO₃	100.09
CaCl₂	110.99
H₂O	18.02
CO₂	44.01

Thus, to dissolve one pound of limestone would require:

$\left[ (1 lb.\, CaCO_3) \cdot \left( \dfrac{mole}{100.09\, lb.} \right) \right] \cdot \left[ \left( \dfrac{2\, mole\, HC_2H_3O_2}{mole\, CaCO_3 }\right) \cdot \left( \dfrac{60\, lb.}{mole} \right) \right] = 1.2\, lb. \,HC_2H_3O_2$

On a volumetric basis, for the case of a 15 percent HC₂H₃O₂ solution (specific gravity = 1.037 at 60º F; density 8.64 $\tfrac{lb.}{gal}$ ), one gallon of acid dissolves $\tfrac{8.64 \ \cdot \ 0.15}{1.2}= 1.08\, lb.$ of limestone. An identical concentration of HCl dissolves 1.84 lb. of limestone. (see Table 1).

Formic Acid

Formic acid is the simplest of the organic acids. It is stronger than acetic, yet weaker than HCl, and is most frequently used in combination with HCl as a retarded acid system for high-temperature wells. Formic acid can be easily inhibited, but not as effectively as acetic acid at high temperatures and long contact times. The percentage of formic acid generally used is 8% to 10% by weight.

A 10% solution of formic acid will dissolve as much limestone as an 8% solution of HCl. The chemical reaction of formic acid and limestone is:

$2HCHO_2 + CaCO_3 \rightarrow Ca(CHO_2)_2+ H_2O + CO_2$

Acid type	Concentration %	CaCO₃ dissolved per gal. acid- lb.	CO₂ formed per gal. acid cu. Ft.	CaCl₂ formed per gal. acid- lb.
HCl*	15*	1.84	6.99	2.04
	20	2.50	9.47	2.75
	25	3.22	12.20	3.57
Acetic*	15*	1.08	4.09	1.71
	20	1.43	5.41	2.25
	25	1.80	6.82	2.84
Formic*	15*	1,42	5.38	1.84
	20	1.90	7.20	2.47
	25	2.40	9.09	3.12

Reaction of three acids on limestone at various concentrations.

* The most commonly used strength for HCl is 15% and 10% for acetic and formic acids. In this table, however, the HCl reaction rate on limestone is compared to the equivalent strength rates for acetic and formic acid rather than to the latter’s commonly used strengths.

Formic acid is frequently used in reservoirs with extreme temperatures or low injection rates. At high temperatures, organic inhibitors work more effectively on formic acid with HCl than on HCl alone. This property minimizes the danger of hydrogen embrittlement of steel associated with HCl treatment in high-temperature formations.

Organic acids have less dissolving capability than HCl and must be used in greater volumes for comparable results, making such treatments more expensive.

The dissolving power of various concentrations of HCl, acetic, and formic acids is shown in Table 1. The cost of acetic acid per unit based on dissolving power is more expensive than either HCl or formic acid. With the exception of fracturing acidizing treatments, dissolving power is not often the primary consideration in acid selection.

The most commonly used strength for HCl is 15% and 10% for acetic and formic acids. In Table 1, however, the HCl reaction rate on limestone is compared to the equivalent strength rates for acetic and formic acid rather than to the latter’s commonly used strengths.

Hydrofluoric-Organic Acid Mixtures

Acid mixtures of HF and either acetic or formic acid are used to slow the reaction of the acid on sand and clay, and to reduce corrosiveness. These acid mixtures can be effectively inhibited for up to 16 hours at 400° F. Mixing HF with organic acids can provide deeper penetration and, therefore, effective removal of deep formation damage. Organic HF mixtures are recommended at temperatures of 200° F and above; below 200° F they can cause the formation of undesirable reaction products.