Introduction to Acidizing

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Acid Treating Solutions

Whatever their specific formulation, all acids have hydrogen ions that react with carbonate rock (principally limestone and dolomite) to form water and carbon dioxide, and with siliceous minerals (sandstone and clays) to form fluosilicic acid and water.

Mineral Acids

Hydrochloric Acid

Hydrochloric acid (HCl) is the workhorse of the stimulation business, finding extensive use in both carbonate and sandstone acidizing. Chemically, hydrochloric acid is hydrogen chloride in aqueous solution. Composed of hydrogen and chlorine gas, it is gaseous in the chemically combined state. Hydrogen chloride gas is readily soluble in water. Concentration is described as percent by weight of HCl gas dissolved in water. HCl acid is commercially available; its concentration has been standardized at 20° Be, i.e., 31.45% acid, and it has a specific gravity of 1.160.

Many acidizing treatments employ HCl to some extent. Usually, it is used as a 15% solution when treating carbonate formations. This concentration, commonly referred to as regular acid, was originally chosen because of inadequacies in early inhibitors and difficulty of preventing corrosion of well tubulars by more concentrated solutions.

Fifteen-percent HCl is used most frequently because it:

costs less per unit volume than stronger acids and is less costly to inhibit;
is less hazardous to handle than stronger acids;
retains larger quantities of dissolved salts in solution after spending.

Although acid concentrations higher than 15% may also be used in carbonate formations, lower strengths are more suitable where acid dissolving power is not the sole consideration. In sandstone acidizing, for example, 5 to 7.5% HCl is often used ahead of hydrochloric-hydrofluoric acid mixtures to prevent precipitation of formation-plugging reaction products.

The principal disadvantage of HCl is its corrosive action on wellbore tubular goods.

HCl reacts with limestone (CaCO₃) to form calcium chloride (CaCl₂), water (H₂O) and carbon dioxide (CO₂):

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

Compound	Molecular Weight
HCl	36.47
CaCO₃	100.09
CaCl₂	110.99
H₂O	18.02
CO₂	44.01

Thus, to dissolve one pound of limestone, we would need

$\left [ \left ( 1\ lb\ CaCO_3 \right )\cdot \left ( \dfrac{mole}{100.09\ lb} \right ) \right ]\cdot \left [ \left ( \dfrac{2\ mole\ HCl}{mole\ CaCO_3} \right )\cdot \left (36.47\dfrac{lb}{mole} \right ) \right ]=0.729\ lb\ HCl$

On a volumetric basis, for the case of a 15 percent HCl solution (specific gravity = 1.076 at 60°F; density = 8.96 $\tfrac{lb}{gal}$ ), one gallon of acid will dissolve $\dfrac{8.96\cdot 0.15}{0.729}=1.84\ lb\ CaCO_3$ .

The reaction of HCl on dolomite is very similar to its reaction on limestone, except for the presence of magnesium. The chemical reaction of HCl and dolomite CaMg(CO₃)₂ is:

$4 HCl + CaMg(CO_3)_2 \Rightarrow CaCl_2 + MgCl_2 + 2H_2O + 2CO_2$ $\quad$ (1)

Hydrofluoric Acid

Hydrofluoric acid is the primary dissolving chemical used in sandstone acidizing. In these applications, HF is usually mixed as a dilute solution with HCl, or an organic acid. Its principal use is to dissolve siliceous minerals.

Within the chemical industry hydrofluoric acid (HF) is available commercially as a relatively pure material in anhydrous form (where it is fuming and corrosive) or as a concentrated (40 to 70%) aqueous solution. Anhydrous HF is not used in the field because its low boiling point of 66.9° F is often exceeded by ambient temperatures. HF reacts with silica and silicates, such as glass and concrete. It also attacks natural rubber, leather, cast iron, and many organic materials.

HF is the only acid that reacts either siliceous minerals such as sand and clays. The reaction of HF on quartz (SiO₂), a primary component of sand, is:

$4HF + SiO_2 \rightarrow SiF_4 + 2H_2O$

$SiF_4 + 2HF +\rightarrow H_2SiF_6$ $\quad$ (2)

Here, the end reaction product, fluosilicic acid (H₂SiF₆), is soluble in water, but its potassium and sodium salts are insoluble. HF contact with formation water containing CaCl₂, MgCl₂, NaCl or KCl thus must be avoided near the wellbore. The only salt solution compatible with HF is ammonium chloride.

The reaction of HF on bentonite clay (Al₂(Si₄O₁₀)(OH)₂) is:

$36HF + Al_2\left ( Si_4O_{10} \right )\left ( OH \right )_2 \rightarrow 4H_2SiF_6 + 2H_3AlF_6 + 12H_2O$ $\quad$ (3)

Hydrofluoric acid also reacts with carbonates; its reaction with limestone is:

$2HF + CaCO_3 \rightarrow CaF_2 \downarrow + H_2O + CO_2$ $\quad$ (4)

HF will react with calcium ions from any source to form the insoluble precipitate calcium fluoride (CaF₂, which can cause sever plugging problems in the formation. The portion of the reaction that creates calcium fluoride is:

$Ca^{++} + 2F\,^- \rightarrow CaF_2$ $\quad$ (5)

CaF₂ formation can be avoided through the proper use and optimal placement of HCl in the acid treatment.

Hydrochloric-Hydrofluoric Acid Mixtures

Hydrochloric-hydrofluoric acid mixtures are the chief solutions used in matrix acidizing of sandstone. These acids are very often used as mixtures because HCl ties up and displaces anions, which in turn prevents reaction of these anions (e.g., Ca⁺⁺, Mg⁺⁺, etc.) with HF. Although HF is the only acid effective on siliceous minerals, it is ineffective as a stimulation agent on carbonates because its reaction forms insoluble CaF₂.

The usual HF solution used in the field contains 3% HF and 12% HCl. This solution is commonly called regular mud acid. However, concentrations of HF in HCl solutions range from 0.5 to 9%, and some operators prefer 1-1/2% HF and 6% HCl.

These mixtures may be formed from the dilution of concentrated solutions of HF or, more frequently, from the reaction of ammonium bifluoride (NH₄HF₂) or ammonium fluoride (NH₄F) with HCl. Usually, 15% HCl is used, and enough NH₄HF₂ or NH4HF is added to create a solution containing 3% HF. Consumption of hydrogen chloride by these reactions leaves 12% HCl remaining in solution. Similarly, 1.5% HF can be prepared from 7-1/2% HCl solutions where the final HCl concentration is 6%.

Corrosion characteristics of HCl–HF mixtures are comparable to those of HCl alone, and similar corrosion inhibitors are required. Both HCl and HCl–HF acid mixtures are stronger and more reactive than organic acids.