An acid is any substance that will release some or all of its hydrogen ions when added to water. By releasing hydrogen ions in solution, acids lower the pH of that solution.
Some acids are stronger than others and strong acids burn. Strong acids will completely release all the hydrogen ions in their molecule when dissolved in water. Weak acids, on the other hand, will only release some of the hydrogen ions in their molecule. The stronger the acid, the more potent it is.
In the past, acids in nature were identified by a characteristic sour taste. This is why the term acid (latin word for acere) really means “sour”. It is, however, not recommended to taste substances in order to tell if they are acid or not. Sour taste is not the only quality of acids.
Corrosive Influence of Acids
Acids used with drilling fluids can react with carbonate rocks to release salt, water and CO2 gas. The salt could plug pores of the reservoir rock and the CO2 released in the presence of water is highly corrosive to drillpipes and other bottom-hole assembly. Even surface facilities made with steel will not be spared when it comes in contact with CO2 in the presence of water.
If exposed to acids long enough, metals can completely break down into hydrogen gas. In the laboratory, acidic solutions turn blue litmus paper red. On the pH scale, solutions with a pH below 7 are acidic.
A solution with a pH of 1 is more acidic than a solution with a pH of 2. This means that the stronger the acid, the lower its value will be on the pH scale.
Useful Application of Acids in Matrix Acidizing
Acids play an important role in the oil and gas industry. One example is in matrix acidizing. Matrix acidizing involves pumping hydrochloric acid (HCL) or a mixture of hydrochloric acid and hydrofluoric acid (HCL:HF) down the tubing to the reservoir. The acid will improve the permeability of rocks by dissolving fines and scales that may deposit on pore spaces within rocks.