Page 400 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

378 CONSEQUENCES OF CORROSION

Figure 5.73 Ductile iron pipe with large corrosion pit (19).

4. Extent of metal corrosion
5. Compatibility of acid with reservoir fluids
6. Density and viscosity of spent fluids

As common with any acidification, the temperature, acid concentration, amount
of acid, velocity of injection, viscosity of acid, and fluid loss properties of formation
have profound effect on the reaction rate.
Acidizing involves treatment with mineral acids such as HCl or HF or organic
acids such as acetic or formic acid. The acids are generally used in combination with
inhibitors. The general reactions involved are:

2HCl + CaCO → CaCl + H O + CO
3 2 2 2
4HCl + CaMg(CO ) → CaCl + MgCl + 2H O + 2CO
3 2 2 2 2 2
SiO + 4HF → SiF + 2H O
2 4 2
2HF + SiF → H SiF
4 2 6
2CH COOH + CaCO → Ca(CH COO) + H O + CO
3 3 3 2 2 2
2HCOOH + CaCO → Ca(HCOO) + H O + CO
3 2 2 2

Iron control agents such as citric acid and organophosphorus compounds form iron
complexes and prevent the formation of ferric hydroxide precipitate.
During acid treatment, sludge consisting of asphaltenes, resin, paraffin, and other
high molecular weight hydrocarbons is formed. Addition of oil-soluble surfactants
can prevent the formation of sludge.
The most commonly used metals/alloys in drilling and well stimulation are
low-alloy carbon steel (API grades I-55, L-80, N-80 coiled tubing); quality tubing
(grades QT-70, 80, 1000); chrome alloy steels (ASM grade Cr-13, duplex steel).