Acid sludge

What is acid sludge?

Acidizing pumps acid, usually hydrochloric acid, into a well to dissolve damage or rock and improve flow. With some crude oils, the acid does not simply pass through: at the boundary between acid and oil, heavy components of the oil, mainly asphaltenes and resins, destabilize and precipitate. They form a semi-solid sludge that does not dissolve in either the acid or the oil.

Because it forms inside the formation and near the wellbore, sludge can block the flow paths the treatment was meant to open, leaving the well worse off than before. It is a form of formation damage.

What makes sludge more likely

• Crude oil composition. Oils rich in asphaltenes are more prone to sludging.
• Dissolved iron. Ferric iron, picked up from tubing, rust and scale as the acid is pumped, strongly promotes sludge formation.
• Strong acid concentrations.
• Acid-oil emulsions, which often form alongside sludge and are also hard to break.

How it is prevented

• Laboratory compatibility testing of the planned acid system against a sample of the well’s crude before the job.
• Anti-sludge agents, typically surfactants that keep asphaltenes dispersed at the acid-oil contact.
• Iron control agents, which keep iron in a form less likely to cause sludge.
• Mutual solvents and aromatic preflushes that separate the acid from the oil or improve compatibility.

Frequently asked questions