Experimental Investigation of Acid-induced Sludge Precipitation: Using Acid Additives in Iran

Dilute hydrochloric and hydrofluoric acid are used in the fracturing and acidizing of compact oil formations to dissolve the undesirable carbonate and silica deposits or scales, which interfere with the passage of oil in tubing or in the formation itself. A typical acid formulation is based on 15% hydrochloric for carbonate reservoirs and 12/3 wt% hydrochloric/hydrofluoric (regular mud acid) for sandstone reservoirs. But higher acid strengths are also used. Contact of acid with the crude oil generates a precipitate known as acid-induced sludge. Acid-induced sludge is becoming an increasing cause of oil well stimulation treatment failure. Acid stimulation experimental tests were conducted on the Marun crude oil sample from the Marun oil field for quantifying the amount of sludge precipitation as a consequence of an acidizing job. The potential of sludge precipitation by acid additives, such as iron ion concentration, standard dosage of mutual solvent, corrosion inhibitor, corrosion aid, suspended agent, and surfactant of the Marun crude oil sample, were experimentally investigated. The results show that, by increasing the concentration of acid and iron ion, the potential of sludge precipitation increases. The threshold of acid concentration for serious sludge precipitation is 17% for hydrochloric and 12/3% for mud acid. Iron ions promote the sludge precipitation. A 7,500-ppm iron ion is confirmed to be the threshold for serious acid-induced sludge. The standard dosage of all additives used in the current study (additives were prepared from Iranian Cymanband Company) was found to cause sludge precipitation.