Acid stimulation experimental data collected in the laboratory on 231 crude samples from various oil fields were used in this study to formulate an empirical model for quantifying the amount of asphaltic sludge precipitating as a consequence of an acidizing job. Throughout the experiment, individual components of the acid system were introduced separately in order to single out their effects on sludge. The standard API procedures were slightly modified in order to mimic downhole conditions. Data showing the individual effects of acid strength, acid type, solvent preflush, iron control additives, ferrous and ferric ion concentrations, mutual solvent concentration, corrosion inhibitor type, and surfactant types have been used as the basis for formulating an empirical model for the weight percent of asphaltene sludge. The acidinduced sludge model has been validated with stimulation data from the Endicott field. The model is then used to study the effect of various factors on sludging.
A 15% HCL concentration is found to be the threshold for serious acidinduced sludge. Hydrofluoric acid exhacerbates sludging except at low concentrations less than 3%. Asphaltene sludging is promoted by the presence of ferrous and ferric ions. However, Ferric ions cause more sludging. The addition of iron control additives like citric acid, EDTA, and NTA is found to cause sludging. Erythorbic extractant causes less sludging than any of the iron control additives commonly used. The three surfactant types investigated (Fluoro-surfactant, Non-emulsifier, wetting agent) are found to have an identical effect on sludging. The use of a mutual solvent concentration greater than 5% is found to increase the sludging tendency.
Acid-induced asphaltene sludging is becoming an increasing cause of oil well stimulation treatment failure. The proposed model predicts quantitatively the effect of various acid additives on crude oil sludging, and may be used as a designing tool for an acidizing job with minimum sludging potential.
Sludging, acid, expert system
