Damage evaluation of acid-oil emulsion and asphaltic sludge formation caused by acidizing of asphaltenic oil reservoir

Acidizing is one of the most effective techniques to remove the formation damage and restore (or even increase) the permeability of the near wellbore region. Although this technique is widely used to resolve the skin problem, it could initiate new damages itself that hinder the fluid flow and decrease the well production, consequently. Acid-oil emulsion and sludge formation are known as two major induced formation damage and the main reasons for the oil well acid treatment failures. Despite its critical effects, no comprehensive study has been addressed this specific type of formation damages in details. In this study, the acid-oil emulsion and asphaltic sludge formation were evaluated quantitatively. Multiple compatibility tests were conducted using HCl and six Iranian acid sensitive crude oil samples. The acid concentration, ferric ion presence, and acid-mixture ratio were selected as the main factors affecting the emulsion stability and the amount of sludge. To mimic the real condition the interference of the effects of these factors was noticed besides the crude oil type which has not been addressed before. The results revealed that controlling the iron ion presence, choosing the best acid concentration, and regulating the acid-mixture ratio would drastically reduce the risk of emulsion and sludge formation without any preventative additives presence. The quantitative values derived from this experimental work showed that the weight of dried asphaltic sludge formed was diminished to 7%, 5.6%, 2.4%, 22%, 7.5% and 10% of the most critical sludging condition for the crude oil samples A, B, C, D, E, and F respectively. Besides, for the first time, the volumetric measurement of the massive asphaltic sludge was performed to approach more realistic damage evaluation. The results confirmed it is possible that the solid sludge occupies 60 vol % of the primitive acid-oil mixture volume at high concentration of HCl and ferric besides high acid-oil ratio condition.