A review of acid-induced sludge formation and dominant parameters

Acid stimulation has proved efficacy in improving the production of crude oil; however, inappropriate design of acid job can lead to a new type of formation damage, including rigid film acid-crude oil emulsion and acid-induced sludge formation, which are caused due to incompatibility of the injected acid and the crude oil. A variety of influential parameters on the amount of formed acid-induced sludge have been identified including crude oil properties and its components, injected acid concentration and its type, iron ions presence, volumetric ratio of acid and crude oil, chemical additives, and temperature. It is indispensable to examine reliably the potential damage relating to reservoir conditions and to adjust the operating conditions of injection process for an appropriate acid job. It has been endeavored in this paper to provide a comprehensive review of acid-induced sludge, its formation and underlying mechanisms, dominant parameters such as the properties of aqueous and organic phases, operating conditions, experimental methods for sludge evaluation and their challenges, and finally approaches to prevent sludge formation. Thorough analysis of these parameters would serve as a guideline for the production engineers to minimize the operating costs and to improve the acid job effectiveness.