Organic acids in carbonate addizing

This paper discusses the use of organic acid as a stimulation fluid in carbonate formations. Organic acids, such as acetic or formic acid, have been used for many years as alternatives to HCl because of the retarded reaction rate, low corrosivity, and reduced tendency to form acid/oil sludge in asphaltene-rich crudes. In Venezuela, organic acids were pumped in acid-fracturing treatments in deep, hot limestone formations. Although these treatments were very successful, optimization of the design was hampered by the fact that models for organic acid/carbonate spending did not exist. Case histories are discussed later. A new model for acid spending is proposed in this paper that can be used for strong (HCl) and weak (organic) acids as well as acid mixtures. Compared to existing (HCl) models, the only new element is the acid dissociation constant that describes the differences between strong and weak acids. The model was verified by comparing simulated reaction rates with rates actually measured in the laboratory with a rotating disk. The results of acid-fracture simulations, comparing the etched length and width of organic acid with HCl acid, are also discussed.