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, due to 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. Case histories are discussed in the paper. 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.
In this paper a new model for acid spending is proposed that can be used for strong acids (HCl) and for weak (organic) acids and also for acid mixtures. Compared to existing (HCl) models the only new element is the acid dissociation constant, describing the differences between strong and weak acids. The model was verified by comparing simulated reaction rates with rates actually measured in the laboratory with the rotating disk. Results of acid fracture simulations, comparing etched length and width of organic acid with HCl acid are also discussed in this paper.
In most carbonate stimulation treatments, above and below fracturing pressure, hydrochloric acid (HCl) is pumped as the main stimulation fluid. HCl is cheap, it has a high rock dissolving power and the reaction products are usually soluble. However, the HCl-carbonate reaction is also fast, especially at the higher temperatures encountered down hole. The rapid HCl spending prevents deep penetration of acid into the formation. In matrix treatments this results in only short worm-holes or even compact dissolution with only marginal skin decrease. In acid fracturing treatments it results in poor etched length, often considerably less than the fracture half-length created with the pad fluid. Other disadvantages of HCl are excessive tubing corrosion and associated high cost of inhibition and the tendency to form acid-oil sludge in asphaltene rich crudes.