A New Technique to Increase the Performance of Organic Acids to Stimulate Carbonate Reservoirs at High Acid Concentrations

Abstract

Organic acids have been used to stimulate HP/HT oil and gas wells. However, these acids cannot be used above certain concentrations, which depend on the type of acid used. For example, 9 wt% is the upper limit for formic acid concentration, whereas acetic and citric acids can be used only up to 13 and 1 wt%, respectively. Using these acids above these limits causes a formation damage results from the precipitation of the acids' calcium salts.A new technique was developed and experimentally proven to increase the concentration of organic acids that can be used in the oilfield without precipitation of the reaction products. This involves the addition of an environmentally friendly acid (gluconic acid) chelant to the selected organic acid under investigation. In a recently published paper (SPE-173751), the authors showed a significant improvement in the solubility of calcium lactate when lactic acid was mixed with gluconic acid to stimulated calcite rocks. The current work investigates the generalization of this idea for other organic acids such as: acetic, formic, citric, glycolic, and boric acids, with an objective to examine the effectiveness of the new acids to stimulate carbonate formation at 150-250°F and determine the optimum injection requirements.Coreflood results showed that mixing gluconic acid with acetic acid increased the solubility of the resulted calcium salt and allowed using acetic acid at 15 wt% without the risk of precipitating calcium acetate. When mixed with formic acid, a minimum of acid pore volume was observed at a gluconic: formic acids molar ratio of 1:7. This allowed the use of formic acid at 12.5 wt% without any observation of calcium formate precipitation. Gluconic-citric and gluconic-boric acid mixtures required as high as 4 PV to breakthrough, indicating that the enhancement of product solubility depends on the type of the acid mixed with gluconic acid. Finally, a gluconic-glycolic acid mixture with a molar ratio of 1:1 showed the least acid pore volume required to breakthrough (2.35 PV) with similar results to what was previously reported for gluconic-lactic acid mixtures.