Organic acid has been widely applied for inorganic scale treatment in oil and gas industry including well stimulation and scale inhibitor. Thanks to its low corrosivity and slower reaction rate with rock, organic acid is considered to offer better performance comparing to strong acid - Hydrochloric Acid (HCl). Yet, proper treatment requires vigorous analysis and experiment in order to meet foremost expectations. Besides, mistreatment of scale could result in formation damage including clay precipitation.

Pre-treatment experiments were performed on Zelda field at South East Sumatera block, that has faced with scale problem for ages. Water sample was taken from flowing Zelda A-08 well to be analyzed for mineral's saturation level. Scale was extracted from three sources including tubing, sand bailer, and Electrical Submersible Pump (ESP) of Zelda A-08. Those scale were treated in X-Ray Powder Diffraction (XRD) for mineral composition, and solubility test that utilized two types of acid system - formic acid (HCOOH) and hydrochloric acid (HCl) for comparison. Anti-swelling test and corrosion test were performed to examine the effectiveness of clay stabilizer and corrosion inhibitor.

As for carbonate analysis, both formic acid 9% and HCl 15% have comparable solubility (98.17% vs 98% for tubing's scale, 91.86% vs 82.79% for ESP's scale, and 70.30% vs 68.07% for sand bailer's scale). Yet, longer reaction is carried out by formic acid 9% (1 hour) comparing to HCl 15% (18 minutes). For silicate analysis, HF-formic acid provided the higher solubility than HF-HCl (8.34% vs 5.67% for ESP's scale and 30.48% vs 25.68% for sand bailer's scale). On anti-swelling test, by reducing swelling tendency up to 62.6%, it proves that examined clay stabilizer works perfectly against swelling potential of clay, despite of high swelling tendency of sand bailer's scale (25.8%). On corrosion test, adding on corrosion inhibitor (pyridine-based) into solution results in regular HCl 15% has corrosion rate 26.279 g/m2.h which is much higher (300%) than HF-HCl (7.977 g/m2.h) and HF-formic acid (8.229 g/m2.h). Based on pre-treatment test, formic acid 9% together with examined corrosion inhibitor and clay stabilizer, can be used as an alternative to regular HCl 15% for stimulation purpose where more areas will be covered that previously left unreachable by regular acid 15%. In addition, potentially more effective squeezed scale inhibitor using organic acid can also be achieved by performing further experiments.

The method presented in this paper for pre-treatment experimental studies of organic acid can provide engineers with intensive guide to meet the best result of organic acid treatment.

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