This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208185, “Single-Phase Retarded Inorganic Acid Optimizes Remediation of Drilling Formation Damage in High-Temperature Openhole Horizontal Carbonate Producer,” by Ahmed Mohamed Fawzy, Noor Nazri Talib, SPE, and Ruslan Makhiyanov, SPE, ADNOC, et al. The paper has not been peer reviewed.

Emulsified acid is one of the preferred choices among retarded-acid options used in Middle East high-temperature carbonate producers. However, it also brings operational complexities. The recent introduction of a single-phase retarded inorganic acid system (SPRIAS) has enabled stimulation with the same benefits as those of emulsified acids while eliminating its drawbacks. A newly drilled oil producer in one of the largest carbonate fields onshore Middle East was selected for pilot implementation of the SPRIAS. The experience gained with this pilot well confirmed the SPRIAS as a reliable option to replace emulsified acids in the region.


The SPRIAS has the dissolution capacity of hydrochloric acids (HCl) of similar concentration. It also has a retardation factor equivalent to that of emulsified acid, which allows the creation of wormholes just as deep into the formation. Such a retardation effect was demonstrated through numerous pore-volume-to-breakthrough (PVBT) coreflow experiments at downhole conditions. Fig. 1 shows such a PVBT experiment with 15% HCl, emulsified acid, and a SPRIAS of similar HCl acid concentration on a 1-in.-diameter×6-in.-long, 8-md Indiana limestone core at 250°F. The PVBT of the emulsified acid and SPRIAS are significantly smaller than that of HCl and are statistically similar.

Because it is not an emulsion but a single-phase aqueous fluid, the SPRIAS also has a viscosity within the same order of magnitude as HCl. This allows the SPRIAS to be pumped at higher rates than retarded emulsified acid. For the application discussed in this study, the SPRIAS was selected to improve the stimulation of the tighter part of the wellbore because the retarded-acid system can develop deep wormholes at low injection rates.

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