A formic acid precursor (FAP) was used in a carbonate reservoir in offshore Qatar to achieve uniform filter-cake breaker treatment over the entire openhole section to improve wellbore productivity. Wells with a moderate wellbore length have been stimulated using a FAP with a delay time of ±3 hours, which is sufficient for spotting the precursor pill into the openhole reservoir section before the filter cake is broken by the treatment reaction (resulting in downhole completion fluid losses). In late 2014, a FAP cake-breaker treatment job was designed and performed for a 3741-m (12,270 ft) section of an 8½-in. open hole. This job required spotting a 159-m3 (1,000 bbl) FAP stimulation pill in the entire openhole section without incurring downhole losses while pumping. Thus, the delay time required for the precursor to convert to acid had to be increased to 8 hours. Extensive laboratory tests were performed to increase the delay time for mixing and spotting the precursor pill before converting to acid and causing a significant reaction with the reservoir. These tests showed that an additional 5 hours of delay time could be achieved by a redesigned formulation. Mixing time also improved by delivering the liquid precursor in tote tanks, rather than in drums. By performing the new fluid design and handling precursor material in tote tanks, the job was successfully run with an actual delay time of 8 hours, as compared to 5 hours for previous jobs. In addition, the retarded acid precursor pill was mixed and spotted into the entire open hole in 5 hours without downhole losses while pumping, indicating a successful mixing and spotting plan for the job. The downhole loss rate increased from zero at the end of spotting the precursor fluid to 2.5 m3/hr after 3 hours, signifying that the acid precursor was releasing acid, which reacted with the filter cake. An increase in downhole losses was observed after three more hours, when the loss rate increased from 2.5 m3/hr to 20 m3/hr. This increase indicated that the entire filter cake had reacted with the acid and that the acid had begun to enter the formation, acting as a stimulation fluid and enhancing well productivity. This job was classified as the largest acid-delay precursor job performed globally to date by volume pumped and by interval length. Extensive laboratory tests were conducted to redesign the precursor fluid to achieve a sufficient delay time to mix and spot 159 m3 (1,000 bbl) of the FAP pill into the entire open hole before the acid reaction began. Using a liquid-precursor material in tote tanks helped to minimize the mixing time by 50% and resulted in a more efficient use of limited rig deck space.
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