In recent years, it has become even more challenging to qualify acid systems that satisfy stringent corrosion acceptance criteria. This paper describes a unique approach of combining the effect of cooldown and effective treatment design to perform scale removal treatments in high-temperature (HT) wells in the Norwegian sector of the North Sea. The method helps minimize the use of corrosion control additives and its associated cost without compromising the integrity of the well components.

Controlling corrosion is a key consideration in designing acid recipes for HT wells. Achieving sufficient cooldown can further help ensure that inhibition lasts for the entire treatment duration. In this work, temperature modelling software was used to determine the volume required to cool down the well from 135°C to a specific temperature. Static weight loss corrosion testing was used to design the inhibition chemistry on 13Cr-L80 alloy. A corrosion loss of ≤ 0.02 lbm/ft2 for the time tested with no pitting observed at 40x magnification was considered acceptable.

The corrosion tests were performed in both organic and inorganic acid mixtures with varying acid concentrations over a temperature range of 40 to 135°C for a period of 3 to 9 hours. This paper discusses the acid recipe composition, results of corrosion testing, and microscopic analysis to determine pitting. The simulation results of the temperature modelling show that wells can be easily cooled down to as low as 40°C by pumping a sufficient volume of preflush at a certain rate, and examples of such treatment designs are presented in this paper. The qualified acid system was used for carbonate scale removal in the Norwegian sector of the North Sea. The treatment post-job analysis showcases an improvement as a result of the acid treatment.

The approach presented serves as a cost-effective guide toward designing scale removal or acid stimulation treatments for HT wells. The details shared in this paper are aligned with the drive of the oil and gas industry toward using cost-effective solutions and environmentally acceptable chemicals.

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