Development cycle for the oil and gas offshore fields at the closing stage provides for the obligatory well abandonment operation, conducted by the company-operator. The article generalizes Vietsovpetro experience on implementing the process solutions for offshore well abandonment: seawater, polymer-clay drilling mud and calcium chloride solution. Statistics data analysis of technical conditions for suspended/abandoned production strings for the last 10 years showed that the most corrosion activity belongs to seawater. Proportion of abandoned wells having corrosive wear signs, filled with seawater, regardless on oxygen scavenger and corrosion inhibitor additives, is 55 %. The signs of corrosive wear were identified in 45 % of wells, abandoned by polymer-clay solutions, treated with biocides and caustics. Preserving liquids based on calcium chloride mix with corrosion inhibitor additives have almost no corrosion impact on production strings material.
The authors stated main requirements to process solutions for temporary suspension/abandonment of wells on sedimentative stability and corrosion activity. It is indicated, that mostly highlighted criteria met by salt brines based on calcium bromide, formiate of potassium and caesium. Such solutions do not reveal acid properties and have low corrosion activity. For preserving liquids with density 1400–1800 kg/m3 it is recommended to use solutions of potassium formiate and calcium bromide; with density 1800–2000 kg/m3 – mixture of potassium and caesium formiate. Implementation of preserving liquid based on zinc and calcium bromides mixture is not recommended due to possible hydrogen-induced cracking of steel, referred to aggressive types of corrosive wearing.
References
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