Improving well cementing efficiency in complex mining and geological conditions through the use of a sub-preventer sealer

The purpose of this work is to improve the safety and cost-effectiveness of the counter-cementing process of casing strings under conditions of lost circulation and the risk of gas, oil, and water shows, using the Yurubcheno-Tokhomskoye field as an example. The causes of frequent failures of the universal preventer during the counter-cementing with the closed universal preventer in order to create a vacuum effect were analysed. The study was given to the geological and technical conditions of the Yurubcheno-Tokhomskoye field, which led to 28 cases of equipment failure in 2023–2024. The design and operation principle of the sub-preventer sealer as an alternative technological solution were considered. A comparative technical and economic assessment of the sealer implementation was performed and the discounted cash flow, payback period, and profitability index were calculated. It was determined that the main cause of universal preventer failures was due to the operation of the sealing element in an aggressive cement slurry environment under the influence of a vacuum. It led to the loss of the second safety barrier. The proposed solution (to install a sub-preventer sealer between the casing head flange and the blowout preventer crosspiece) allows to seal the annular space of the casing string without the use of a universal preventer. The feasibility study showed that a one-time investment of 16 million rubles for equipping 5 drilling rigs covers the expenses during the first year of operation providing significant annual savings by eliminating the repair cost of the drilling rigs. The introduction of a sub-preventer sealer makes it possible to eliminate a major technological risk of second barrier failure during counter-cementing, thereby increasing the industrial safety of work. The technical solution is economically feasible since its yield index is <1 and its discounted payback period is less than a year. It can be recommended for use in fields with similar complex mining and geological conditions.

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