Nigeria's First Open-Water Riserless Cement Evaluation and Well Securing Operation

Abstract

Isolating hydrocarbon-bearing and over-pressured reservoirs in subsea wells is traditionally complex and costly, often requiring drilling rigs or marine vessels equipped with risers. This paper presents Nigeria's first deepwater riserless well-securing campaign on four suspended appraisal wells, delivering significant reductions in cost and time while meeting industry best practices. The approach effectively mitigated well integrity risks posed by hydrocarbons and over-pressured sands with flow potential behind casing. The wells contained open-hole intervals suspended with mechanical and/or cement plugs, and a brownfield survey confirmed no hydrocarbon seepage, enabling a riserless solution. The scope included an innovative corrosion cap retrieval and reinstallation using the vessel crane and sandlines, supported by a bespoke ROV tool; behind-casing cement integrity evaluation; and placement of permanent cement plugs via coiled tubing to achieve rock-to-rock isolation. These activities were executed in optimized batch operations for efficiency. Key considerations included sea-state impact on depth control for slickline and wireline, seawater corrosion on wireline, coiled tubing buckling, and cement contamination risks. This approach delivered 40–50% cost savings, eliminated safety hazards associated with riser-based methods, and provided a replicable workflow, decision matrix, and benchmarks for time and cost—offering a safer, more efficient model for future deepwater well-securing campaigns.