During a drilling campaign in the southern North Sea involving installation of two additional conductors inside a mono-tower, significant challenges to the operator were encountered. The mono-tower is a hydrocarbon production platform with six slots. The slots are located inside the foundation pile, extending above sea level, and were driven into the seabed using an impact hammer. Structural integrity of this foundation pile over the platform operating lifetime is paramount.

The basis of the well is its conductor, which is typically installed using the drill/drive method with an impact hammer followed by soil drillout. Offset installation challenges in the mono-tower deemed using the drill/drive method ineffective for this new well. An alternative method for installing conductors is the drill/grout method. However, drilling an open hole increases the possibility of massive washout or hydraulic fracture. The fluids provider designed a water-based mud (WBM) system customized for rapid fluid-loss prevention, thus strengthening the wellbore, maintaining hole integrity, and preventing hydraulic fracture.

In subsea or conventional platform wells, fluids return at the seabed without additional hydrostatic pressure. Because of drilling inside the foundation pile, the lowest achievable elevation for fluid returns is at the top of the foundation pile. Fluid flow into the formation can lead to degradation of the soil-bearing strength around the single foundation pile and adjacent wells. This could potentially lead to subsidence of the mono-tower or buckling of the existing wells. In case of experiencing ever losses a contingency plan was in place with a decision tree used to determine the most appropriate formulation and sequence of lost circulation material (LCM) pills, with setting cement plugs as a last resort. Installing the two additional conductors with that rig could have impaired the structural integrity of the platform foundation pile. The drilling mud design played a vital role in managing this risk.

The uniqueness of the fluid design helped deliver the section without wellbore instability issues, hence avoiding losses or hole collapse, requiring reactive measures. The results demonstrated that wellbore pressure containment and full hole integrity can be achieved when drilling unconsolidated sands.

The novelty of the technical process and fluid design approach acted as a barrier to help prevent losses and proved the theory of wellbore stabilization and strengthening in unconsolidated sands through field applications.

This paper focusses only on discussing the application of an engineered fluid solution through a rigorous technical approach, which contributed to the success during such critical operations.

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