One of the riskiest phases of completing a deepwater slimhole well is the wellbore displacement. For a recent series of deepwater Gulf of Mexico slimhole completions, two pressure issues combined with one major economic issue to necessitate some creative problem-solving.

In Well #1, the operator used 15.4-ppg synthetic oil-base mud (SOBM) to drill a sidetrack

A conventional staged displacement would have taken at least 30 hours longer than a weighted, direct displacement because of the low rates possible given rig pump limitations. (Graphic courtesy of BJ Services)
with an aggressive trajectory to a target depth of 16,700 ft (5,094 m). After displacement to 15.2-ppg ZnBr2 completion fluid, the well was to be completed with a frac pack.
In a conventional displacement, mud is displaced to seawater and then to the desired completion fluid. This series of wells required a different solution for several reasons:
• Enormous hydrostatic pressure differences (more than 6,000 psi) between the mud and sea water;
• Limited rig pump safe output pressure (4,500 psi);
• Tapered workstrings;
• Large completion fluid volumes; and
• Rig costs upwards of US $300,000 to $650,000/day.

The hydrostatic pressure difference would require extremely high pump pressures to achieve annular velocity required to achieve good cleaning efficiency (see figure). The rig pump limitation dropped the possible rate significantly, adding some 30 hours of displacement time to the job, not counting any additional time that might be required for cleanup due to the reduced efficiency.

Instead, BJ engineers designed a displacement procedure that would meet both pressure and rate limitations. First, the riser would be conventionally displaced (mud to water to completion fluid). Then, a weighted displacement fluid system would enable direct displacement of the well bore from mud to completion fluid without exceeding rig pump limits and without dropping the rate below the level required to achieve timely displacement with good cleaning efficiency.

The objective of weighted displacement is to achieve a direct mud-to-brine displacement with premixed cleaning spacers, and to clean the well bore without time-consuming filtration cycles, which minimizes waste disposal and reduces rig time and, therefore, rig costs.

High-efficiency operation
The recipe for the displacement included 17.5- and 12.0-ppg barite spacers aimed at helping equalize hydrostatic differential pressures to displace the heavy mud, eliminating the need for high pump pressure or additional pumps while minimizing costs. The spacers would be followed by surfactants, solvents and viscous fluids designed to help remove wellbore mud, drilling solids and other debris, and restore the tubulars to a clean, water-wet state.

In Well #1, the displacement was pumped as designed and resulted in a
30-bbl fluid interface. Total pump time was 8 hours, with estimated 15 hours of non-productive time savings valued at around $200,000.

In Well #2, the 14.6-ppg water-base mud was displaced to a depth of 15,000 ft (4,575 m) with 14.8-ppg ZnBr2 completion fluid. Using the weighted system for direct displacement of the well bore, a 25-bbl interface was created. Total pump time was 9.5 hours.

In Well #3, 13.5-ppg SOBM was displaced to a depth of 26,000 ft (7,930 m) with 11.5-ppg CaBr2 completion fluid. Displacement created an interface of 30 bbl. Pump time was 9.5 hours.

The operator estimated that each efficient direct displacement had eliminated at least two days of circulation time for a rig spread cost savings of nearly $2 million.

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