Completion Types

A completion is everything installed in a well to allow it to produce safely and controllably. The design is driven by the reservoir, the fluids, the recovery plan and the intervention philosophy — and it is largely irreversible, because changing it later means a workover costing a substantial fraction of the original well.

Open hole versus cased hole

• Open hole — the reservoir section left uncased. Maximum flow area, no perforating cost, and no damage from cement. But no selectivity: individual zones cannot be isolated, water or gas cannot be shut off, and the hole must be mechanically stable. Used in competent formations, and common in horizontal wells with segmented completions.
• Cased and perforated — casing or liner cemented across the reservoir, then perforated. The dominant approach because it allows selectivity: choosing which intervals to open, isolating water or gas zones later, and stimulating individual intervals. The cost is a flow restriction through the perforations and the risk of formation damage from cement and completion fluids.
• Liner with external packers — a middle route used extensively in horizontal wells, dividing the lateral into isolated compartments without cement.

Single, multiple and smart completions

• Single string — one tubing, producing one commingled stream. Simple, reliable, cheapest.
• Dual or multiple string — separate tubing strings producing separate reservoirs, allowing independent control and allocation. Complex, congested, and harder to intervene in.
• Selective single string — one string with sliding sleeves or similar, allowing zones to be opened and closed by intervention.
• Intelligent (smart) completions — downhole flow control valves and permanent gauges operated from surface, allowing zones to be choked or shut without intervention. Very valuable where intervention is expensive — subsea and deep water above all — and where zonal management materially affects recovery. The trade-off is complexity and the consequence of a downhole failure that cannot be repaired.

What drives the choice

• Reservoir properties, heterogeneity and the number of zones.
• Expected water and gas breakthrough, and how it will be managed.
• Sand production potential.
• Fluid properties, including corrosion and scaling tendency.
• Artificial lift requirement now or later — a well that will need a pump should be completed so one can be installed.
• Intervention cost and access. A land well that can be worked over cheaply justifies a simple completion; a subsea well in deep water justifies spending heavily on a completion that will never need to be pulled.
• Well life and abandonment requirements.

Completion design is where the subsurface plan meets a permanent commitment. It should be settled with the reservoir and production engineers together, and before the well is drilled — not after.