Background
Field: Deepwater Gulf of Mexico
Well Type: High-pressure, high-temperature (HPHT) gas well
Challenge:
- Required completion fluid density of 11.2 ppg to control reservoir pressure (~8,500 psi).
- Water-sensitive shale formations present in intermediate casing sections.
- Low seabed temperatures (4°C / 39°F) risked brine freezing during displacement.
Fluid Selection Comparison
| Parameter | CaCl₂ Brine | NaCl Brine |
|---|---|---|
| Max Achievable Density | 11.6 ppg (1.38 SG) | 10.0 ppg (1.20 SG) |
| Shale Inhibition | Excellent (Ca²⁺ stabilizes clays) | Poor (Na⁺ worsens swelling) |
| Freezing Point | -50°C (-58°F) | -21°C (-6°F) |
| Corrosivity | Moderate (controlled with inhibitors) | Low (but inadequate density) |
| Cost | $2.10/gal (for 11.2 ppg) | $3.50/gal (with barite weighting) |
Operational Results
CaCl₂ Brine Performance
- Well Control:
- Maintained stable hydrostatic pressure with 11.2 ppg brine, preventing gas influx.
- Shale Stability:
- No wellbore collapse observed during 7-day completion phase.
- Low-Temperature Handling:
- No freezing issues during subsea pumping.
- Cost Savings:
- Saved $250,000 vs. NaCl + barite system.
NaCl Brine Limitations (Simulated Scenario)
- Insufficient Density: Required barite addition, leading to:
- Solids sedimentation in risers.
- Formation damage (barite invasion into pay zone).
- Shale Instability: Caused tight spots during tool runs.
- Higher Logistics Cost: Barite handling increased rig time.
Key Takeaways
- Density Matters:
- CaCl₂ achieved target density without solids, avoiding formation damage.
- Shale Protection:
- Ca²⁺ outperformed Na⁺ in clay-rich zones.
- Cold Readiness:
- CaCl₂’s ultra-low freezing point ensured operational safety.
- Economics:
- 30% cost reduction vs. weighted NaCl systems.
When NaCl Might Still Be Preferred
- Sulfate-bearing zones (CaCl₂ risks CaSO₄ scaling).
- Low-pressure wells where 10.0 ppg suffices.
- H₂S-rich environments (NaCl has better sulfide compatibility).
Conclusion
This case demonstrates CaCl₂’s superiority in deepwater HPHT completions, balancing performance, stability, and cost. NaCl remains viable for simpler wells, but CaCl₂ is the engineer’s choice for challenging environments.
