Ann: Cuba Block 9 Update, page-1619

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Hi Andrew, I think best chance of success would be drilling under balanced Considering the massive mud intrusion regardless of the special RDF formulation. Not sure about availability of specialty manifolds and other special surface equipment required to drill and flow at the same time but I hope it’s under serious consideration for all future wells. Please refer AI chat commentary here

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Yes, drilling underbalanced (UBD) can be a good approach in this scenario, particularly when dealing with a highly fractured and vuggy formation that’s sensitive to mud invasion and damage. Let’s break this down and address your key considerations:

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Why Underbalanced Drilling (UBD) May Be Better Here
1. Minimizes Formation Damage:
• In fractured and vuggy carbonates, drilling overbalanced often leads to deep invasion of drilling fluids and solids into the formation, especially when barite-laden muds are used. This clogs pore spaces and fractures.
• UBD allows formation fluids to flow into the wellbore, reducing mud invasion and thus preserving permeability.
2. Improves Productivity:
• By preventing the formation of damaging filter cakes and emulsions, UBD can significantly improve initial production rates and ultimate recovery.
3. Early Reservoir Evaluation:
• UBD can allow real-time inflow testing and better reservoir characterization while drilling.

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Challenges and Mitigation
1. Well Control (Blowouts & Kick Management):
• Yes, a properly sized and rated Blowout Preventer (BOP) is essential and can enable safe UBD if integrated into a broader well control strategy.
• In UBD, the well is intentionally allowed to flow, so BOP alone isn’t sufficient. You also need:
• A rotating control device (RCD) to manage annular flow.
• A choke manifold system to control surface pressures.
• Possibly a separator if formation fluids (oil, gas, or water) are expected during drilling.
2. Operational Complexity:
• UBD is more complex than overbalanced drilling — requires real-time monitoring, specialized crew training, and good reservoir knowledge.
• If gas influx is expected, nitrogen-based UBD may be used to control hydrostatic head precisely.
3. Hole Stability:
• If the formation is mechanically unstable, drilling underbalanced could trigger wellbore collapse. But if your prior overbalanced case resulted in severe formation damage, this may be a worthwhile tradeoff.

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Practical Recommendation

If the previous well experienced emulsions, significant invasion, and formation plugging, UBD should be strongly considered for the reservoir section (600m–1000m).

Yes, a sufficiently rated BOP combined with RCD and surface control systems can make UBD feasible and safe.

You might also consider:
• Displacing to UBD just for the reservoir section (from 600m) after drilling the upper hole conventionally.
• Running UBD with managed pressure drilling (MPD) as a hybrid if continuous inflow is too risky.

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If you want, I can help you map out a high-level UBD setup for your well or compare UBD vs MPD options based on your formation specifics.

Great — let’s build a high-level plan comparing Underbalanced Drilling (UBD) and Managed Pressure Drilling (MPD) for your 1000 m well targeting the net pay from 600 m to 1000 m, through a fractured, vuggy, and damage-prone formation.

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1. Overview of Options

Feature Underbalanced Drilling (UBD) Managed Pressure Drilling (MPD)
Bottomhole pressure Intentionally less than formation pressure Precisely controlled to be slightly below or equal to pore pressure
Fluid Influx (Reservoir Flow) Expected and managed at surface Typically avoided, but system can handle small kicks
Damage Control Excellent — minimizes or eliminates mud invasion Very good — minimizes fluid loss with precise pressure control
Well Control Requires RCD, BOP, separator, choke, gas handling system Requires RCD, automated choke, pressure control manifold
Cost & Complexity Higher due to gas injection or lightweight fluids Moderate — uses conventional mud, fewer operational changes
Best Use Highly damaged formations where productivity is key Tight pressure window, risk of kicks/losses, stability concerns



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2. UBD Setup for This Well

A. Drilling Strategy
• Surface hole (0–600 m): Drill conventionally with weighted mud and casing set at ~600 m.
• Reservoir hole (600–1000 m): Drill underbalanced.

B. Key UBD Equipment

Component Description
Rotating Control Device (RCD) Seals around the rotating drill pipe, diverts flow to surface separator.
Choke Manifold Maintains backpressure, controls reservoir influx.
Surface Separator Separates formation fluids and gas from drilling returns.
Gas Injection System (Optional) Nitrogen or natural gas used to reduce mud weight.
Drilling Fluid Lightweight fluid or gasified mud (aerated, mist, or foam), low solids.
BOP Stack Rated for anticipated maximum shut-in pressure (based on reservoir pressure plus margin).

C. Operational Considerations
• Require a trip gas management plan and real-time monitoring.
• Plan for flare or storage of produced fluids (depending on H2S, oil/gas volumes).
• Team needs training in underbalanced operations and kick management.

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3. MPD Alternative

If full UBD is deemed too complex or risky, MPD offers a controlled compromise:

MPD Setup
• Use standard mud system with automated pressure control.
• RCD + MPD choke system used to maintain near-balanced or slightly underbalanced conditions.
• Can ramp down pressure gradually to limit invasion without allowing full influx.
• MPD also allows easier transition to conventional drilling if necessary.

Advantages in Your Case:
• Lower surface handling requirements than UBD (no separator or produced fluid processing).
• Better suited if gas influx or fluid disposal is a concern.
• You can maintain slightly underbalanced condition to reduce damage without fully flowing the well.

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4. Recommendation for Your Case

Given your description:
• Formation is highly fractured and vuggy.
• Prior well experienced severe damage from mud invasion and emulsion formation.
• You need to preserve reservoir deliverability.

Recommended Option:

Drill surface conventionally → transition to UBD or MPD at 600 m.

UBD preferred if:
• You can handle produced fluids at surface.
• Formation pressure and influx are manageable (no extreme sour gas or pressure spikes).
• Maximizing productivity is critical.

MPD is a safer fallback if:
• You have limited surface infrastructure.
• You want more flexibility in controlling downhole pressure without producing.

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If you’d like, I can help model the fluid system (e.g. UBD fluid density or MPD choke pressures) or review a casing/BOP design for either approach. Let me know how deep you want to go.