New Well Tie-In: Design and Monitoring Options Under Uncertainty
A new well is being tied into an existing 8-inch carbon steel flowline that currently carries sweet gas with 0.1 mol% CO2. The new well delivers 2.5 mol% CO2, 30 ppm H2S, and a water cut that is expected to rise from 5% to 40% over two years. The existing line has no corrosion inhibitor injection and no monitoring points beyond the inlet. The tie-in must be online in 90 days. Options under consideration: (A) install inhibitor injection and monitoring now, (B) install monitoring only and defer inhibition until water cut rises, (C) replace the line section with corrosion-resistant alloy. Budget is constrained, and the operator wants to minimize capital spend.
1. Compare the three options against the expected operating envelope and identify the key risks of each.
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- Option A (inhibitor plus monitoring now): highest upfront cost but addresses the CO2 increase and rising water cut before they bite. Risk: inhibitor may be unnecessary in the early low-water period, and dose optimization requires data. Mitigation: start at low dose and tune.
- Option B (monitoring only, defer inhibition): lowest upfront cost but highest corrosion risk. With 2.5 mol% CO2 and water cut rising to 40%, the line will move from dry to wet service, and without inhibition the corrosion rate can climb quickly. Monitoring without a response plan is not control. Risk: by the time monitoring shows a problem, wall loss has occurred.
- Option C (CRA replacement): eliminates internal corrosion risk for the replaced section but is the highest capital cost and does not address the rest of the line. Risk: galvanic or preferential corrosion at the transition joint if not designed correctly, and the rest of the line remains carbon steel.
Key judgment: the decision hinges on the timing of water breakthrough and the consequence of failure, not just capital cost.
2. What design and monitoring features should be included regardless of which option is chosen?
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Regardless of option:
- Monitoring points at the inlet, midpoint, and downstream of the tie-in, with coupons and ER probes, plus iron counts and water chemistry.
- Injection point and mixing designed for future inhibitor addition even if inhibition is deferred, so Option A can be implemented without a shutdown.
- Sour service assessment for 30 ppm H2S, including weld hardness and material verification.
- Pigging capability to manage deposits and water holdup.
- A defined decision trigger: e.g., when water cut exceeds 10% or coupon rate exceeds 1 mpy, inhibition is activated.
These features preserve optionality and ensure that whichever option is chosen, the program can respond to the rising water cut.
3. Recommend an option and justify it, including how to manage the budget constraint.
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Recommendation: Option A, phased - install monitoring and the injection infrastructure now, but defer full-dose continuous inhibition until water cut or coupon data justify it.
Justification:
- The line is moving from dry to wet service with higher CO2; the corrosion risk is increasing, and monitoring alone does not control it.
- Installing injection and monitoring now is cheaper than retrofitting later under time pressure, and it satisfies the 90-day schedule.
- CRA replacement is disproportionate for a line that can be protected by inhibition, and it leaves the rest of the line unprotected.
Budget management: phase the spend - monitoring and injection skids now, chemical spend ramped with water cut. Set a trigger (water cut >10% or coupon >1 mpy) to start continuous inhibition. This gives the lowest lifecycle cost while preserving the ability to respond to the expected change in operating conditions.