Batch drilling and completion improve field development efficiency in deepwater subsea environments, but well completion operations may be suspended for a long time while waiting for a subsea production system to be installed.
An Angola deepwater field development plan called for five oil producers, two water injectors, and two water-alternate-gas injectors. Using a fast-track approach, the drilling campaign started in parallel with the subsea production system manufacturing. Seven of the nine wells were drilled and completed with reservoir isolation valves (RIVs) that were expected to suspend the wellbore for several months and then remotely open the wellbore without intervention. However, the RIV remote opening mechanism was compromised due to well conditions and compatibility issue in two wells, prompting contingency operations including coiled tubing and electric-line interventions and a redesign of the upper completions for the remaining wells.
The lower completion strings were designed to be isolated with an RIV equipped with a remote opening system based on a nitrogen precharged spring chamber and tubing-applied pressure cycles. After pressure cycles not trigging the remote opening mechanism to reopen the valves in two wells, root cause analysis determined that the combination of bottomhole temperature (120°C), fluid (CaCl2 brine), and long suspension time (12 to 22 months) prematurely aged the RIV elastomers, causing the nitrogen section to loose pressure integrity. As a contingency, intervention operations were performed in the two wells with compromised nitrogen section RIVs, and engineering designs were modified to optimize the upper completion strategy for the five remaining wells. The RIVs in the first two wells were successfully opened using coiled tubing milling (first well) and electric-line mechanical shifting intervention (second well)—the first such operations in the world for the full-ball type of RIV. The paper explains the tool selection and testing processes for the intervention operations, compares the economics of the different intervention approaches, and describes how engineers optimized the new completion design on the remaining wells.
By sharing lessons learned from root cause analysis, intervention operations, and new completion design, this paper adds critical experience to a relatively limited body of work related to engineering completions and contingencies for long-term well suspension and reentry for completion.
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