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Case Study: Long-Term Zonal Isolation in UGS Wells

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Multiple microannuli created during laboratory testing illustrate the repeatability of FUTUR active set-cement technology.

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Isolation Scanner cement evaluation service and CBL-VDL logs across dir 155, showing optimal cement placement.

ENI uses FUTUR active set-cement technology to ensure long-term well integrity in Italy

Challenge
Create long-term isolation in underground gas storage (UGS) wells in area with strict environmental regulations and where surface leaks are common occurences.

Solution
An integrated solution that included the FUTUR active set-cement technology to protect against the loss of well integrity.

Results
No surface leaks detected after completion and an excellent cement bond across the gas injection interval that is expected to last for the life of the well.

Isolation critical

ENI has many UGS wells in Italy. Its Stogit wells are very close to urban areas, and stringent regulations require leak-free wells. In this area, sustained casing pressure (SCP) and its effects on well integrity can lead to premature abandonment if leaks exceed the low threshold set by authorities. To avoid this waste of resources, ENI sought a long-lasting solution.

Innovative solution

For its Cortemaggiore 155dir well, the operator selected the revolutionary FUTUR active set-cement technology. Used in conjunction with good cementing practices and the GasMigrationAdvisor software package, this technology offers excellent protection against the loss of well integrity.

When hydrocarbons come into contact with the FUTUR sealant, it responds by swelling to close gaps and flow paths. Within hours, the cracks or fissures are healed, and the hydraulic seal is reestablished. The active set cement rapidly forms a complete barrier that continues to reseal if annular integrity is further compromised.

FUTUR active set cement can be strategically placed—from surface to reservoir depth, in any string of the well—to distribute an effective long-term seal throughout the well. The Cortemaggiore 155dir slurry was formulated to mitigate the concerns specific to that well. Those risks included

a well deviation of 48°, with expected washouts in depleted formations
a high risk of gas percolation
slow cement compressive strength development owing to low BHST (44 degC [111 degF])
a high risk of set-cement mechanical failure caused by cyclical stresses related to production and injection phases.

FUTUR active set-cement technology was used to cement two strings on the same well: around the 7-in liner across the injection/production zone and around the 7-in casing tieback.

Successful cementing

Once the cement was placed, it was evaluated using a cement bond log-variable density log (CBL-VDL) wireline logging measurement. The CBL-VDL readings were enhanced with readings from the Sonic Scanner acoustic scanning platform, which generates a solid-liquid-gas map of the material behind casing. These analyses show an optimal cement bonding across the treated interval, ensuring that the FUTUR active set-cement technology will be able to react efficiently and self-repair if the cyclical injection and production stresses cause a loss of isolation.

ENI plans to use the FUTUR technology as part of a multiyear UGS well construction project.

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