Consistent leaks in storage wells
UGS wells help store natural gas and reduce the risk of service
interruption. They often cross multiple hydrocarbon zones and have long
lifetimes (80+ years). In Europe, these sites are mostly depleted gas
reservoirs into which gas from other sites has been injected.
Operating on behalf of Stogit, Eni worked with Italian depleted gas
production reservoirs at average depths of 1,000 to 1,500 m [3,280 to 4,921
ft]. High injection pressures, high production rates, and frequent changes in
temperature and pressure caused sustained casing pressure (SCP) that
compromised well integrity. Leaks were common. Regulations also required low
SCP thresholds near urban areas (facilities are located just 80 km [50 mi] from
Milan, Italy).
Eni sought to improve its cementing strategy with better isolating
properties.
Self-healing cement blend
To counteract gas leakage caused by changes in pressure and temperature
after the cementing process, Eni chose to use FUTUR cement, a cement blend with
intrinsic self-healing properties automatically activated by hydrocarbon
exposure. This technology protects against hydrocarbon leaks and SCP at the
wellhead. Pumped and placed as part of a primary cementing operation, the FUTUR
cement sheath forms an added isolation barrier above the reservoir. If the
sheath becomes damaged, FUTUR cement responds within hours by swelling to close
gaps and flow paths.
For the Eni wells, a 1,700-kg/m3 [14.2 lbm/galUS] FUTUR system was used,
including the D600G GASBLOK gas migration additive. The team placed 14 m3 [118
bbl] of slurry behind a 17.78-cm [7-in] liner. Over a 3-year period, 26 wells
were cemented with FUTUR self-healing technology.
Restored well integrity
The FUTUR system provided the required level of zonal isolation for the
26 wells. The hydraulic integrity of each was completely maintained—no SCP
was experienced after the implementation of FUTUR technology. These successful
operations also eliminated the costs related to remedial cementing operations
and lost inventory.
