Achieving Long-Term Isolation for Thin Gas Zones in the Adriatic Sea Region

Abstract

Abstract This paper describes the application of a flexible cement system to isolate pay zones, spaced 1 m apart, in the Barbara gas field in the Adriatic Sea, off the coast of Italy. This field presents several challenges to achieving long-term zonal isolation—crossflow between pressured zones, changing mud weights, and a high-pressure cold-fluid injection fracture treatment. One or more of these conditions may contribute to a loss of isolation; hence, there is a need for an effective sealant material that can handle these challenges. Conventional cement systems have been unable to address zonal isolation at such a scale. In view of these various challenges, the team in charge of cementing these wells redesigned the cement program to mitigate the risks in the field. The main objective was to minimize the Young's modulus (maximize flexibility) and maximize the compressive strength of the set cement. Thus, an extensive program was carried out to measure the mechanical properties of the set cement. The new cement program used a flexible, expandable slurry system that isolated five zones behind 7-in. casing in the Barbara gas field. A cement-bond log evaluation confirmed isolation of the pay zone. No losses were observed during cement placement across the low-fracture-gradient formations. The new slurry system brought substantial value to the operation by enabling use of 7-in. casing as the production casing. It also eliminated any interzonal pressure communication between formations and the need for an additional 5-in. liner, which saved rig time and rig costs.