Abstract
The use of lightweight materials (ceramic or glass beads) is not new in our industry. But mainly, they have been used to control shallow water/gas flows. There are very few documented cases where lightweight slurries have been pumped across gas producing zones (normally deeper than the previous ones).
British Gas' offshore Scarb Field was producing from the 8.5" open-hole gravel pack completion, below the 9-5/8" casing, until this pay zone was depleted. The plan was then to plug and abandon this zone, side track from the 9-5/8" casing and run a 7" Production Liner to 1,900 mMD approximately. In this depleted field, each well was expected to produce around 60 MMscf of gas. Scarb Field is mainly a gas field, producing considerable amount of gas from the 1,600 – 1,900 meters interval. The pore pressure is between 2,000 – 3,000 psi at the zone of interest.
The dynamic gradient at the shoe depth was limited to 0.65 psi/ft, and the maximum allowed differential pressure over the zones to be cemented was restricted to 500 psi (fracture gradient around 0.7 psi/ft). To overcome this challenging scenario, 12.5 ppg cement slurry (based on ceramic beads) was proposed. As any slurry to be placed across a production zone, this had zero Free Fluid, low Fluid Loss and high Compressive Strength.
The other obvious concern was the quality of the cement bond. Due to the small volume involved, the slurry was batch mixed and all the basic good cementing practices followed.
This paper summarizes how the first two jobs were executed and the results obtained from the CBL/VDL logs.
