The fluid and debris isolation sub (FIS) is an enhancement of the standard cone-type debris-circulating sub for heavy mud environments or large amounts of debris. The longer circulation slots allow greater amounts of relatively large debris to fall out rather than accumulate on the ceramic disc. Alternatively, debris can be circulated out. Because heavy annulus fluids trap the clean fluid in the space between the cone and the firing head they cannot displace the clean fluid and the firing head operation is unaffected by mud and debris.

Underbalance enabling

Before the packer is set, a lighter fluid can be circulated through the tubing to create the required underbalance condition. After firing the guns, the ports in the FIS let the formation fluid flow to the surface through the tubing. The ports are sized to provide a minimum unrestricted flow area equal to the tubing flow area.

Debris removal from dirty strings

Whenever possible, “dirty” strings should be periodically circulated on the way down. Circulation prevents debris accumulation above the cone, which often results in plugged tubing. The frequency of circulation depends on the initial cleanliness of the string. Typically, at least one tubing volume is circulated before setting the packer. If plugging occurs, reverse circulation can be employed to unclog the ports.