The fluid/debris isolation sub is an enhancement of the standard cone-type debris-circulating sub for heavy mud environments or large amounts of debris. The circulation slots are longer to allow a larger amount of relatively large debris to fall out rather than accumulate on the ceramic disc. Also, heavy annulus fluids trap the clean fluid in the space between the cone and the firing head. Thus, the lighter fluid cannot be displaced by the heavy annulus fluid, and firing head operation, therefore, is unaffected by mud and debris.
The cone stops falling debris in the tubing and prevents it from settling farther down on the firing head. The conical shape of the sub allows most of the debris to slip through the sub ports into the annular space. Alternatively, debris may be circulated out.
Underbalance and production
Before the packer is set, a lighter fluid may be circulated through the tubing to create the required underbalance condition. After firing the guns, the ports in the sub allow formation fluid to flow to the surface through the tubing. 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 possible debris accumulation above the cone, which often results in plugged tubing. Frequency of circulation depends on the initial cleanliness of the string. Circulate at least one tubing volume before setting the packer. The typical pump rate is 2 bbl/min. If plugging occurs, reverse circulation may unclog the ports.