The circulation ball drop–activated firing (CBF) head has a small diameter and is used in CT operations. It allows you to deploy a perforating gun or other explosive device, such as a severing tool, or a cutting device on a CT string, into a well. It is the most commonly used of the various CT firing heads. For wells being abandoned, the CBF head allows you to circulate the well until it is clean and then to perforate, spot a cement plug, and circulate again to clean after the plug has been placed—all during one trip in hole with the CT. The CBF head can be used to shoot guns underbalanced and to drop guns or perforate without killing a well to minimize formation damage. The CBF head is available for high-pressure, high-temperature applications, and it supports multiple perforating techniques and the combination of other deployment systems.
The head uses the same hydrostatic firing mechanism as tubing-conveyed perforating firing heads, requiring a minimum hydrostatic pressure to fire. The initiation of the downhole detonation is controlled from the surface by first circulating a ball down to the CBF head. When the ball reaches the head and closes the circulating path by sealing the ball seat, the firing mechanism is activated by the differential pressure buildup across the operating piston rather than by the absolute bottomhole pressure. For this reason, a well can be perforated in overbalanced or underbalanced conditions and when existing perforations are present. Because the operating piston moves upward in the CBF head to release the firing pin, the head is unaffected by water-hammer effects and insensitive to vertical drops.
The CBF head is available in two sizes with multiple adapters to accommodate the various CT dimensions and gun sizes. The firing head holds a maximum of twelve 500-psi [3.4-MPa] shear pins and is recommended for gun sizes of 2.00 to 4.5 in. Use fill sub with guns 2 7/8 in or greater or heavier than 15,000 lbm [6,804 kg]. CBF fill subs are available to isolate the CBF head from the tensile strength of the conveyor string. The fill sub then carries the load while the CBF head functions purely as a firing head.
- Circulation flexibility—Circulation is allowed in both directions before and after the guns are fired. Underbalanced perforating conditions are easily established. The head also allows circulation after the guns are fired.
- Pressure-balanced system—The CBF head is insensitive to water-hammer effects and vertical drops.
- CT compatibility—The CBF heads can be connected to any CT string by means of a crossover connector and can be adapted to any type of Schlumberger perforating gun.