Mechanical ball seat is a device that temporarily plugs the running string to provide the pressure-tight chamber necessary to hydraulically activate the liner hanger or release the running tools.
The temporary plug is created by dropping a setting ball that lands in a steel ball seat that is sheared out and retains the ball when the plug is no longer needed. The mechanical ball seat is located inside a sub in the running string, below the hanger and above the liner-wiper plug of the liner-hanger system. Before dropping the setting ball, the ball seat allows passage of a smaller trip ball to activate equipment below the liner hanger and running equipment, such as auto-fill float equipment, or a Defyer™ drill assembly. The primary setting ball is then used by the mechanical ball seat to isolate pressure above the ball seat. Upon completion of the hydraulic events, the ball seat shears with application of predetermined amounts of pressure and locks into the bypass position within the sub. With the ball seat in this location, pressure surge on the formation is eliminated or reduced when the ball seat shears.
If cementing plugs are run, they are located just below the mechanical ball seat and may sustain damage by the pressure surge. In this case, the energy-absorption tool (EAT) is placed between the plugs and the mechanical ball seat to diffuse the pressure surge before it reaches the plugs, saving them from damage. After the mechanical ball seat is deactivated, the ball is trapped in the body, and the mechanical ball seat is opened full-bore for passage of drillpipe darts. In the event the ball seat cannot shear, a backup rupture disk opens to establish circulation.
The mechanical ball seat can operate in both horizontal and vertical wells and is recommended for pressure-sensitive formations and casing designs with close-tolerance annuli. The mechanical ball seat is also valuable in drilldown applications where higher hydraulic setting pressures are required to prevent premature activation of hydraulic set tools by unexpected pressure spikes.
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