Changing out a damaged or inoperative wellhead master value is a typical example of the use of freezing plugging. A jacket containing the freezing medium is installed around the wellhead below the master value then the complete wellhead is pumped full of a thick mud solution which, given time, freezes into a solid block. Once the freeze plug is established it is pressure tested, the downstream side is depressured, and then the master valve can be removed and replaced. With the new master in place the plug is thawed and the wellhead can be placed back into service.

 

The freezing medium is usually dry ice (CO2) however liquid nitrogen can also be used. The length of a freeze plug is directly related to the length of the freeze jacket. Even large diameter pipelines can be freeze plugged but long jackets are required. Double freeze plugs can be used to hold pressure from both sides while a center section, between the plugs, is depressured and opened.

Freezing is also often done in combination with other procedures such as hot tapping and or trash plugging. For example hot tapping can be done prior to freezing to provide an access point for freeze plugging material (mud) or after freezing to inject a well kill fluid down a well below a freeze plug.

Trash plugging is a procedure that can be used to stop flow by introducing a plugging material to jam-up a leak-off area. An example would be an external leak on a cracked valve body. Trash, such as knotted rope, is introduced into the line so that it will either flow to or can be pumped to the crack outlet. The trash is sized so that it will jam in the crack thereby stopping the flow. With the flow now static a freeze plug can be placed upstream of the valve, when it is in place the down stream portion including the cracked valve can be depressured, opened and replaced.

Trash plugging can be amazingly effective. Safety Boss gained extensive experience trash plugging the explosives damaged wellheads in Kuwait. These wellheads often containing numerous and large fractures were successfully plugged by carefully sizing and staging various trash plugging materials.

This is a general description of freezing practices; the full freezing procedure is much more complicated than indicated here.