Snubbing Units Information
Snubbing units, also referred to as hydraulic workover units (HWU), are used to perform well intervention on live or dead oil and gas wells. They are used to drive pipe into a well bore and overcome well pressure that is preventing conventional drill rig operations. Snubbing units consist of hydraulic machinery to provide force on the drill string, as well as hydraulic jacks, slips, and a high basket serving as the control post.
The use of hydraulic jacks along with stationary and traveling slips allows operators to run the drill string on a live well, being a well which is under pressure. The snubbing unit advances or retracts the drill string whether experiencing light or heavy-pipe conditions.
- Light-pipe conditions are experienced when the weight of the drill string is exceeded by the wellbore pressure and without the use of a snubbing unit the drill string would be forced out of the well.
- Heavy-pipe conditions are experienced when the weight of the drill string overcomes the force exerted on the drill string by the wellbore pressure. In heavy-pipe conditions opposing hydraulic jacks are used to control the descent of the drill string into the well.
Snubbing units are large units that can be costly and time consuming to set-up. Traditionally, they served as a means to intervene when conventional drilling operations were inadequate or even impossible due to rising reservoir pressures. Unconventional oil fields have allowed snubbing units to broaden their use in completion and work over roles. The added expenditure of a snubbing unit is warranted when the use of heavy kill weight fluids proves to be severely damaging to well productivity.
API SPEC 7K – General principles and requirements for the design, manufacture, and testing of new drilling and well-servicing equipment.