Cement plugs are used for various reasons, including
- Solving a lost-circulation problem during drilling operations by spotting a cement plug across the thief zone and then drilling back through the plug;
- Plugging a zone or abandonment of a well by sealing off selected intervals, a dry hole or a depleted well;
- Sidetracking or to initiate directional drilling to help guide the drill bit in the desired direction;
- Provide an anchor for an openhole test, particularly when the zone to be tested is significantly off bottom
- Other remedial work
It is essential to these operations that a competent cement plug is placed the first time. Properly placing the designed cement plug helps reduce nonproductive rig time, minimize wasted material, and mitigate the need for additional cementing services.
Plugging oil or gas wells is a very common operation. When required, plugs are designated for a specific place in the well though usually not at the bottom of the wellbore. Thus, the challenge is placing a relatively small amount of cement slurry above a larger volume of wellbore fluid. As a result, a sound engineering design that addresses the major factors affecting plug success is necessary. Factors include the density and rheology of both the cement and the wellbore fluid as well as hole size and hole angle including vertical, deviated and horizontal well orientations. The goal is to secure a seal and leave the top of cement in the location required to address the reason for the plug in the first place. Creating an artificial bottom may be required to effectively spot the plug.