At some stage during the life of a well a cement plug may have to be placed in the wellbore. A cement plug is designed to fill a length of casing or open hole to prevent vertical fluid movement. Cement plugs may be used for:
1. Abandoning depleted zones
2. Seal off lost circulation zones
3. Providing a kick off point for directional drilling (eg side- tracking around fish)
3. Isolating a zone for formation testing
4. Abandoning an entire well (government regulations usually insist on leaving a series of cement plugs in the well prior to moving off location).
The major problem when setting cement plugs is avoiding mud contamination during placement of the cement. Certain precautions should be taken to reduce contamination.
1. Select a section of clean hole which is in gauge, and calculate the volume required (add on a certain amount of excess). The plug should be long enough to allow for some contamination (500′ plugs are common). The top of the plug should be 250′ above the productive zone
2. Condition the mud prior to placing the plug
3. Use a preflush fluid ahead of the cement
4. Use densified cement slurry (ie less mixwater than normal)
After the cement has hardened the final position of the plug should be checked by running in and tagging the cement. There are three commonly used techniques for placing a cement plug:
(a) Balanced plug (Figure 22)
This method is aimed at achieving an equal level of cement in the drillpipe and annulus. Preflush, cement slurry and spacer fluid are pumped down the drillpipe and displaced with mud. The displacement continues until the level of cement inside and outside the drillpipe is the same (hence balanced). If the levels are not the same then a U-tube effect will take pace. The drillpipe can then be retrieved leaving the plug in place.
(b) Dump bailer (Figure 23)
A dump bailer is an electrically operated device which is run on wireline. A permanent bridge plug is set below the required plug back depth. A cement bailer containing the slurry is then lowered down the well on wireline. When the bailer reaches the bridge plug the slurry is released and sits on top of the bridge plug. The advantages of this method are:
• High accuracy of depth control
• Reduced risk of contamination of the cement.
The disadvantages are:
• Only a small volume of cement can be dumped at a time – several runs may be necessary
• It is not suitable for deep wells, unless retarders used.