Cement plugs have been used for decades within the oil and gas industry. Some of the applications include zonal isolation, curing lost circulation, abandonment, and serving as a base for kicking-off or sidetracking. The most common method used for the placement of cement plugs is the balanced plug method using drillpipe, tubing, or a combination of both. The pipe is run in the wellbore to the desired depth of the bottom of the plug. A calculated volume of cement is placed in the well. Usually a volume of spacer is pumped ahead and behind the cement. When the plug is in place, the height of the cement and spacer left inside the pipe is the same as the height of the cement and spacer placed outside the pipe. With equalized column heights and fluid densities inside and outside the drillpipe, the hydrostatic pressure is balanced.

Tubing with a smaller diameter than the drillpipe is commonly run on the bottom of the drillpipe for setting a balanced plug; this smaller diameter tubing is commonly known as the stinger. Using a stinger lowers the height of the cement plug with the pipe in place, prior to pulling out of hole (POOH). The stinger also provides a larger annular cross-sectional area during cement placement. Some operators use a stinger with the assumption that it will minimize the disturbance of the plug while POOH, decreasing the chance of cement contamination.

If a stinger is used, the assumption that all fluids both inside and outside the drillpipe will remain in equilibrium is false. A mathematical analysis of what occurs once dynamic conditions begin by POOH with a small diameter stinger shows that the initially balanced system quickly becomes unbalanced. This analysis of placement technique can be valuable for any situation in which a balanced plug would normally be used for plug placement.

Perhaps the most critical component of most cement plugs, especially kickoff and sidetrack plugs, is compressive strength (CS) development. A cement kickoff plug with a final strength greater than the adjacent formation provides better support to sidetrack in the neighboring formation. Surfactant-laden spacers and drilling mud contamination have a drastic effect on cement CS development. The contamination of the cement plug with these fluids can result in failed strength development, which can result in failure to kick off the plug. The end result is lost time and money for the operator.

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