Technical Paper: Don’t Get Stung Setting Balanced Cement Plugs: A Look at Current Industry Practices for Placing Cement Plugs in a Wellbore Using a Stinger or Tail-Pipe

Society: SPE
Paper Number: 168005
Presentation Date: 2014
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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 drill pipe, 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 height of the cement and spacer placed outside the pipe. With equalized column heights and fluid densities inside and outside the workstring, the hydrostatic pressure is balanced. A stinger with a smaller diameter than the drill pipe is commonly run on the bottom of the work string for setting a balanced plug. 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 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 workstring 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.

By looking at the changing conditions while POOH with a stinger, it can be concluded that using the balanced plug method will result in the contamination and potential failure of the plug. Other methods should be utilized when using a stinger.

Perhaps the most critical component of most cement plugs (especially kick-off and sidetrack plugs) is Compressive Strength (CS) development. The plug must have enough strength to allow the driller to sidetrack off of the plug and into the neighboring formation. Surfactant laden spacers and drilling mud have a drastic effect on CS development. The contamination of the cement plug can result in failed strength development, which can result in failure to kick off the plug. CS testing shows that development times can be from three to five times longer with as little as 10% mud contamination. The end result is lost time and money for the operator.

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