There are several reasons to place a cement plug – from plugging the well back for a sidetrack, preparing for a kickoff, well abandonment, or for curing a lost circulation zone. Successful cement plugs are rarely quick or easy. Placing a cement plug in an open hole horizontal well is especially difficult. Additionally, as most open hole plugging operations are unplanned, it’s essential that nonproductive time is minimized and that drilling gets back on track as soon as possible.
In a technical paper published by the Society of Petroleum Engineers (SPE 113092*) the authors found that an average of 2.4 attempts are necessary to achieve a successful sidetrack. With each plug attempt consuming between 24 and 48 hours of rig time, well costs can increase dramatically. Among the problems identified as factor of plug failure include: plug slippage, plug or slurry contamination, and insufficient cement wait time before attempting to drill the plug.
Common practice in setting a plug in open hole is to use a workstring to place a cement slurry, forming a plug and wellbore seal. The workstring is tripped into the desired depth to the bottom of the plug, then a cement slurry is pumped down the workstring and into the annulus between the workstring and the wellbore. The workstring is then tripped out leaving the cement slurry to harden, thereby forming the seal. However, in horizontal wells, as the workstring is pulled through the slurry, the slurry often is pulled uphole, resulting in stringers of cement and a less-than-competent plug.
Determining the correct slurry transition time from liquid to solid is challenging. If the slurry hardens too fast, the workstring could become stuck in the wellbore. If it doesn’t harden fast enough, the plug could become contaminated or move as the workstring is pulled due to wellbore swabbing. Additionally, a slow-setting slurry can slump in the horizontal wellbore, thereby preventing the plug from properly isolating the wellbore and zone.
To solve this problem, plug placement aids have been developed that allow sacrificial tubing to be left downhole at the completion of the plug job. This allows the workstring to be disconnected from sacrificial tubing at the top of the cement plug and tripped out without disturbing the slurry, and without worry that the workstring may become stuck. These tools also allow longer plug lengths to be pumped at the same time rather than setting multiple short-length plugs on top of one another. Additionally, higher performing cement slurries can be used with much faster cure times, higher viscosities, and higher yield points.
These mechanical devices typically employ a wiper dart that is pumped down the workstring which latches into the disconnect assembly when cement is fully displaced. This creates a pressure increase which activates a sliding sleeve, allowing the disconnect to take place. The tailpipe can be made of drillable composite material, and can be drilled out along with the cement plug if continuing operations are required.
However, no mechanical device is trouble free and fully dependable in deviated wells or short plug lengths where sufficient tailpipe weight is not available to assist with separation. What if the required overpull to disconnect is greater than the sacrificial string weight? Do you wait to achieve cement gel strength to pull against? Do you risk pulling on the cement plug? While the industry has been accepting of this potential failure mode in the tool design, Citadel Casing Solutions recognized this shortcoming and engineered a solution.
The PreNup™ Plug Placement aid is run in hole on drill pipe or tubing with sacrificial tail pipe below.
After being released by a dart, the drill pipe is free to be removed from the well, leaving the tailpipe and a formidable cement plug behind.
Citadel’s PreNup™ line of plug placement aids make complicated separations a thing of the past. The tool includes a pump-off feature that enables hydraulic disconnect with the sacrificial tailpipe by applying pressure rather than overpull from surface. This feature eliminates the risk and anxiety of moving the sacrificial tailpipe through the cement plug, or failing to release entirely during release operations. Now cement plug setting can be accomplished accurately and efficiently on the first attempt. Protect the assets you’ve worked hard for with the PreNup™ Plug Placement Aid.
*Al-Yami et al., 2008. New Cement Systems Developed for Sidetrack Drilling. SPE-113092-MS
About Citadel Casing Solutions
Citadel Casing Solutions produces innovative technology for cementing casing equipment.
We create customer value through our unmatched engineering expertise, broad base of experience, and overall responsiveness. Our goal is to deliver the ultimate in wellbore integrity to protect our customer’s assets.