In Part 1, I mentioned why cement plugs had taken a leading role in this website. It is so relevant to do sound engineering to avoid non-productive time and reduce the risk of even more disastrous consequences, like a placement pipe stuck in hard cement.
In this subsequent post, I would like to introduce a CST tool, designed to avoid the sinking of the cement slurry due to density differential (cement slurry heavier than the wellbore fluid) in off-bottom cement plugs. The CST (Cement Support Tool) is not the only way to prevent this effect; there are other applicable fluid-based solutions, each with pros and cons.
Issues with off-bottom cement plugs:
• Fluid swapping-instability
• Higher density fluid falls down
• Lighter density fluid goes up
With the utmost priority being “get the cement plug right the first time,” it is the engineer’s role to risk-assess job conditions and always opt for the best available solution. The engineer must also include proper prevention, mitigation, and contingency measures to make each (the only one, remember: get it right the first time ) job count.
In a recent discussion with the leading company providing the Cement Support Tool, Perigon, I was enchanted to see their non-biased technical approach in a paper called “HOW TO SELECT THE RIGHT TECHNOLOGY FOR SETTING BALANCED CEMENT PLUGS.“
Starting with a fundamental principle in our industry, before any action or change to an existing process: answering two basic questions: Is this action necessary? (do we need a base for the cement plug) Moreover, What is the associated cost or loss if the action is not implemented or done improperly? (Understand the total cost picture for setting a successful cement plug).
I want to present and comment here on the content of this exciting paper. Before that, let me talk a little bit about Perigon’s experience with this tool as presented to me.
Perigon has sold over 3000 CST’s since its introduction in 2002 with a documented 95% success rate. The CST is a versatile and simple tool that can be pumped through the DP to the setting point when it opens up and fill the hole up to a specific diameter independently of the hole shape.
- Below a performance summary until October 2019 in the North Sea area.
- Total CST jobs reported since 2003; 521 jobs.
- In 88 jobs, a Hi-Vis pill in combination with the CST was implemented.
- Overall, well inclination from vertical up to 97deg.
- 58 jobs in inclination above 75deg, more than 91% success.
- 356 jobs in hole/casing size above 12 ¼”, more than 92% success rate.
- 229 plugs were KOP.
- Unsuccessful job details:
- Eight jobs had losses; CST placed at the wrong depth (above the losses zone).
- Six had operational issues, e.g., top drive issues, wrong pump direction, etc.
- Four had other hole issues.
- Two had DP/XO issues.
- Two had cement slurry or mixing issues.
- One CST issues;
- 18 unknown root cause.
Available technologies for off-bottom cement plugs:
- The easiest choice: High viscous pill
Placing a viscous (thicker) fluid right below the cement plug’s planned bottom is the most common solution to overcome the difficulties of placing a cement plug on top of a lighter fluid.
A high viscous pill is basically a mix of water and a substance (e.g., bentonite), where the substance thickens the water. Increasing the viscosity of the fluid increases the fluid’s ability to support solids in the cement.
However, the thicker the fluid – the more difficult it is to handle.
- Benefits of high viscous pills:
- There is no need to allocate time to perform a pressure test or pull out of the hole with running tool.
- It can be used in cased hole and open hole.
- No need to rent special equipment.
- Relatively low product cost.
- Disadvantages:
- The industry average success rate is approximately 75%, as per Perigon’s paper.
- The placement time of setting one high viscous pill is 3-4 hours, increasing the overall job time and cost.
Comments:
I am basically in agreement here, perhaps just adding other elements affecting the success rate:
- Placement and balancing the pill is usually underestimated.
- Both cement slurry and viscous pill should be designed to maximize stability, e.g., cement slurry rheology, and viscous pill density.
- Use of diverter tool for lateral flow (avoid down jetting the cement slurry into the pill).
- Risk of using a rig-made diverter tool (need a proper one to avoid excess pressure drop leading to cement slurry dehydration).
- In an open hole, unknown hole size affecting both the pill and the cement slurry balanced placement.
- Chemical reactive pill
A chemical reactive pill’s basic idea is to create a chemical reaction between the substance in the pill and the cement. The reaction forms a gel that prevents contact between the cement and the wellbore fluids below the pill.
A reactive chemical pill is somewhat like high viscous pills, but the key difference is that the chemical pill reacts with the cement due to the high pH in the cement and correspondingly low pH in the pill.
- Benefits of reactive chemical pills:
- There is also no need to allocate time to perform a pressure test or pull out of the hole with a running tool.
- It can be used in a cased hole and an open hole.
- No need to rent special equipment.
- Relatively low product cost.
- Disadvantages:
- Mixing and pumping pills require specialized personnel.
- The industry average success rate is approximately 50%, as per Perigon’s paper.
- The placement time can be up to 2-3 hours, increasing the overall job time and cost.
Comments:
Again, I agree; however, I am a bit more concerned about this solution:
- Placement and balancing the pill is usually underestimated.
- It requires laboratory tests to validate the pill design (basically, it has to be incompatible with the cement slurry).
- The concept is based on the chemical reaction between cement slurry and the pill resulting in gel development when they are in contact. Warning: this is supposed to happen away from the end of the pipe.
- Use of diverter tool for lateral flow (avoid down jetting the cement slurry into the pill).
- Risk of using a rig-made diverter tool (need a proper one to avoid pressure drop leading to cement slurry dehydration).
- In an open hole, unknown hole size affecting both the pill and the cement slurry balanced placement.
- This concept has been adapted very successfully for cement plugs placed with coiled tubing using the pump and pull method. Warning: requires expert design.
- Heavy fluid below the cement
It means pumping a heavy fluid from the bottom of the well and up to where the cement plug is set.
- Benefits of heavier fluid below the cement plug:
- There is also no need to allocate time to perform a pressure test or pull out of the hole with a running tool.
- It can be used in a cased hole and an open hole.
- No need to rent special equipment.
- Relatively low product cost
- Disadvantages:
- It is widespread to use barite to increase fluid’s density.
- Potential issues when running in the hole to bottom to set the heavy fluid.
- The industry average success rate is approximately 90%, as per Perigon’s paper.
- The placement time on average is 6-10 hours increasing the overall job time and cost.
Comments:
Perhaps useful to add a few more considerations:
- In front of the production zone, typically in cased-hole temporary abandonment cement plugs, the weighting agent can be formation damaging and difficult or impossible to clean or remove with acid.
- Use of diverter tool for lateral flow (avoid down jetting the cement slurry into the pill).
- Risk of using a rig-made diverter tool (need a proper one to avoid pressure drop leading to cement slurry dehydration).
- Risk of damaging the cementing stinger when running in the hole, especially in deviated wells, unstable wellbore, or with the risk of hole enlargement.
- Bridge plug
A bridge plug is a mechanical plug that anchors in the casing and holds differential pressure. Bridge plugs may be used for temporary or permanent plugging.
Bridge plugs are used as a base for cement plugs. They are normally made of drillable materials such as composite and aluminum, in case the operator wants to remove the cement plug later.
- Benefits:
- The industry average success rate for bridge plugs is 100% (in the context of cement plugs) when the plug is successfully set, making the bridge plug the top performer among the available technologies for setting off-bottom cement plugs, as per Perigon paper.
- Disadvantages:
- Bridge plugs are often used when setting balanced off-bottom cement plugs in cased holes. A bridge plug does not typically apply for open hole applications.
- The installation requires an extra run typically since it is often not desired to have the bridge plug setting tool in the well when performing the cement operation.
- Bridge plugs have a high success rate but are relatively expensive and time-consuming to set.
- There are potential issues when running in the hole to set.
- Bridge plugs also require approximately 1 hour for pressure test.
- Bridge plugs require specialized personnel and rental equipment.
- The placement time is usually 10-16 hours, increasing the overall job time and cost.
Some additional comments:
- In the context of well barriers elements (cased holes), all bridge plugs require some cement on top.
- If considered for open-hole cement plugs, it requires at least a four-arm caliper (known hole diameter).
- The success rate is linked to setting the tool in place.
- Avoiding tripping out of the hole, depending on cement slurry volume and typically for workover operations, some companies include a circulation tool to the placement string—warning: Expert design is required.
- Expandable packer
The principles of using expandable packers are, in some ways, the same as using a bridge plug. The plug is a long steel tube with a rubber gasket on the outside, inflated via ports, just like a balloon. They are installed on the outside of a pipe and expanded by applying pressure to the packer’s inside. The expandable steel and elastomers conform to the wellbore shape and anchor to a certain degree when inflated.
- Benefits:
- They can also be used in open hole applications.
- Unlike bridge plugs, expandable packers do not have slips that anchor or pure rubber elements for pressure integrity.
- Expandable packer inside the casing, the success rate is very high.
- The industry average success rate for expandable packers is 75% (in the context of cement plugs), as per Perigon paper.
- Disadvantages:
- There are potential issues when running in the hole to set.
- Expandable packers in an open hole, the success rate is significantly lower. In many cases, the gasket (balloon) can be damaged due to the internal profile of the open hole and due to limitations with the setting mechanism.
- It requires specialized personnel and rental equipment.
- The cost of the packer itself is at the ‘expensive end.’
- The placement time is usually 10-16 hours, increasing the overall job time and cost.
- Cement Support Tool
Cement Support Tools are used as mechanical bases for balanced off-bottom cement plugs and is designed to create a physical barrier between a cement plug and the fluid below the plug. This barrier aims to prevent the heavier cement swapping place with the fluid below the CST and becoming contaminated. The CST is inserted into the work string at the surface and pumped downhole. When the tool exits the work string, it unfolds just like an umbrella and fills the open or cased hole. When the cement plug is pumped in the next stage of the operation, the tool will become a physical barrier between the cement above and the well fluid below.
- Benefits:
- It is possible to decide to use the Cement Support Tool when the work string is already in the hole (no need to POOH).
- It allows for multiple cement plugs in the same run at different depths.
- Usually, no issues when running the tool in the hole to set them.
- It does not require a running tool.
- There is no need for pressure testing when using a CST.
- If the correct CST size is used (there are five versions) considering the string and hole’s inner diameter, the success rate is at the same level whether the CST is used in a cased hole or an open hole.
- Relatively low cost.
- CST does not require specialized personnel or rental equipment.
- The industry average success rate for expandable packers is 95%, as per Perigon’s paper.
- Disadvantages:
- The additional time is usually only one (1) hour, increasing very little the overall job time and cost.
Additional comments:
- Like any tool application, the CST requires proper engineering and careful deployment following the manufacturer’s recommendations and technical advice. However, it is a straightforward tool with an effective self-activation mechanism. Cement plugs can have a tremendous effect on cost in case of failures; in the case of off-bottom cement plugs, like KOP while drilling or other critical application, the engineer must implement the best possible job design. In this context, the CST is worthy of consideration to maximize success and ensure getting the job right the first time, saving precious rig time and money.
- I invite all to visit the Perigon’s website for any further information and enquires; they are really helpful and will assist with pleasure in selecting the best solution for your well.
Cheers,
L. Diaz
Usama says
Hi Lenin,
The Pros and Cons written for each cement plug are quite precise, too the point. We recently have heavy complete losses in a Well and I am guessing the team pumped around 18-30 plugs into this 2000 ft of section where we had issues at multiple depths.
After nearly all of the plugs the updates were usually same as ” Did not find cement , encountered losses, pull back…..etc) One of the reason we identified was not able have a good base for cement to be stationary and develops strength. Due to Covid-19 scenarios, mobilized equipment and pesonell was a risk and the team went back to drawing board and we place and a reactive calcium pill to get solid base. Tha worked with good success.
Lenin Diaz says
Hi Usama, glad to read your comment and your case story. The most important action in cement plugs, starting for the first one is to identify the challenges preventing success, and take action. If success is not achieve after the first plug, inmedately double check the operation to see if your design premises were kept, and if so then you review your design and implement changes. Here try to change one or max. two elements at each time, or you may get lost with the multiple variables involve.
I am here to help anyway I can, please don’t hesitate to reach me, perhaps I can provide a fresh view as an outsider.
Cheers
L. Diaz
Usama says
Hi Lenin,
The Pros and Cons written for each cement plug are quite precise, to the point. We recently have heavy complete losses in a Well and I am guessing the team pumped around 18-30 plugs into this 2000 ft of section where we had issues at multiple depths.
After nearly all of the plugs the updates were usually same as ” Did not find cement , encountered losses, pull back…..etc) One of the reason we identified was not able have a good base for cement to be stationary and develops strength. Due to Covid-19 scenarios, mobilized equipment and pesonell was a risk and the team went back to drawing board and we place and a reactive calcium pill to get solid base. Tha worked with good success.
Lenin Diaz says
Great job Usama, please see my previous reply
Muthanna says
Hi Lenin,
Very interesting to know more about the CST, as an example for 12 1/4″ hole what will be the max diameter for the tool? Does the tool easy to drill by PDC or Tri-Cone bits and easy to circulate the junk out to surface or may need for junk baskets runs due to the big amount of junk?
Lenin Diaz says
Hi Muthana, glad you are interested in the CST, It is a great tool. Please for information, contact pa@perigon.no and cc me on dlenincementer@gmail.com, to follow up and assist you in your case
Muthanna says
Lenin,
Thanks for your reply, will be in touch with Perigon team for more details.
Lenin Diaz says
No problem, Muthanna. All the best
Ibrahim Dao says
Dear Lenin,
First of all thank you very much for all these information.
I have two questions :
1- How easy can this tool be drilled ?
2-Why its better than the conventional bridge plugs ?
3- Can this tool be run in OH as well ?
Regards
Ibrahim Dao
Lenin Diaz says
Hi Ibrahim, thanks for your support.
Regarding your questions:
1- How easy can this tool be drilled ? Yes the tool is drillable without any problems or need for spacial tools
2-Why its better than the conventional bridge plugs ? The tool will no replace, depending on local regulation, a bridge plug with cement on top for the objective of zonal isolation during P&A
3- Can this tool be run in OH as well ? Yes, it can work in OH
Hope this helps, let me know if you have any additional questions or feedback
Cheers
L. Diaz