Here are the primary considerations that need to be top of mind when dealing with cement plugs. We think that all the most relevant ones are here, and can serve as a revision checklist. We’d welcome any additional suggestions to add to the page.
- Use a cementing stinger and cover both the cement and spacer lengths. Doing this will minimize the risk of contamination and the fluids intermixing.
- Always set a base for off-bottom cement plugs. This can preferentially be a bridge plug (if cased hole) or a viscous pill. In the cases where you use a viscous pill, add a side jet diverter sub at the end of the stinger to avoid jetting the flow downwards.
- Displace at the maximum rate possible, ECD permitting. The cementing contractor should provide a computer simulation for optimized placement.
- The maximum recommended cement plug length that can be placed in a single attempt is 200 meters. (As a ‘general rule’).
Slurry Contamination & Cement Plugs
- The main reason for failed cement plugs is cement slurry contamination:
| Cause of contamination | Action |
|---|---|
| During placement, inside the tubing or DP, at the front and rear interfaces of the cement slurry | - Sufficient Cement slurry volume. The cementing contractor should provide a recommendation based computer simulation to minimize cement slurry contamination. - When there is risk of cement slurry intermixing with brines or WBM/OBM/HPWBM compatibility tests shall be performed following API RP 10B-2. |
| Unstable base | - Place a hard base, like a bridge plug - Cement density > pre-flush / spacer density > control fluid - Viscous pill as a base (require proper design) - For low density brines the pre-flush can be water |
| Over-displacement | - Physical volume count. Do not rely on stroke counters. - Double-check (plug placement) displacement calculation |
| Pulling out of the hole too fast, swabbing while POOH or unknown hole size (OH) | - POOH slowly while the working string is inside the cement - Under-displace by 1-3 barrels to POOH dry - Hole caliper, preferable 4-arm  |
Reverse Circulation
- Reverse circulation is ‘preferred’ to normal direct circulation. The reason for this is because conventional (bottom-up) circulation takes a longer interval of time.
- Do not circulate out at the top of the cement plug. The drill pipe should be pulled back at least 20 to 50 meters above the expected top of cement before attempting to reverse or circulate out.
- Beware of the ECD when reversing out if there are open perforations at the top of the liner or an exposed open-hole. Reverse circulation applies more pressure on the formation than direct circulation.
- Whenever we employ reverse circulation, both the circulating pressure and the slurry thickening time must be considered and carefully monitored.
- In the use case of open hole cement plugs, the preferred method is direct circulation to avoid high ECD.
Please help me expand these guidelines. Are there any suggestions that should be added to the list based on your experience?
Cheers
L. Diaz
1-The design of cement stringer is another important aspect ( If sacrificial string is not used) a mule shoe design with slotted windows on the stringer for like 30 to 45 ft.
2- If the volume of plug is small as comapre to the displaced fluid like a 6” 200 ft plug in a 13000 ft well. Use foam balls or wiper plug etc.
Thanks Usama for your input. The use of a mule shoe is of great help, particularly in deep wells, to prevent any damage on the stinger. I fully agree as well with the use of mechanical separators, like foam balls. This is from my own experince please see: “SPE/IADC SPE-119415-PP, Lenin Diaz “Innovative Computer Model Increases Success Rate When Placing Deep Kickoff Plugs in Southern Mexico” Presented at the SPE/IADC Drilling Conference and Exhibition held in Amsterdam, The Netherlands, 17–19 March 2009″
Hi Lenin,
Thanks for your post. I would like to add few things based on my experience. I hope it will be useful.
Tailpipe (Check of friction and differential pressure during POOH):
My experience about, based on some studies we have performed in HPHT conditions (Compare friction pressure of 5” DP vs 2 7/8” DP using computer simulations) is that smaller pipe ID is, more force is needed to push the fluids out of the pipe during POOH. This increases the possibility of having the fluids to be dragged inside the drill pipe resulting in a flow back.
The hydrostatic balance of cement plug:
Sometimes it recommended to the hydrostatic balance technique instead of the conventional balanced plug technique for cement plug placement. The hydrostatic balance technique allows you to only increase the spacer volume behind the slurry in order to ensure non-contaminated slurry with mud inside the pipe (Adjust the volumen based on the computer simulations). No need to increase the spacer volume ahead (compare to volumetric equilibrium). It also helps you regarding POOH issues (Flow back).
Interaction treatment fluids – Formation:
Design treatment fluids having a good understanding of their interaction with the formation (Shales, salt etc…).
Very Interesting Bikouyi, I always remembered you as smart fellow. It is a pleasure to see your contribution here.
I know exactly what you mean by hydrostatic balance, very elegant. Something similar, but lightening the fluid behind instead, or under-displace, can work for LCCP (Lost circulation cement plug)
Lenin,
As we are aware that typically under displacement is done in plug jobs at calculated volume based on the stinger geometry. I always wonder, when we do under displacement it is typically a closed system (from pump frac valves thru the HP lines thru the tubing to annulus) but when the cement lines are disconnected (bled off), the air from outside would be sucked inside the well and the balance would happen quickly before the tubing POOH has begun. Which in other words practically we are doing a conventional plug job rather than the enhanced job (giving a chance of contamination). Please let me know if I am predicting it correctly and asking a right question here. kindly help me in understanding what exactly is an under displacement scenario in real case.
Keshav, thanks for coming back with more questions.
Certainly, as you said, under-displacement in a cement plug job helps reduce contamination while POOH at the end of the displacement to account for different string diameters.
The vacuum effect you hear/feel at disconnecting the treating iron from the DP is normal, but it doesn’t mean that full balance quickly happen and all the under-displacement is lost before even POOH. What happens is that the falling velocity start very fast (you feel the suction) but then slows down progressively and endures even during the POOH.
I also want to mention that the validity of the under-displacement only applies if the wellbore diameter is known, so for cement plugs inside casing or if open hole caliper is available it works, but in open-hole plugs where the diameter is not-known then we don’t know where is the TOC (with the DP in), so if in this case we feel/hear the vacuum effect, this is considered a good sign. But, if we see back flow or the string is coming wet them we know the plug top could be compromised.
Hope this helps
Cheers
L. Diaz
*compared
Good post Lenin! I agree with many of your points, particulalrly the use of viscous plugs or bridge plugs as a base to prevent cement sloughing. Even if feed rates are high, I still recommend that a base is set to ensure good cement. As an engineer, I’ve never bought the theory that cement would flow directly into perfs at high rates without some type of stringers or sloughing happening. Nobody has any proof of this, despite what they say.
Reverse circulation is also desired, but I would caution that backwashing is done well above the cement plug to reduce the risk of disturbance/contamination as in Canada at least, a solid pressure test and tag of cement top is required for compliance. Having a soft top or a pressure leak in the cement will require repair and may completely undue what may otherwise have been a stellar cement squeeze.
Some of my disagreements would be with ECD. Generally, I agree that a maximum circulation is desired if the formation can ahndle it, but maximum ECD is not always required for a good operation. A thorough understanding of not only the wellbore design, but the condition of the openhole section after drilling completed and the behaviour and composition of the formations is also critical for success.
I also have consistently placed 250m cement plugs at a time on many occasions. 200m limit is a good general rule for those not very familiar with the procedure or inexperienced field crews. However, if everything is well planned, engineered and supported by an excellent field team, 250m cement stage plugs are absolutely realistic with no greater risk than a simple bradenhead of retainer squeeze.
Thanks Will, nice contribution.
I fully agree with you about the maximum length of cement plug. I did mention 200 m as a general “rule” and that was kind of hard for me to say, because I know what a “rule” can truly means in this industry. In this context this so-called “rule” is definitely linked to the performance of the crew. As you mentioned.
Personally, I have seen high performance crews that are hard to believe … those crews, I can tell you can do almost anything.
Lenin,
As usual you make some good points.
I would like to add several points concerning methods beyond balancing, such as the use of an IBOP, and displacing the cement near the end of the pipe and allowing the differential pressure to close the IBOP while pulling out. This is especially useful when hole dimensions are not clear (washouts etc.). There are many considerations for this method, and probably worth it’s own post.
I would like to address the “best practice” of using a stinger. Depending on the hole and pipe size a Stinger may not be necessary and in fact may create an annuls that is too large for typical cement rates to fully mobilize the mud. A general rule of thumb that I have used for Stinger sizing is: no larger than half the hole size, and no less than 1/3. This covers a lot of hole and pipe sizes, but mainly serves to identify when the pipe is too small. I have had people try to effective set a plug in a 20″ hole using a 3 1/2 inch Stinger. It may work in many situations, but at 5-8 bpm if the mud has any viscosity there is a chance that a good portion of it is not mobile at those rates, especially when you consider the low likelihood that the Stinger is centered.
If possible it is good to use all the same size pipe to better manage the balance and avoid the known issue of over-displacing a balanced plug when sufficient under-displacement is not calculated for the differences in pipe size. You may recall a demonstration that I did years ago that validated under displacement modeling.
If no Stinger is deemed necessary then a Diverter is needed to slow the free-fall rate while placing.
Plugs are always a good subject to discuss. Everyone has success and failure stories, and the key is to treat every plug as a new exercise. The same rules don’t apply for every situation. Determine what the risks are and design based on that. Every rule of thumb has a situation when it should have been given the finger.
Take care,
Jeff
Jeff, it is always a pleasure to have your input here.
As usual you have some very goog points to enrich the discussion and give people a broader view over the topics presented.
I fully agree with you about the use of stinger, and there are many things to consider, including contamination, free-fall, under-displacement; but the effect on annular velocity (annular gap) is definitely also a key one to keep in mind.
The important thing overall is, as you mentioned, to treat every plug as a new exercise.
Cheers
L. Diaz
Way back when I worked for BJ Services in the 1990’s they had a small tool department and in that tool group they had a tool called Parabow.
https://www.bakerhughes.com/news-and-media/resources/brochures/parabow-cementing-tool-brochure
I see BHI still has info on it. It creates a barrier to prevent the slurry from slumping downhole due to density differences. It was made with fiberglass ribs and a nylon fabric so the differential it could withstand is on the low side.
I have no practical experience with its use (like I said – BJ’s Tool group was small). Perhaps some of you folks may have seen it in use.
My old rule on under displacement was 1 bbl for each inch of diameter in the workstring.
Could be it doesn’t work very or perhaps it’s cursed with NIH (not invented here). Maybe some of you folks that are more active in the cementing world can comment.
Dan, Thanks for your contribution to this post and discussion.
Personally I have not seen an operation using the Parabow tool, but I have heard of it several times as a valid alternative, however in every single occasion the tool was not really available or in stock at the moment.
The logic of the tool makes a lot of sense and it appears to be very easy to deploy.
In practice, the cement doesn’t really need too much help not to fall or sink, that is a time-sensitive process in which the increased gel strength of the cement would be doing much of the job eventually, so a tool like this would only truly need to provide an “obstacle” to allow the cement to finally set in place for a successful plug.
For off-bottom cement plugs, I have used several times, with DP and also with Coiled Tubing, a reactive-viscous pill underneath the cement. This king of pills are a combination of viscosity and an accelerator, so that as soon as the cement sinks a little gets in contact and develop gel strength at the bottom of the plug supporting the rest of it until finally set.
In our location we use a CST tool especially in big diameter well bores. It looks like an upside down umbrella and this tool is pumped ahead of the spacer through the drill pipe. When leave the DP it unfolds preventing the slurry from sinking/ flowing down the well.
In smaller well bore diameters we achieve a competent cmt plug with a high displacement pump rate. If the annular velocity is so high that an exchange of well bore fluid past the pumped cement slurry is not possible the contamination will be minimized and the result is a competent cement plug.
This is very good experience with CST tool. Thanks Thomas. In the locations I worked, We never had that tool available for one reason or another. If you can please tell us more about it
More info about CST by the link – https://www.perigon.no/products-the-cement-support-tool-cst?hsCtaTracking=f19f7539-efdb-4084-8104-bd67163c5246%7C2b4d7e71-26c3-4cf8-a4d7-6c6e2aa9e3d4
Thanks Sergey
Hello Lenin.
Another good practice worth to mention is rotate the drill string during cement plug placement. For this purporse you need to have cement swivel on the rig. Mechanical separators were mentioned already, but if you plan to set several plugs in one run ( like abandonment plugs on top of each other) better to use open ended DP and separate properly. In case mule shoe is planned to be used it is not recommended to pump foam balls as most of the times balls get stuck on the uppest slot. Maximum on foam ball ahead of cement slurry can be used for the last cement plug.
After circulate out excess cement, also circulate out with scouring pill to remove any gelled cement from DP( cement rings) and pump one foam ball as well to clean up DP inside. Rotation should be practiced during circulation as well.
For horizontal plugs design fluids with focus on friction pressure hierarchy rather than density hierarchy and 0 Free fluid. Be ready to use pump and pull method as well in case wet pull observed for first stand and POOH slowly 5 min from slips to slips.
Hi Fuad. Thanks a lot for your very valid comment.
In Mexico, in 2007, when I first used foamed balls (SPE/IADC SPE-119415-PP, Lenin Diaz “Innovative Computer Model Increases Success Rate When Placing Deep Kickoff Plugs in Southern Mexico” Presented at the SPE/IADC Drilling Conference and Exhibition held in Amsterdam, The Netherlands, 17–19 March 2009), We developed the practice and continually used foamed balls ahead and/or behind the cement slurry even if a mule shue or diverter tool was used. Almost in every ocasion the foam balls were found at the bottom of the mule shoe (typically one meter long — I know not standard and not necesarily the safest for deep plugs due to the risk of damaging it while runing without noticing it). Depending on rate the balls where sistematically pushed down the tool just to open enough opening to allow flow, like a valve. The practices to place plugs were different if one or two foamed balls were used ahead and/or behind. We did a lot because deep cement plugs were quite common there. The good thing is that our succesful rate was close to 100%. But it was thanks to the plug advisor software that allowed us to see what was going on and implementing a remedy.
Thank you Lenin….!
Thanks for the tip that we should use a cementing stinger for the cement plug that we’ll use for the water well so we can build a new one. I think the water bed in our field is already dry, so it’s time to retire it. For this reason, I’ll hire someone to do it since I don’t know how to do this by myself.
Always glad to help Angela. Thanks for your feedback.
Cheers
L. Diaz
Hi Lenin Diaz,
Is it common to include excess cement in a slurryvolum when calculating HOC w DP in hole for a balanced plug. Or should HOC exclude excess?
Hi Petter,
When in open hole, you always have the choice to use excess or not. Excess % is an arbitrary slurry volume added to cover for uncertainties in open hole size, length of contaminated cement, proper cement plug support (off-bottom cement plugs) or simply to ensure (based of the engineering team criteria) that cement is found and tagged (as per job objectives).
If you have caliper, you should include your excess in all calculations, but if you don’t have caliper and your hole size is guessed, in addition to add your excess % you should check as well your plug length with and without excess to safely design your cement plug, TOC, POOH depth, etc.
Cheers
L. Diaz
could you give procedures for cement plug placement and we have total losses in open hole ,deemed differential sticking , and how use spacer in total losses issue ,please you recommended highly appreciated
Hi Mustafa, please take a look at this post Balance to succeed: Cement Plugs in losses situation
There you can find a procedure I successfully use is total losses situation many times.
Regarding the position of the DP, in this procedure, it is placed to the bottom of the losses zone or it is considered to be placed across the losses zone, like a conventional cement plug, using tail pipe or cement stinger.
However in some situations, where full DP is used to place the plug or when cement is pumped thru the drilling bit, the possibility of differential sticking due to total losses exists, in this case the end of the placement pipe should be placed above the losses zone, at least 10 to 20 meters. This position would also depend on the aggressivity of the solution intended to be used, including some type of in-situ reaction, and/or shorten thickening time; to prevent the possibility of leaving the pipe stuck in cement.
Cheers
L. Diaz
I want to ask if measure static fluid level relative to displacement volume ,if so ,how to calculate displacement volume or balance plug and is relative to U-TUBE concept and this level is not constant while pumping ,please advice me full in details .thanks
Hi Mustafa,
The whole idea is to reduce the hydrostatic portion of the ECD mainly towards the end of the job, and obviously the fluid column above the cement plug during WOC (depending on well control and the risk of fluid influx – e.g. gas migration).
Let’s assume the fluid level is static before the job and its depth is known (previously measure by filling the well – approximated – or a more precise measured like echometer ), the fluid level, if static, is a measured of the formation pressure which gives us a reference for estimating the under-displacement volume, basically as explained in this post, the idea is to calculate what volume of drilling fluid (inside the placement pipe, DP) is required to have a bottom hole pressure exerted by cement and drilling fluid equal to the formation pressure.
This approach implies several assumptions, as you might have realized already, for example:
1) The fluid level at the end of the job in the annulus is equal to the fluid level inside the placement pipe. This is not necessarily the case, but the bigger the losses zone (the lower the friction pressure at the losses zone) the closer both level will tend to equalize,
2) The additional ECD caused during cement placement, during free fall, (depending on cement slurry density) is ignored. Additional ECD means higher losses rate .. in some cases, in severe loses it is possible the majority of the cement could be lost during placement, so the hydrostatic weight estimation is overestimated and the fluid level would be actually higher,
Each case is different and the final job design is often the combination of several losses mitigation practices and solutions, like use of LCM, In-situ downhole-fluid-reactions, fluid rheology, fluids density, etc.
An important element, is to understand the contribution of the friction pressure at the losses zone. For example, if at some high pumping rate returns are gained (typically when using water). If this is the case, then any method to maximize the downhole friction pressure (use of LCM, high rheology fluids) would provide at advantage by lowering the dynamic losses of cement slurry during placement.
Cheers
L. Diaz
Hello Lenin,
Indeed very fruitful discussion on Cement Plug placement. Just would like to have your expertise here, is suggesting 9.6ppg mud LCM Pill ahead of Cement (13.5ppg Thixo) in well condition of 50-60% circulation w/ ~400bph Dynamic ~240bph Static Losses would be good idea?, the reason asking is here the general concept is that LCM Pill might disturb Cement Plug and pumping LCM ahead may plug off 4″ OEDP, thus, we should not suggest pumping LCM ahead of cement, my point of view among team here “the whole objective is to cure the losses, pumping LCM would not effect Cement Plug placement, as the well is already loaded with LCM”. Please do share your thoughts, as you may have case histories pumping LCM Pill ahead of Cement Plug to cure mud losses.
Hi Tariq, thanks for your contribution to this discussion and for your contribution.
This is an interesting case, and I will try to address your inquiry in the best possible way considering I don’t have the whole picture, but based on the information you are providing; it doesn’t seem of added value the addition of the LCM-loaded spacer ahead of the cement slurry. First of all, as you mentioned, the mud in the wellbore is already loaded with LCM and there is partial circulation.
Now, the “amount” of disturbance to the cement plug, if I understand correctly what is referred by you as “disturbance”, will be dynamically and depend on plug setting depth. The deeper the cement plug (particularly > 2500 – 3000m) the risk of contaminating the cement slurry inside the DP on the way down is higher; and the possibility the LCM-loaded spacer mix with the slurry (losing the benefits of both) is higher and the higher the LCM loading and the viscosity the higher the risk of this intermixing.
The risk of plugging the DP is certainly there but very low, if open-ended and well is not deviated.
In this case the use of a cement slurry is the LCM measure, You can add LCM to the cement slurry, in the form of fibres, flakes or granulated material.
The integrity of the slurry is very important and proper measures must be taken to address this dynamically and statically, including: hydraulic placement program, estimation of BHCT, thickening time, compressive strength development, POOH, need to fill the well or not before 50 psi is reached (what is the risk of a well control event, influx, etc)
Hope this helps
Cheers
L. Diaz
Hi Leni,
In regards the Hi-Viscous pill, what would be the minimum height or volume that can be recommended?
Would it depend on the slurry density and cement column height?
thank you
Hi Karol,
Yes there are some recommendations for the viscous supporting pills. In terms of height/volume at least 1.5 to 2 times the volume of the cement slurry. Then density of the pill should be half way between the cement slurry and the fluid in the well.
Cheers
L. Diaz
Thank you Leni 🙂
If you set a permanent bridge plug with Hydraulic Setting tool #20, What is the best practice if you want to place cement above the BP? The ID of setting tool is very limited after stroke all the way long.
Hola Daniel,
Yes, as you mention, the issue is the setting tool’s ID, which adds a risk associated with the possibility of cement dehydration due to the additional friction pressure. I have seen and use a circulation tool as part of the BHA to eliminate the risk. The other possibility is to mitigate the risk, and for that, it needs to be quantified. Can you advise the ID and length of your tool? If possible, the length/capacity of the placement string or DP and the volume of cement.
Sorry for the late reply
Best Regards
L. Diaz
I’m new in this mail thread and have not read all the historic information, but I saw that the CST was mentioned in one of the comments. I represent the company who manufacture the CST and we have used the tool in more than 3000 plugs since 2002 and we can document a success rate above 95% when using the CST.
When the CST does not work we have seen this is related to operation issues, hole condition, or losses below the set depth of setting the CST.
The CST can be pumped through the DP (work string) and is extremely flexible as one tool can work in a hole size between 7 and 16″, while other CST’s can work in hole sizes up to 42″.
Hi Kristian, thanks for this contribution. This is certainly the idea of this and any discussion in this website, a place to share knowledge and experience. I would like to invite you to have a whole post about the benefits and succes of the CST, please contact me by email or on my WhatsAPP anytime.
L. Diaz
Hello Mr. Lenin
Is the open end stinger preferred for cement plugs or perforated side-close end stinger?
Dear Mustafa, thanks for your question and support. Answering your questions, it depends, open-ended is no problem for on-bottom cement plugs, while perforated side-close end stinger is best for off-bottom cement plugs. Please take also in consideration that a side-close end stinger is a weak point and it can get damaged in in open hole plugs, if there are wash-outs or hole enlargement and in deviated wells.
Cheers
L. Diaz
Thank you, I’ve recently been searching for info about this topic for a while
and yours is the best I’ve came upon till now. However, what about the conclusion? Are you
positive about the supply?