I have to be honest, it is only recently that I had the opportunity to place frequent cement plugs in such conditions and purpose.
Yes, I did those only occasionally before …
However, “Frequency” is important, and beside what everybody knows (so-called best practices) Frequency is how the significance of all the variables can be really appreciated.
You might ask yourself sometime; Why if I have implemented all the best practices in the book, still the cement plug wasn’t successful?
Well, eventually, anybody can normally get it … after all, probing (trial and error) is a way of learning, yet always costly and not necessarily long lasting.
Let’s go back to the creed …
Do your best initial assessment of the circumstances … (a) Understand your objectives, the well, the challenges and hole conditions;
build your best model and be ready to adjust … (b) Engineer your job;
deploy as planned … (c) Controlled, supervised execution and the most important;
understand the outcome … (d) Take the learnings through post-job analysis, sustain or change.
This is particularly true for cement plugs in Highly deviated or Horizontal sections. But what is it Highly deviated? A wellbore at an angle from vertical that exceeds 80 degrees. Here what looms us:
- Plug cementing in Highly deviated and Horizontal wells is increasingly challenging, particularly as the reach and depth increase;
- Hole cleaning (static cuttings) and “proper” before-job conditions;
- Mud displacement becomes more challenging as inclination and horizontal section length increase (longer sections result in higher ECD limiting displacement rate); and
- Density hierarchy does not play a significant role.
Now, let’s start from the beginning … Can We place a cement plug using the Balanced Plug Technique across Highly deviated or Horizontal well section?
Not really, because there is very little to no balancing with Gravity pulling perpendicularly (or close to) and making anything heavier than the mud to slump low side, unless the diference in fluid properties comes to an equilibrium at some time.
Remember: the force to balance the cement plug is the hydrostatic differential pressure between the inside of the placement pipe and the annulus, and this force needs to overcome the resistance to flow given by the mud and the cement slurry.
Calculate here the length of the slump
NOTE: Change fluid properties if SLUMP is negative. It means the cement plug slurry would continue to flow down and/or up the hole.
NOTE: Slump lenght at top and bottom are not necesarrily the same. However, considered equal here as a simplification.
The cement slurry will slump underneath the mud both below and above the plug. The cement volume must be enough so that there is sufficient section of hole covered with cement. The minimum cement volume being approximately equal to two times the volume slumped plus your desired cement plug length (volume). In other words:
( Hole Diameter )^2 * ( 0.000054 * Slump Length + Desired Plug Length / 1029.4 ) [bbl]
NOTE: Slump and Plug lenght in feet; Hole Diameter in inches
Your “cementing engineer” mind, already in motion, now knows what to do to mitigate the slump: a) make cement slurry flow more difficult; and b) increase cement volume. In other words, play with the cement slurry yield point and/or cement excess.
Wait a minute, but We still need to POOH the pipe, correct? …
Yes! Pull the pipe out of the cement and in the same direction it is slumping! … That doesn’t sound too good, does it?
The problem: how can I remove and clean/wipe my pipe minimizing disturbance of the cement in place?
You have 3 choices (from more to less beneficial):
- Leave the pipe (tail pipe) in place. Disposable pipe (aluminum or fiber glass sacrificial pipe) alone or with disconnecting tool;
- Pump and Pull method. Mimicking a cement plug with CT, Pumping the hole capacity in a joint length at the same time the joint is pulled out of the hole; and
- Plug placement tool or similar, however the higher the Tool OD and hole dimeter ratio the more swabbing.
And a fourth … yes! Another “possible” alternative:
Pump the job like a conventional cement plug. If placed on-bottom and the ratio between hole diameter and plug length is less than 0.04, but more than 0.02, as a reference. These are empirical numbers, cementing engineers are called to use proper judgement here depending on specific well conditions.
In any case consider all the precautions for cement plugs, the need for cement excess and select the correct tail pipe, e.g., PIPE OD = 0.49 * HOLE DIAMETER + 0.17 [in] and round to the closes existing OD.
Other things to consider:
- A small stinger is beneficial but increases the resistance to move out the cement behind and inside the pipe, for the Pump-and-Pull and the conventional method;
- rotation could be favorable, depending on hole geometry and stinger OD, while circulating and cleaning hole and whilst placing the cement slurry;
- for holes in excess of 85 degrees the recommended cement slurry yield point is around 25 to 35 lb./100 ft^2;
- under-displacement has to be considered on case-by-case basis, e.g., cement slurry bottom hole behavior (as seen in the TT chart, including static time), use of tapered pipe, implementation of the Pump-and-pull or conventional method;
- the Pump-and-pull method assumes the mud is not balancing itself when pulling out of the cement plug, therefore it needs to be pumped down for every stand pulled out to allow the cement slurry out of the stinger. This methodology allows placement of longer cement plugs, as long as: the end of the stinger is always inside cement (do not consider the slump), there is cement inside the stinger (under-displacement is contemplated) and the hole volume in a joint length is pumped out at the same time the joint is pulled out of the hole. It is important to mention that the Pump-and-pull method requires proper job planning and supervised execution.
Some more …
- It is fundamental to have the hole properly cleaned prior the job;
- Consider cement slurry contamination to run the UCA to estimate the WOC before tagging or testing. In some cases, the use of mechanical separators (wiper balls) could be necessary;
- CORRECT Temperature selection, meaning a well-estimated BHST and even better assumed BHCT;
- Thickening time shall be Job time with safety margin, as per the company policy. In some cases, perform the Thickening time test with go-no go;
- Pull out of the hole the working string slowly;
- Batch mix the slurry. The Pump-and-pull method might allow pumping-on-the-fly for larger cement slurry volume (Warning, good/expert cementing engineering is required); and
- a diverter tool is not as important as for vertical wells.
I really appreciate your comments and feedback