Some time ago I put together a short paper about the principles of squeeze cementing. Below is an extract of this document.
Successful squeeze cementing relies mostly on selecting the correct cement slurry to form an effective cement cake or node. We base the decision shall on the injectivity test performed before the squeeze operation.
There are two main controlling variables during squeeze cementing that affect the rate of filtration. These are the fluid loss of the cement slurry and the permeability of the formation or porous media.
a) The higher the cement slurry fluid loss, the higher the amount of cement filtrate available for filtration. The fluid loss of the cement slurry is a property of the slurry that we can modify with chemicals (cement additives).
b) The higher the injectivity rate, the higher the volume of cement filtrate that the permeable media will be able to receive. Before every squeeze cementing operation, we always perform an injectivity test. This injectivity is directly proportional to the effective permeability from Darcy’s Law.
If we combine (a) and (b):
- High fluid loss + High Injectivity = very high rate of filtration = premature plugging. Not recommended.
- Medium/Low fluid loss + Medium/High Injectivity = rate of filtration controlled by cement slurry fluid loss. A margin for squeeze pressure.
- Medium/High fluid loss + Medium/Low injectivity = rate of filtration controlled by the permeable media. A margin for squeeze pressure.
- Low fluid loss + Low injectivity = very low rate of filtration = long operation. Not recommended.
2. and 3. are common scenarios for squeeze cementing with conventional slurries system, while 1. and 4. may require non-conventional slurries types or additional chemicals/materials.
More details in following posts.
L. Diaz
What is this groups experience with micro-fine “Matrix” cement for squeeze jobs? Is it worth the considerable additional cost, in your opinion?
This was a nice question posted on linkedin group Oilfield cementing Professionals.
These micro-cement slurries do not work on the principle I described and based on FL/filtration/filter cake/node formation. In consequence, these cement slurries have a potential application in any case where the cement slurry filtration process is not practical.
I am of the opinion that micro-cement cement (performance will vary on particle distribution and average particle size) has an application in medium injectivity and low MASP (max. allowable surface pressure). Just to give you numbers, injectivity of 1 bpm or less and MASP of less than 1000 psi. Apart from that, a conventional slurry would do just fine.
There is another potential application for injectivity lower than 0.5 bpm and high squeeze pressure (> 5000 psi). However in this case, knowing that to get penetration the slurry needs to displace the fluid already there, the additional friction caused by the cement slurry in addition the low injectivity would not likely permit sufficient penetration for an effective seal. It is my opinion that in this case a conventional cement slurry with medium FL (aprox. 150 to 250 ml/30min) would perform better … because it will be cement filtrate displacing the fluid in place.
Can we also say that micro-matrix cement usage shall depend on the economic factors such as purpose of squeeze and the ultimate monetary advantages of its use in special cases?
Kedar, thanks for your question. It is actually a very interesting one. Economics of remedial cementing is a tricky one, particularly if we consider new or old wells. For new wells the objective is to stop the NPT and save cost and in most cases the use of costly cement, like micro-matrix, is just a fraction of the rig time cost. In work-over, I think economics makes more sense and the use of costly cement slurries can only be justified if technically the best choice, but as you know in economics you need as well to consider the probability of not being successful in one attempt
One more question…a rather late one….but what are the typical ranges for ‘low, medium and high’ injectivities?
Thanks.
I developed for a field application the following flowchart some time ago. Please let me know if this helps.
Thanks a lot. That was very helpful.
Dear Lenin Diaz, please advice regarding the right and side of the flow chart,
1. So generally, if injectivity test result is > 3.5 bpm, can we assume that we have loss? and Thixotropic cement is neeeded?
2. Compare between middle and on the righ where injectivity is 2.5 and 3.5 bpm, I think if injectivity is higher, the filtration rate should be lersser.
Please advice.
Dear Thirayu, thanks a lot for your comment and for visiting my website. As I always mention I am here to help you achieving your well cementing goals.
Now regarding point No. 1. If injectivity is as high as 3.5 bpm we are likely in a losses situation if we consider our objective is to perform a cement squeeze. My prefer way to deal with this is to use, if available, a low-density high performance cement slurry. i.e., cenospheres or a foam/nitrogen based system combining low density and high solid content to reduce the hydrostatic head during placement and have more control over the squeeze process.
With a thixotropic cement, we are targeting an increase in friction pressure with a gel strength developing at low rates to static conditions, however thixotropic slurries density to have good characteristics are around 14 ppg which in most cases would not allow the gel strength to develop as quickly as needed in actual conditions.
For point No. 2, as I explained in the post. High injectivity + High rate of filtration can lead to premature plugging. That is mainly valid for rates up to 2.5 rpm, but for higher injection rates up to 3.5 pm the plugging affect can be beneficial to help achieve the objective of the cement squeeze.
Cheers
L. Diaz
Having issues with cement retainers “washing out”. The zones are on a high vacuum and suck the tubing dry in s matter of seconds. Is there something we could pump prior to cement to slow the vacuum
Hi Brian
Thanks for your comment. That is certainly a big issue in Low Pressure Wells or when there are total losses.
In some parts, Shutting off intervals in Low pressure wells (Fluid level almost half way the available column) produced several solutions, they included a combination or stand alone application of nitrified fluids, modified cement slurries, a form of DOBC slurry, gels and LCM. I believe the more successful involved creating a network of LCM able to act as a filter lowering the injectivity only enough to allow filtration rather than losses.
Cheers
L. Diaz
Hey Lenin!
Good seeing you again :o) I never knew you were running this EXCELLENT webpage, but now since I’ve found let me fire away my questions:
So we often discussed to adjust FL in order to handle different injectivities. Under what circumstances though would you modify the cement viscosity? Like if I had a very high injectivity zone, could I not leave FL but increase viscosity? Certainly not when doing a CT squeeze, but when doing a P&A operation maybe?
Looking forward to hearing back from you,
Cheers,
Matt
Hi Matthias
So great to know from you, hope all is going well !
Please let the team know that I am surely missing our talks and work together
And as I remember, you always asking the right question … this one is a good example
Please keep in mind, that during squeeze we are forcing the cement filtrate (liquid phase in the cement slurry) out, and along with any liquid ahead, through a porous media (Cement Filter cake and Formation). This is why there is relationship between cement fluid loss and permeability (as related to injectivity).
As described by Darcy’s Law viscosity plays a role (inversely proportional). In this case the viscosity of the cement filtrate.
However, if there is very high injectivity, as you mention, and the gap is in the form of fractures big enough to allow cement slurry flow through. Then certainly a way to take advantage of the friction pressure inside the flow path is to increase the viscosity of the cementing fluids. For example, in Low Pressure wells / depleted, viscosity of the cement slurry and the fluids ahead (like gels) was a key parameter to retain some cement in the wellbore. A measure of the required viscosity (friction pressure) can be derived from the fluid level in static condition and the required pumping rate to get returns, while pumping high viscosity pills.
Thanks for the contact and please do let me know if I can be of further support.
Cheers
L. Diaz