Cement slurry as mixed on rigs for oil well applications is a pumpable liquid consisting of water and various particles in a suspension. If this liquid is exposed to a differential pressure across a filter medium, water tends to filter through the medium leaving particles behind. This effect with its severity is called fluid loss.
So, what is, or can act as, a filter medium? Well, any permeable formation is. Hence, for a cement slurry you plan to use for example inside a casing, fluid loss is not a relevant factor in the design. Now, fluid loss property of a cement slurry is also connected to free water and stability of the slurry, so for a useful cement slurry even inside casing, good fluid loss (normally, meaning low) is indirectly a factor.
So, what if you lose some fluid - what's the big deal?
Well, let's look at an example where a slurry is moving up a narrow annulus, and you lose fluid across a permeable formation. The remaining cement slurry becomes thicker as you lose fluid, and this will increase your pumping pressure. This could, of course, lead to more loss of fluid and possibly a change in flow regime and less efficient placement. In severe cases, the slurry can bridge off and prevent you from getting all the cement placed, or it could lead to formation fracking and losses, again leading to low TOC (top of cement). A cement slurry with less water will also normally set up earlier, so your thickening time will be reduced. All this can lead to a severely messed up cement job that is hard to remedy.
In larger casing jobs, fluid loss is less of a concern. Usually, it is not an issue for jobs in open hole either unless it is a high-density plug across a permeable formation where you could end up with a lower TOC than planned. For squeezes, the concern is somewhat different. For these jobs, you very often want a certain level of fluid loss so that the cement will bridge off on the leakage points and seal, then allowing flow diverted to other parts targeted for the squeeze. In cement slurries used for lost circulation, a high fluid loss would be a benefit since the cement then would tend to bridge off towards the loss zone.
How do you determine fluid loss for a cement slurry?
Fluid loss is simply measured in the lab (as per API-RP-10B), (see also Miguel Diaz' article Lab tests to review when designing a "gas tight" cement slurry) where a slurry is mixed and poured into a small cell with a filter medium (325 mesh screen at the bottom). The cell is pressured up to 1000 psi, and then a valve is opened at the bottom and the amount of liquid coming out within 30 minutes is measured. The measured amount, multiplied by 2, is the fluid loss of that particular cement slurry. This is the standard way of measuring static fluid loss. In some cases, the slurry will leak off all the fluid before the 30 minutes and then just blow air through. Then you don’t have much fluid loss control and a simple formula based on the time air starts blowing through, is used to calculate the 30-min. fluid loss (see API-RP-10B)
What is good or bad fluid loss?
It depends on the application, of course. Most operators will accept fluid loss values below 200-250 ml./30min. as reasonable good fluid loss control, but for critical jobs like a narrow annulus liner or casing job across permeable hydrocarbon zones, they typically require fluid loss values below 50 or even 15. A basic 15.8 ppg cement slurry with no chemicals to improve fluid loss will usually have a fluid loss value higher than 1500 ml./30 min. (ref. Well Cementing, Nelson & Guillot).
How do you control fluid loss?
All service companies providing cementing service on rigs use several chemicals to improve fluid loss control. They work in different ways and are dependent also on other chemicals and the properties of the cement slurry. A heavy cement slurry will automatically tend to have a lower fluid loss than a light-weight one. The same with a well-dispersed slurry versus a not so well dispersed one. The actual fluid loss chemicals work in basic ways:
- Particles of certain size and distribution that packs and makes a less permeable filter cake, in some cases an extra film on the filter cake is also created
- Viscosifying chemicals for liquid phase that decreases flow rate through the filter cake
- A combination of 1 and 2.
Now which type of chemical and approach you choose to improve your fluid loss is not normally that important, only the effectivity is. Very often the type of fluid loss control system you will use is dictated by other factors, like the slurry density and other chemicals used for other purposes.
Have you heard about fluid loss control in resins used in oil wells? Is there even such a thing? Read my next blog on this topic.
