What is Rheology?
One of the most important things we, as oilwell cementers, must understand is rheology. In our line of business, Rheology is the key to cement coverage and zonal isolation; which is the sole goal of well cementing. However, as simple as this can be, it is incredible how easy it is underestimated and sometimes ignored entirely in our industry.
Unsurprisingly, it is not rare to hear cases of ‘unexpected’ cement returns in primary cementing, ‘unexpectedly’ higher TOC in liners, ‘unexpected’ lack of zonal isolation, etc. Only on very few occasions, the ‘unexpected’ designation of these situations gets attributed to Mother Nature or any other variable out of our control (no mystery here). More commonly, ‘unexpected’ would be the result of lack of awareness of the importance of rheology and fluids compatibility.
It is this reality that motivated me to write a post with some fundamental knowledge about rheology.
What is rheology? In general, we define rheology as:
The science of flow and deformation of material.
However, during my engineering studies, I remember that I liked this particular definition better:
All that happens in between Newton’s Law and Hooke’s Law.
The study of materials with both solid and fluid characteristics.
In well cementing, rheology affects:
- Cement slurry mixability and pumpability
- Cement coverage (mud displacement)
- Friction pressure estimation, ECD, losses prevention
- HHP requirements
There are two types of flow:
- Laminar flow
- Turbulent flow
The one we shall master as cementers is no doubt laminar flow. Why? It is merely because a turbulent flow is rarely achievable for any fluid. It’s even rarer for cement slurries, in situations where there is a high risk of channeling (like surface and intermediate casings)
I also have to mention plug flow…
… Which is sometimes heard out in the oilfield. Just one phrase for you: forget about it!!
In Laminar flow, the velocity at the wall (casing and formation faces) is zero, and the maximum velocity is at the center of the annular gap.
In other words, essentially there is a natural tendency for fluid to channel when it flows laminarly. What should we do? Just prevent it channeling too much! How?
First of all, the standoff has to be maximized so that the flow path is maximized all around the annular gap.
Now, let’s see the mathematical representation of the shear stress (friction pressure) vs shear rate (flow rate). Or in simpler words how the fluid flows in response to force (pressure). There are four accepted models:
- Bingham Plastic
- Power Law
- Herschel–Bulkley (provides a better fit in most occasions for cement slurries)
The use of computer simulation software
Most computer simulation software used in well cementing provides a representation of the flow models for each fluid’s rheological data entered. This data comes from laboratory measurements following API RP 10B-2 / ISO 10426-2 procedure. In some simulation software, you even have the opportunity to select which model better fits experimental values manually. Proper selection of the model is an essential step as it affects the validity of the simulator output.
The following chart is perhaps the first most important tool (simulator output) we have to ensure proper fluid placement (rheology). Here we must understand that the Oilwell Cementing Engineer should work closely with the laboratory to generate the best-fit solution for fluids design (spacer and cement slurries).
A common error these days is that design engineers do not provide the lab with a starting ‘target’ fluid rheology. This means that whatever comes from the lab is assumed to be ok and entered in the software, but there is no check of the following chart to see if fluids will progressively displace one another. Rheological properties are just as crucial as thickening time and density, they have a direct effect on job success (cement coverage = zonal isolation).
This image, in particular, is an ideal representation, where each fluid has higher rheology (friction pressure) than the previous (ahead), starting with the mud. This additional friction pressure in each fluid behind is vital to prevent channeling.
The maximum allowable ECD sets the limitation
Here are some recommendations:
- Lower the rheology of the mud as low as practically possible (‘practically possible’ usually means, that the mud retains some carrying capacity or to prevent solids settling) only after the hole is clean. It is a common practice for cementers to request/suggest a certain Yp (yield point) value in the mud. We must understand that Yp for the mud engineer is not the same as the Yp for the formed. Why? Because Yp for the mud engineer is “Θ300 – Pv”, but for the Cementing Engineer (using software) it’s a parameter (for Bingham Plastic; the Offset at 0 shear rate) in a mathematical equation. The Yp value from the simplified method used by the mud engineer will be typically higher than the one obtained from the mathematical model. Conclusion: When the mud engineer tells you the Yp, ask for the readings instead. Don’t input the Yp value directly in the software.
- For surface and intermediate casing, simplify your fluids. For instance, if we use a WBM, consider removing the spacer. Compatibility between mud and lead slurry won’t likely be an issue (always check).
- Get a well-documented FG or losses margin.
- Pick fluids density wisely. Don’t limit yourself to generic values.
- Choose the TOC for the Tail slurry as low as possible, if no requirement to cover a formation in particular.
- I have to say; pumping rate should not be compromised, always try to keep it as high as 7 to 8 bpm, especially when the cement is in the annulus.
Now, some words about fluids compatibility
Fluids compatibility is the change in rheology that happens down-hole due to fluids intermixing, and it can quickly make your job design (simulation) obsolete. It has particular significance in the presence of OBM, high density or high salinity muds; more common for deeper sections.
We need to test fluids compatibility following API RP 10B-2 / ISO 10426-2 section 16. However, the best approach is to avoid any incompatibility between mud and the cement slurries, even if we use a spacer (don’t trust the spacer will keep your slurries away from any contact with the mud).
Any incompatibility will alter the flow of fluids creating viscous or increased-gel byproducts every time an interface is formed as a result of channeling (higher viscosity or gelled residues are left behind bringing new fresh fluids in contact again as flow continues).
I hope you find this post helpful.
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