Rheological Hierarchy vs Density Hierarchy

In today's oilfield the term Rheologies is used a lot for a lot of things, but primarily its a term very well used in Oil Well Cementing. When you hear the term Rheological Hierarchy what comes to mind, even Fluid Density Hierarchy? Well first, lets define the word "Hierarchy" which is an arrangement of materials placed in order according to intrinsic values.

Since we have grasp a little bit better understanding about the term hierarchy, lets flip the script and talk about how these term fits into the world of cementing. When designing a cement job, the end goal is to place fluids in the wellbore in a specific order to achieve zonal isolation.

If the principles of wellbore fluid hierarchy is ignored, then different fluids may intermix or move past one another or they may have the tendency to settle out to a location thats different than what the cementing design has called for in regards to final fluid positions.

This is dependent upon static condition or dynamic condition, but both conditions could give adverse effects if our wellbore fluid hierarchy are not kept in check throughout the designing phase, also throughout cementation at the well site.

Let's Talk Fluid Density Hierarchy shall we, If you were to take two fluids of different densities (water & diesel) and placed them in a container of some kind such as a bucket. You would quickly observe the diesel is floating on top of the water.

Why? Simply because water is more dense than diesel, so you have a density differential, and the greater the differential the more pronounced fluid separation will become.

When you hear the term Fluid Density Hierarchy understand each fluid pumped during a cement job has a specific function or responsibility that is directly related to its relative position in the wellbore to the other fluids, so you would want to maintain a density hierarchy of fluids, so that each fluid pumped into the wellbore is heavier than the one above it, especially after cement placement under static conditions.

The purpose for achieving an adequate fluid density hierarchy is to minimize the amount of contamination between fluids primarily cement, however because what we know to be true is fluids of different densities will have some degree of intermixing at the interface of where two fluids meet.

Having a superior fluid density hierarchy helps to reduce the effects of contamination.

Next, lets talk Rheological Hierarchy. Rheological Hierarchy pertains to fluids in a dynamic state (flowing) which means in order for fluids to keep their relative position to each other in a dynamic environment, the rheological hierarchy profile of each fluid is defined by it's friction gradient (psi/ft).

So much like the fluid density hierarchy, we would want the friction gradient for each fluid in the wellbore to be less than the fluid beneath it. In regards to rheological hierarchy, there no defined best practice for relative value differences between the friction gradients, therefore keep in mind, the closer the numbers are to one another the chances for intermixing and contamination to take place are greater downhole considerably.

Remember, since the relative positions of fluids in motion are governed primarily by Rheological Hierarchy instead of just their densities. With this being said, this allows us to design cement jobs with fluids that are lighter than the fluids they are displacing, primarily spacer with low densities.

As long as a low density spacer has a higher friction gradient than any drilling mud system it will displace it or keep it aligned as expected for an appropriate rheological hierarchy profile. Rheological Hierarchy is dependent upon hole geometry, pump rate and rheology readings and so forth.

As an individual who worked as an operational employee for a particular cementing service provider. I feel it is very important for those who execute the cement design or should I say make it come to realization from simulation at the well site.

To really understand the importance of these terms and not just the technical professional who designs the job.

If we all understand technical and operational values from both perspectives it definitely put us in a better position to meet customer demands and help resolve their challenges and most importantly sustain the highest level of service quality prior to, during and after the cement job.