Mud Displacement, the ‘mystery’ factor in gas and fluids migration

Very often I find myself helping people having issues with fluids migrating following primary cementing. In all these cases, it is always fascinating to see the frequency of this problem nowadays. Especially with the involvement of expert services companies and the availability of state-of-the-art engineering and products solutions.

I have already written here about fluid and gas migration, and this is definitely not a new topic in our industry.

More importantly gas and fluids migration should not be a ‘mystery’ nowadays. However, in some cases, it appears to be.

The cause is univocally simple:

Fluids migrate because the annulus is in underbalance

Why?

First, let’s remove the obvious triggers:

• Losses.
• Faulty or unknown actual Pore Pressure profile or model.
• The next thing is the cement slurry transition period. (The intermediate self-supported gelled structure that lowers the cement slurry internal pressure to water density). NOTE: What is not so obvious, is that the conditions (let’s forget about having a proper slurry able to control gas migration or not) for this to happen are: 1) flow through the cement slurry should be the only (easiest) path (which translates to good cement coverage) for gas migration, and; 2) the cement slurry setting process actually happens.

If we remove the above, the only thing remaining is merely to achieve the main objective of primary cementing. Not only to prevent gas and fluids migration but in general for any casing cementing operation:

Cement Coverage (mud displacement)

This is the real challenge today.

In most primary cementing jobs, not achieving cement coverage (mud displacement) ends in a bad cement log and/or a lack of zonal isolation. This is already bad enough, but when there is a risk of gas or fluids migration, it might end up in a well control situation.

Why can a lack of cement coverage (channelling) lead to gas or fluids migration?

NOTE: This is what you really need to worry about in the first place. So, please do not ask for LATEX or some other specialty (costly) chemical and think that you are covered… Well, you are NOT. What you need to ask first to your cementing company is: Channelling prevention, please.

Why?

1. Minimum ECD below Pore Pressure. This is the case when a Newtonian fluid is introduced (because the ‘best practice’ says so – supposedly!). Here the Newtonian fluid does what it does best: It channels. Then depending on the length of the channel, the fluid column becomes underbalanced during the job allowing formation fluids to enter. Then, this lowers the hydrostatic pressure even further.
2. Damaged fluids properties:
   • Mud. Longer cement channels lead to increased cement – mud contamination (intermixing). Cement, an active contaminant for WBM, leads to loss of mud properties such as rheology (flocculation) and carrying capacity. Finally a reduction in density will occur.
   • Cement. Cement-mud contamination delays the setting process of the cement slurry. This not only prevents the formation of the gelled structure (forget about your gas migration slurry), but it might stop the cement slurry setting entirely. And what happens if the cement doesn’t set? What happens is that; Solids (cement particles) settling, and lighter fluids (water) floating. Together, these effects eventually lower the annulus hydrostatic pressure. This will allow formation fluids migration.
   • Don’t forget about cement slurry density control. One of the requisites for gas migration control slurries is Zero free-water. However, if a sufficiently large volume of cement slurry is mixed at a lower-than-designed density, the free-water increases. The result might be a destabilization of the cement slurry. This leads to loss of density in both the cement slurry and the mud.
   • The mixing of two successive fluids may lead to the formation of a viscous mixture. The viscosity of the mixture is often higher than the average viscosity of the two fluids. In turn, this increased mixture viscosity is known to be an undermining factor for mud displacement.
3. Mud Displacement. Intermixing of Mud-Spacer-Cement Slurry or Spacer-Lead-Tail slurry. Intermixing causes a deterioration of fluid’s rheology and density. This is not accounted for by any software simulation, (assuming perfect piston-like displacement), hence leading to an obsolete hydraulic program. (Actual dynamic and static pressures may be different from the predicted model). Potentially such a program is unable to fully displace mud and/or remove gelled/partially dehydrated (filter cake-like stuff) mud.

In summary

• Cement placement is fundamentally the most important gas and fluids migration objective.
• Two main defects arise from channelling (in the case of gas and fluids migration this can be potentially dangerous) in primary cementing.
  • An un-displaced fluid. This is when a fluid is left behind, either in static (gelled mud attached to casing or formation) or flowing mode. (Typically in the annulus).
  • Fluids intermixing. Active mixing of fluids resulting in mixed-products having different properties from the individual mixed fluids. It can happen both inside the casing and in the annulus.
• When gas or fluids migration is a risk, the main focus is to design and deploy a cement program that avoids any form of channelling. In other words, a cement program that seeks to attain the maximum possible cement coverage. Then, secondarily is the cement slurry gas migration control property (in the case of gas).

If you have any comments or questions, please contribute here.

Cheers

L. Diaz