What Causes Micro-Annuli

In terms of oilfield cementing, micro annuli in cement is a common issue. Micro annuli usually lead to production reduction and expensive remedial cost. The problem is caused by many factors and this article will focus on the most common reasons and how to deal with them.

There are many leaking wells in the world which can be challenging in many aspects. A lot of wells are shut off even if they can still be used for many years. Thus, rather than losing money, well owners could get a return of their investment when they can stop the leakage instead of shutting the well down completely.

Understanding Micro-annuli

These are micro-separation between pipe and cement as a result of a temperature or pressure change during the cementing setting process or after setting the cement. Also, micro-annulas can take place between cement and formation. This is less common; however, it can lead to a lack of bonding to the casing or formation and a potential communication channel.

Causes of Micro Annuli

During the setting of the cement, it is important to keep an eye on the cement behavior. The constant exposure of the slurry to forces and obstructions while being in the wet state paves the way for the occurrence of micro-annuli.

Below are the common causes:

  • Cement cracks. Cracks in cement can take place due to stress. The forces which occur during setting can result in de-bonding and create micro fractures in the cement. Over time, cement shrinkage can do the same.
  • Contraction and expansion. Dynamics associated with this can result in debonding from the formation. This could only be a gradual reduction of pressure under well construction, after a decrease in reservoir pressure. Also, this could be an abrupt change when displacing the well from mud to water or stress which happens during a frac job.
  • Dirty annulus or formation. If the layers of mud or other materials on the wall of the formation or annulus are not removed, the bonding will weaken.

  • Less hydrostatic head. Setting cement in a zone with less hydrostatic head on the formation will cause high gas pressure from the bottom. In turn, this will lead to gas migrating during the setting process. As the gas finds its way through the mud cake, a fluid column in the cement will be created.

Despite the commonality of micro-annuli in the cementing process, its occurrence can be reduced with proper planning that includes process and material selections. Also, it is necessary to have proper contingencies in place to deal with problem areas right away when micro-annuli take place.

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