Cementing Problems

Gas Migration in Cement: Challenges and Solutions

In Canada, it is estimated that approximately 57% of shallow to moderate-depth wells experience leaks after primary cementing, which highlights a significant challenge in ensuring effective zonal isolation.

Key Technologies for Achieving Zonal Isolation During Cementing

1. 1. Stable Wellbore Conditions
      Achieving a stable wellbore, free from fluid losses or gains, is crucial before running casing. A consistent wellbore environment reduces the risk of gas migration during cementing.
   2. Adequate Annular Circulating Flow Clearances
      Ensuring adequate circulating flow clearances, particularly when selecting the bit size in relation to the intended casing size, is vital. For situations involving liner overlap, expandable casing may help avoid cement channeling in tight annular spaces.
   3. Proper Spacer Design
      The design of spacers, including their weight and volume, is essential for ensuring effective displacement of drilling fluids and preventing contamination of the cement slurry.
   4. Casing Centralization
      Correct casing centralization ensures even cement placement. It’s important to use neither too few nor too many centralizers, as either extreme can affect the quality of the cement job.
   5. Effective Drilling Fluid Conditioning
      Well-conditioned drilling fluids are critical for efficient mud removal and cement displacement. The proper conditioning ensures a clean wellbore and effective bonding between cement and the formation.
   6. Tripping Best Practices
      Following best practices when tripping both drill pipe and casing minimizes the risk of disturbing the wellbore and creating channels for gas migration.
   7. Optimal Drilling Techniques
      Employing sound drilling techniques throughout the well construction process contributes to minimizing wellbore instability and subsequent cementing challenges.
   8. Continuous Well Monitoring
      Continuous monitoring from start to finish ensures that any anomalies or changes in pressure or flow are quickly addressed, minimizing the risk of cementing failures.
   9. Proper Waiting on Cement (WOC) Time
      Adequate WOC time is necessary for cement to cure properly. Rig operations during this period should be managed to ensure cement integrity and prevent early disturbances that could cause gas migration.
   10. Hydrostatic Pressure Management
       Maintaining sufficient hydrostatic pressure during the cement curing period, coupled with cement mixes designed to resist gas migration, is essential to prevent the formation of channels that could lead to leaks.Static Gel Strengths in a Cement Slurry.

       The development of static gel strengths in cement during its curing process presents a significant challenge globally. As the cement hardens after displacement, gel strengths begin to form as part of the hardening process. Once the gel strength reaches its “critical gel strength,” the gel structures start to support the cement above, much like how a bowl of Jello can maintain its shape without collapsing. As the cement continues to cure, the gel structures, along with fluid loss into the formation, begin to degrade the hydrostatic barrier against the wellbore. Eventually, this may leave only the hydrostatic pressure from the makeup water. If the hydrostatic pressure of the makeup water is insufficient to resist formation fluid pressure, gas migration may occur through the water phase of the unset cement, potentially creating permanent “worm holes” within the cement sheath. This can lead to flow or pressure transmission through the set cement.

       The critical gel strength, referred to as Zero Gel Time, typically begins around 100 pounds per 100 square feet and ends at approximately 500 pounds per 100 square feet. The time between these two gel strength levels is known as the transition time.

   11. Mechanical Barriers
       In certain situations, utilizing mechanical barriers such as packers or plugs can provide an added layer of protection against gas migration, ensuring long-term well integrity.

   By addressing these factors, the industry can mitigate the risks associated with gas migration in cement and improve the overall success of cementing operations.

   For further information on this or other well control subjects, consult my Well Control Manual at: https://learn-well-control.com/product/well-control-manual-by-edwin-ritchie/