| Popis: |
In the western Canadian sedimentary basin, tens-of-thousands of wells currently are leaking gas between the surface and production casings. Often this leakage manifests itself as a surface vent. While much work has gone into preventing gas vents during primary cementing, little has been done to improve the chances of successfully sealing existing leaking wells. The work described in this paper focuses on new materials and techniques that have been developed to seal vent flows. The paper describes the theoretical physics behind the process and gives case histories to demonstrate the successful application of the technology. Introduction Microannular gaps as narrow as a few microns can allow gas leakage depending upon the differential pressure. The flow paths that allow for the leakage may be present at either the pipe/cement or cement/formation interfaces. In order to penetrate and seal such narrow gaps, special optimized microcement systems have been developed. The properties of a cement slurry required for placement into such narrow gaps are small particle size, efficient fluid loss control both axially and radially, a very thin filter cake, low rheology, zero free water and no sedimentation under down hole conditions. The set cement properties required for long-term sealing are extremely low permeability and mechanical properties sufficient to resist stress cracking. The placement technique of the slurry is also a key parameter in the success of sealing vent flows. Placing the slurry at extremely low rates, often less than 10 L/min, decreases the friction pressure generated in the gap. This in turn reduces the differential pressure across the slurry and decreases the probability of bridging. Once the slurry is in place it must remain undisturbed until it sets to form a permanent seal. This is accomplished by continuous pumping until the slurry thickening time has been reached and the cement sets. |
