Due to pressure and temperature changes during the life of the well (from drilling to production); globally many Casing-Casing Annular (CCA) leaks are observed in the annuli between surface and intermediate casings. A new approach to well cementing had to be developed, and advanced cement technologies considered, taking into account both short and long term design parameters.
To assess the effect of pressure and temperature cycling on the long-term well integrity, finite element modeling of sections of interest was performed to analyze the stresses applied on the cement sheath throughout the life of the well. Such modeling requires detailed knowledge of the pressure and temperature cycles, as well as the mechanical properties of the formations, casings and cement.
The output is a failure analysis of the cement sheath in terms of compression, tension and micro annulus development. Once the failure mechanisms are well understood, further sensitivity analyses permit to select the most adequate cement system for the application, based on target windows of compressive and tensile strengths, Young's modulus, Poisson's Ratio, and expansion capacity.
The paper will discuss a case study where a fit-for-purpose durable cement system was designed to meet the required mechanical properties as suggested by the stress modeling performed on a candidate well. It was placed as a Tail slurry on the 9 5/8" Tieback casing to provide 1000 ft. of long-term hydraulic isolation in the cased annulus should the liner top packer fail to seal the high-pressure water formation below. A post-job analysis using job data and cement evaluation logs showed that placement was executed as per design and that the durable cement system provided excellent bonding and annular coverage. The well was later drilled to total depth, completed, fractured and delivered to production without CCA pressure at the wellhead.
The new approach consists of performing finite elements stress modeling during the well planning phase to custom design cement systems that will withstand the anticipated loads.