Abstract
Achieving zonal isolation for the lifetime of oil and gas wells is crucial for well integrity. Poor zonal isolation can detrimentally affect well economics and increase safety-related risks because of pressure buildup with unpredictable consequences. Additional local regulations prohibiting production of a well with positive pressure in the annulus made sustained casing pressure a major challenge for operators in the North Caspian Sea. An innovative cost-effective solution was required to resolve this challenge.
Historical well analysis proved that previously applied cementing approaches were ineffective. Several modifications were required to define the effective solution. Implemented changes included revision of the casing setting depth, optimization of the drilling fluids and spacer formulations, and implementation of the self-healing expanding cement. Carefully engineered placement of the self-healing cement system was the key to success. If cracks or microannuli occur and hydrocarbons reach the cement and flow through the cracks, the system has the capability to repair itself, thus restoring integrity of the cement sheath without external intervention. This technology has been used in 11 extended reach wells in two fields with excellent results.
The collaborative approach with drilling engineers eliminated the challenging sustained casing pressure issue in two major offshore fields in North Caspian Sea. In addition to the existing cementing best practices available in industry for mud removal efficiency enhancement and successful cement placement, the newly implemented methodology included potential requirements for well trajectory adjustments, implementation of the real-time control during cementing job execution, engineered placement and optimization of the self-healing expanding cement system formulation, and a specifically developed "initially required" bleedoff schedule that allows acceleration of the self-remediation cement capability. The self-healing cement was designed with low Young's modulus for maximum flexibility. Expanding additives were also incorporated into the design to minimize the risk of set cement integrity failure due to microdebonding from bulk shrinkage after setting.
Adherence to the mutually developed flowchart for the drilling and cementing stages improved the zonal isolation of the critical hydrocarbon zones in the extended reach wells and increased the success ratio of the wells with no pressure buildup from 30% to almost 100% within the last 5 years. As a result, the self-healing cement technology and developed approach, which is discussed in this paper, have become the standard for both fields for all future wells.
The complex engineering approach described in this paper expands the existing best practices in the industry for zonal isolation improvement of the extended reach wells and provides a new effective solution for eliminating sustained casing pressure problems. The design strategy, execution, evaluation, and results for two sample wells are discussed in detail to help to guide future engineering and operational activities around the world.
