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.
Well cementing in high temperature and hydrogen sulfide (H2S) corrosive environment presents challenges in preventing cement compressive strength retrogression and selecting weighting agents inert to H2S. This paper presents the development of a cementing system for high pressure high temperature (HPHT) well with bottom hole static temperature in excess of 165°C, a drilling fluid density of 2.19 SG and a high concentration of H2S. A major operator in the Caspian Sea region accepted the cement design and successfully used it on the production liner section of the HPHT well. Cementing of the production liner was complex due to the requirement for a high-density cement system, narrow margin between the pore pressure and frac gradient, HPHT conditions and 18% H2S concentration in the formation fluid. Comprehensive laboratory testing was performed to evaluate the properties of the cement system including measurements of thickening time and compressive strength evaluation using a UCA and destructive method using ultra-HPHT curing chamber for cube sample curing. The presence of H2S limited the use of conventional weighting agents such as hematite and hausmannite, and the high temperature environment dictated the need for quartz silica. These factors required a nonstandard approach to cement blend formulation and flowability assessment. During cement system optimization, the target slurry density was achieved using barite which has a lower density compared to other common weighting agents and significantly reduces cement content in the blend but also is inert to H2S corrosion. A further challenge encountered during cement system optimization was strength retrogression that could not be prevented by the conventional approach of adding 30-40% quartz silica by weight of cement into the system. To overcome strength retrogression, much higher concentrations of silica were required. As a result, the low cement content led to insufficient compressive strength development at liner hanger depth. A solution was found by adding a Vinylamide/Vinylsulfonated polymer (VA/VS) polymer in a certain proportion to the slurry design. Thus, at elevated temperatures, it was observed that the VA/VS polymer tended not to delay compressive strength development while still extending the slurry thickening time. The developed heavy weight cement system was successfully implemented to isolate the 7-in liner on HPHT well. All the stages of job planning, design and execution, along with the slurry optimization process are presented.
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