Abstract
Complete and durable zonal isolation is the foremost goal of the cement job. In the deep and high-pressure environment, obtaining such goal is particularly critical, but also challenging due to the additional factors associated with the high drilling fluid densities that limit mud removal efficiency, narrow margins between fracture and pore pressures that cause loss circulation and differential sticking, and cement sheath exposure to downhole stresses during construction and production phases that compromises its integrity.
Careful planning is required to ensure all risks are captured and mitigated during the design stage, taking into consideration not only the construction phase, but also post-placement downhole conditions changes caused by temperature, pressure fluctuations, and mechanical shocks during perforation and stimulation operations.
Data analysis of the offset wells located in the eastern section of the Caspian shelf showed that conventional cement systems and previously applied cement job designs had limited success in addressing those challenging complex requirements. Thus, a new approach was required. This approach was used in 20 wells in the field with excellent results.
Two wells were used to demonstrate the improvements obtained in zonal isolation behind production liners upon implementation of new engineered methodology. The innovative complex approach involved not only the revision of the previously used cement and spacer fluid designs, but also required revisiting and evaluating every aspect of cementing practices to achieve the desired results. Fiber-based spacer technology was introduced to enhance mud displacement and an engineered flexible and expanding cement system to achieve and maintain well integrity.
Numerical analysis modelling was used to simulate the stresses that the cement sheath will experience over the well's life and calculate the minimum required mechanical properties of cement to be able to withstand these stresses. The set cement mechanical properties were then customized using a proprietary trimodal particle-size distribution technology to accommodate the expected downhole stresses. Hydraulic isolation improvement was achieved and confirmed by downhole logs.
