The favored technology for shallow-water flow (SWF) mitigation in deepwater has traditionally been foamed cement. These slurries exhibit expansive qualities and low Young's modulus, which enhances mechanical resiliency. They also deliver a high yield per sack of cement while improving displacement efficiency. The inherent pore pressure exerted by the nitrogen fraction is believed to compensate for hydration reduction that occurs as slurries set, minimizing pressure losses due to volume reduction.9,10 These slurries have been proven worldwide and have become quite common for SWF mitigation as technology has simplified the means of adding the nitrogen and foaming agent.2 Research indicates it is possible to maintain a margin of accuracy of 99% or better for adding the nitrogen and foaming agent, minimizing large density swings while mixing cement. Additionally, the system used for additive addition is automated, removing a large portion of the "human error" factor. This application is normally utilized for cementing the tophole section while drilling riserless, with the excess cement being returned to the seafloor. Typically, shallow-water flow problems arise only in deepwater environments, but some rare, shallower offshore locations also encounter this problem. One such area is in the Caspian Sea. Due to tectonic uplift and rapid sedimentation, the Caspian Basin exhibits geohazards atypical for its shallow water depths. Challenges often found in water depths of only 120–150 m include mudline temperatures averaging 5°C, a narrow pore-pressure/frac-gradient window, very little formation integrity, geopressured sands, and mud volcanoes. An operator's preparations for the development phase of a Caspian Sea project included predrills conducted with a typical semi-submersible rig. On several occasions SWF was encountered after cementing the 20-in. strings because the water- and hollow sphereextended slurries could not control SWF. During the pre-drills, the flows were simply allowed to dissipate naturally. However, after a production facility was established and platform drilling commenced, this practice caused the complete loss of one sub-sea template. This catastrophic event forced the operator to consider using foamed cement. Concerns quickly arose because the technology had never been used
in water so shallow with a riser in place;
on a platform producing over 200,000 BOPD;
with synthetic oil-based mud; or
in the Caspian for over a decade.
This paper discusses the challenges faced in implementing foamed cementing technology in a new area: The challenging conditions faced when cementing, the importance of automated foam system accuracy; the steps taken to prove system accuracy and reliability; key steps taken by the operator to ensure job quality and safety; an overview of how slurries were optimized; and case histories
