Offshore drilling operations that encounter shallow gas formations must consider the potential annular gas flow that may occur following primary cementing. Many specialized cements and procedures have been developed to combat gas migration, but the complexities of gas migration control still challenge operators worldwide. In offshore shallow environments, additional complications can arise with the presence of weak formations and cold temperatures. In such conditions, lightweight lead cements are employed to avoid fracturing the wellbore. Although lead cements are often viewed simply as "filler" materials, shallow gas control slurries must far exceed that role as they become the mechanism to help isolate the movement of gas up the annulus.

Presented in this paper is a review of the properties of gas control cementing systems specifically related to lightweight lead slurries. The importance of fluid loss control, rapid gelation, and compressive strength at the time of drillout is stressed. Silica fume cement and newer cementing additives such as colloidal silica and small particle cement are highlighted as means of helping prevent shallow gas migration. Several offshore cementing operations are documented which confirm the success of applying the prescribed designs and methods.

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