This paper describes development of a cement slurry composition designed to cope successfully with gas migration problems. The gas blocking effect is obtained by addition of microsilica, a pozzolanic material of extremely fine particle size. Numerous tests have been performed under laboratory conditions to investigate the phenomena of gas migration through cements, and the corresponding amount of specially selected microsilica needed to effectively prevent gas flow.
Gas flow behind casing after cementing has been a common problem in the industry since the inception of oil well cementing. After 1970 a comprehensive understanding of the mechanism involved in gas migration has been obtained through laboratory testing and field trials. Different concepts pertaining to solution of the problem have been suggested. Extensive reviews of literature on gas migration mechanism and methods to prevent gas migration up to 1985 have been given by Sutton and Faul and Cheung and Beirute.
In Statoil's operation the need for gas tight cement was dictated by two frequently occurring phenomena in exploration and appraisal drilling in the Gullfaks field area, block 34/10 in the Norwegian North Sea: Gas migration through cement from a shallow gas zone outside 20" casing, and migration of gas from the paleocene and cretaceous formations outside 13 3/8" and 9 5/8" casings during the setting process of the cement. Both problems have presented serious, costly and time consuming consequences. In 1983 a research program was initiated to develop a lightweight gas tight cement for shallow gas problems. During the research period microsilica was found to have a suppressing effect on gas migration. Further study showed that provided a certain minimum dosage of microsilica was added, the slurry would exhibit gas blocking properties.
The first field test with microsilica was performed in 1985, when 20" casing was cemented through a shallow gas zone. No gas migration occurred. Since this initial test, some seventy casing strings have been cemented with microsilica based slurries with density variations from 1.54 g/cm 3 to 1.95 g/cm3. A total success has been achieved by tailoring each slurry composition to adapt to existing field conditions.