The problem of annular gas migration following primary cementing is analyzed, and the main mechanisms responsible for its occurence within the cement matrix or at the cement to formation interface are described.
The emphasis is put on a special technique that has proved, through extensive field usage, to solve the problem in widely varying conditions, such as shallow gas from weak zones, very high pressure formations, interlayered gas and water sensitive zones. Such conditions involve slurry density from 10 to 22 lbm/gal, downhole temperature from 10 to 180° C, and mix water from fresh to 18% NaCl.
This technique is based first on the proper displacement of the drilling mud through good cementing practices, and then on the placement in the annulus of a well dispersed, non gelling, impermeable cement. The key of this system is a latex additive, that, through its film forming property, locks the cement permeability to gas when the gas attempts to permeate under a differential pressure.
For such a system to perform optimally, cement pore pressure drop is delayed as much as possible, through the use of proper dispersants that inhibit premature cement gelation. Then the transition time between the liquid and the set state is shortened by chemical stabilizers of the latex. Finally, cement permeability to gas is controlled during cement thickening and setting, and cement to pipe and to formation bonds stay durable for the whole life of the well, and this is achieved through the latex itself.
Field applications of this system are presented through several case histories illustrating typified normal and extreme situations.