The problem of annular gas migration following primary cementing is analyzed, and the main mechanical responsible for its occurrence within the cement matrix or at the cement to formation interface are described.
The emphasis is put on a special technique that had proved, through extensive field usage, to solve the problem in widely varying conditions, much 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 degrees 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 much 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.
The primary cementation of a well aims at, first holding the casing in place in the hole, and second at providing a durable and tight isolation in between the different producing zones and up to the surface. This annular cement seal must be totally efficient for the whole life of the well in order to work anytime under safe conditions, prevent casing corrosion, improve production and optimize any future stimulation treatments. It has long been recognized that the most critical problem is the percolation of gas through the cemented annulus that leads to the development of wide and very conductive channels in the cement sheath. This flow of gas, so called gas migration, has proved first to be very difficult to prevent, second extremely dangerous, especially when H2S containing gas is involved, and finally extremely long and costly to repair. Over the years, many attempts have been made to solve this serious problems. These have included the use of annular back pressure, multi-stage cementing, external casing packers and compressible cement slurries. it is generally accepted that in certain circumstances, each of these techniques can meet some degree of success, but that they all present limitations. This paper is not intended to review all these techniques, but to present a different system based on the addition of a safe liquid material to the basic slurry which, when coupled with good cementing practices, has proved to be extremely successful in the field and applicable in almost any situation of potential gas migration. This technique is referred in the following of the paper as Impermeable Latex Cement or ILC.