Causes and preventive measures of annular air channeling after cementing

1. Harm of air channeling

After cementing, annular air channeling refers to that when the cement slurry is transformed from liquid state to solid state, it is difficult for the cement slurry to maintain the pressure on the gas reservoir or the cement slurry channeling causes poor cementation quality, and the gas in the gas reservoir flows into the cement matrix, or into the gap between the cement and the casing or between the cement and the well wall, resulting in inter layer channeling or even into the wellhead.

The main hazards of air channeling are as follows: 

a) It directly affects the cementation strength of cement stone;

b) It leads to interlayer channeling, which directly affects the test and evaluation of oil and gas reservoir and pollutes the oil and gas reservoir.

c) Reduce oil and gas recovery;

d) It has adverse effects on the follow-up operations of oilfield development, such as water injection, acid fracturing and layered production;

e) In serious cases, oil and gas may flow out of the wellhead, and even blowout accidents may occur after cementing. Even if cement extrusion and other remedial technologies are used, they are difficult to work. 

2. Mechanism of annular air channeling

         a)"Bridge blocking theory"

After the cement slurry enters into the annulus, the water cement ratio drops sharply due to continuous filtration, which results in the obstruction of static pressure transmission of the cement slurry, and gas channeling occurs when the pressure of the cement slurry column in the annulus is less than that of the gas reservoir. Therefore, the filtrate reducer of cement slurry is widely used to prevent gas channeling.

         b) "Theory of cement slurry loss of cementation"

After the cement slurry enters the annulus, when its static cementation strength is 48 ~ 240pa, it belongs to the transformation period from liquid to solid. The cement slurry gradually loses the ability to transfer the liquid column pressure, which is also the period of gas channeling. Therefore, thixotropic cement, delayed gelling cement, right angle thickening cement slurry system and non permeable anti gas channeling cement slurry system have been researched and developed by relevant organizations at home and abroad.

         c) "Theory of pore pressure reduction"

During the solidification of the cement slurry, the pore water in the cement slurry decreases with hydration and filtration, which makes the pore pressure in the cement slurry decrease continuously, and then causes the gas to invade into the cement slurry to generate the annular air channeling. For this reason, relevant institutions at home and abroad have developed gas cement and foam cement cementing technology. 

3.Technical measures to prevent air channeling

a) Annulus pressurization Technology

The technology of annulus pressurization is to add a certain pressure in the annulus after the completion of cement injection (the value of field pressurization is generally 2-3mpa), so as to ensure that the pressure of the liquid column in the annulus is greater than the pressure of the gas reservoir, so as to achieve the purpose of preventing gas channeling. Annulus pressurization technology is the earliest, most widely used and most economical technology measure to prevent gas channeling, and it has also achieved a certain effect. But because of its limitations, it is more used as an auxiliary technology to prevent gas channeling.

b) Technology of packer outside pipe

The technology of outer casing packer refers to adding outer casing packer between adjacent gas layers, expanding the casing packer after the completion of cement injection, and forcing each gas layer to separate. This technology is generally used when conventional chemical methods can not work, and it is only suitable for wells with a small number of gas reservoirs.

c) Pulse cementing technology

Pulse cementing technology is a simple and low cost technology, which is conducive to keep the cement slurry in liquid state, so as to maintain static pressure and prevent gas channeling, improve the cementing quality between cement and casing, eliminate micro annulus, and improve the cementing quality.