Trapped air inside casing and effect on cementing « Thread Started on Apr 5, 2010, 1:39pm »
When cementing a casing string using either a mechanical plug set (Subsea Release System) or drill pipe stinger, with the best intentions in the world, there will always be a certain volume of air in the casing (or in the case of the plug set - above the plugs). What real affect does this have on the cement job and in the case of the plug set, are there concerns of a vacum being drawn after the top plug launches?
Re: Trapped air inside casing and effect on cement « Reply #1 on Apr 9, 2010, 2:54pm »
The only air that should get into the cement slurry would be during the mixing part of the job. A small amount of air maybe entrained in the slurry but most companies add antifoaming agents to reduce this. When dropping the plugs and pumping out the lines before displacing the system is always closed, Ie. valvles are closed before another is opened to prevent any air getting into the lines. Any air that did get in would be compressed down to a negligile amount of space due to the hydrostatic pressures from the fluid column.
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Re: Trapped air inside casing and effect on cement « Reply #3 on Apr 17, 2010, 8:44pm »
Air is inert and has no affect on the cement chemistry, in addition when you test lines you are purging the air out of lines to ensure a closed system (or as holdril mentioned, ideally) but the affects of trivial air does nothing...
Re: Trapped air inside casing and effect on cement « Reply #4 on May 17, 2010, 3:15pm »
In 2 stages cement job, after pumping the first stage slurry, cement head cap is opened to drop the shut off plug (plug not designed to be pre-loaded). Therefore air is drained downhole due to vacuum effect. During displacement and once the plug catches the cement, this amount of air is either pushed down into the cement or remains trapped or pushed up above the plug (inside displacement fluid). I would agree on the last option otherwise the shoe track will be found empty while drilled out.
Some people believe this is the origin of microannulus in cement bound (option1) but I don’t think so.
Re: Trapped air inside casing and effect on cement « Reply #5 on May 17, 2010, 5:51pm »
On a 2 stage cement job, the plug for the first stage should be loaded into the plug loading head so ther eis no chance of getting air into the casing at this point when the plug is released. After the first stage plug is bumped and the float is confirmed as holding, the cap is taken off the drop the opening dart. The casing at this point is a closed system so there is no possiblity of air entering the casing. The well is circulated anywhere from 1-2 bottoms up or maximum about 8 hrs. The cap is then removed to load the closing into the plug loading head. Because the fluid inside and outside is the same density there would be no chance of sucking air into the system as the plug is put in the head. As far as teh comment of "in a perfect world" if your cementeres are not following prper procedures for releasing the plug and pumping out the lines, you might look at replacing them. It's not following proper procedures is how BP's goat roping incident out in the GOM happen and the industry as a whole takes the hit.
On a 2 stage cement job, the plug for the first stage should be loaded into the plug loading head so ther eis no chance of getting air into the casing at this point when the plug is released. After the first stage plug is bumped and the float is confirmed as holding, the cap is taken off the drop the opening dart. The casing at this point is a closed system so there is no possiblity of air entering the casing. The well is circulated anywhere from 1-2 bottoms up or maximum about 8 hrs. The cap is then removed to load the closing into the plug loading head. Because the fluid inside and outside is the same density there would be no chance of sucking air into the system as the plug is put in the head. As far as teh comment of "in a perfect world" if your cementeres are not following prper procedures for releasing the plug and pumping out the lines, you might look at replacing them. It's not following proper procedures is how BP's goat roping incident out in the GOM happen and the industry as a whole takes the hit.
Firstly regarding the air in the cement. The plug launching mechanism is not the only place that air entrainment occurs. Batch mixing and airtight plug launchers help. But air is there and it is a fact acknowledged by the industry who insist on the use of pressure scales to weigh slurry.
Secondly, it is too easy to sit in judgment from afar. Please provide the world with detail of which proper procedure BP/Transocean failed to follow before this tragic accident occurred?
Re: Trapped air inside casing and effect on cement « Reply #7 on May 20, 2010, 8:16am »
What's the big deal with a little air in the casing?
It will probably get dragged down with the slurry and compress in downhole conditions, perhaps separate from the slurry and rise up once it's out of the shoe; air rising through the slurry will create micro-channels, but those should disappear just as easily as they were created.
if air stays inside the casing, above or below the plug, it will compress to a negligible volume when holding the bump pressure.
What's the big deal with a little air in the casing?
It will probably get dragged down with the slurry and compress in downhole conditions, perhaps separate from the slurry and rise up once it's out of the shoe; air rising through the slurry will create micro-channels, but those should disappear just as easily as they were created.
if air stays inside the casing, above or below the plug, it will compress to a negligible volume when holding the bump pressure.
This thread has been examining why plugs do not bump. Your considered opinions is one of the explanations, i.e., in spite of the air finally compressing to a neglible volume the quantity of displacement fluid actually placed in the casing behind the plug is less than that required.
Re: Trapped air inside casing and effect on cement « Reply #9 on May 22, 2010, 1:10pm »
Quote:
boots99 wrote: On a 2 stage cement job, the plug for the first stage should be loaded into the plug loading head]
I have used cement heads single & double plug for CSG and plug dropping head for liner, but never heard about the above to load shut-off plug. Any link or file?
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