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Effect of Applied Pressure at surface during cement hardening
19 July 2010
All

Need your advice

Does anyone here experienced with the applied pressure at surface during cementing hardening. The reason behind this is we have a better reading of cement log (USIT) rather than conventional i.e. close cementing valve and WOC without pressure. In addition, the sub-surface team is pleased with the way we have as the oil is produce with minimal water-cut.
There is no different cement recipe that we used except the mechanical effort as we mentioned above.

I would be grateful if you can advice us with sound engineering background
Regards,
Gustioro
Medco Indonesia
3 answer(s)
prosdenv
Senior Drilling Engineer
Repsol
Total Posts: 1
Join Date: 18/11/12

Hi Gustiro
we had such practice in one of my past applications.
What we were doing was closing BOP while WOC and connecting cementing pump unit to annulus to keep some extra pressure in it.
The reason for that was a phenomenon when hydrostatic pressure in annulus while WOC is equal to water pressure (something around 1sg) because cement slurry is getting harder and hard structure is built whereas among the hard cement clusters we still have "free water". The water cannot withhold our pore pressure and gas/oil/water (whatever you have in the reservoir) starts moving up from the formation toward to surface, creating kind of worm holes (channels)which worsening your cement isolating properties and you may see it on your CBL.
Hope it will help.
Best regards
Denis 

david_white
Well Engineering Manager
NexenCNOOC
Total Posts: 30
Join Date: 30/06/08
Gustioro, as cement goes through it's curing process there comes a time when the cement thickens to a point where it loses it's ability to transmit hydrostatic pressure.  At this point the effective density is equivalent to water. 

A potential problem with some cement blends is that although the cement is curing properly it may not have the required properties to resist the influx of water, gas or oil from the formations penetrated by your well.  If you have encountered formation pressures higher than a water gradient fluid can enter the cement and start to channel to surface. 

Closing the annular and putting pressure on the top of the cement column is an attempt to replace the hydrostatic pressure required to keep the fluids in the reservoir.  The method can be successful in some situations.  Most service companies that supply cement for oilfield use will design the blend so that the time between losing hydrostatic pressure and gaining sufficient strength to resist influx is very short. Additives can also be added for Self Pressuring or expansion. 

Other companies have developed services to pressure pulse the cement column as the cement sets to maintain the ability to transmit hydrostatic pressure.  I've seen good results from the holding pressure technique but it can be challenging to convince your rig crew that they need to wait when they want to get on with rigging out and moving to the next well.  The technique is not always successful as the cement will not always transmit the applied pressure either. 

So, I usually recommend that well designers work closely with the cement service company to get the blend right and then do appropriate quality control checks to make sure that the blend performs in the well. 

Cheers
Dave

ifyclinton
Wellsite drilling engineer
Nigeria Petroleum Development Company
Total Posts: 4
Join Date: 07/01/13
Hi,
My exeperience in applying pressure during cement hardening (WOC) is to prevent U tubing effect (due to difference in weight of cement and displacing mud) as result of failure of both float shoe and collar during casing cementing. Thus applied pressure at surface will prevent cement placed in the annulus from coming into the casing string when float shoe and collar have failed.This will ensure  less cement inside the casing string to drill out and proper isolation at the shoe because adequate cement is placed above the shoe.
Regards,

Clinton.
(Benin City Nigeria)
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